Accelerator beam data commissioning equipment and procedures: Report of the TG-106 of the Therapy Physics Committee of the AAPM

AAPM WGTG51 Report 374: Guidance for TG-51 reference dosimetry

Practical guidelines that are not explicit in the TG-51 protocol and its Addendum for photon beam dosimetry are presented for the implementation of the TG-51 protocol for reference dosimetry of external high-energy photon and electron beams. These guidelines pertain to: (i) measurement of depth-ionization curves required to obtain beam quality specifiers for the selection of beam quality conversion factors, (ii) considerations for the dosimetry system and specifications of a reference-class ionization chamber, (iii) commissioning a dosimetry system and frequency of measurements, (iv) positioning/aligning the water tank and ionization chamber for depth ionization and reference dose measurements, (v) requirements for ancillary equipment needed to measure charge (triaxial cables and electrometers) and to correct for environmental conditions, and (vi) translation from dose at the reference depth to that at the depth required by the treatment planning system. Procedures are identified to achieve the most accurate results (errors up to 8% have been observed) and, where applicable, a commonly used simplified procedure is described and the impact on reference dosimetry measurements is discussed so that the medical physicist can be informed on where to allocate resources.

AAPM MEDICAL PHYSICS PRACTICE GUIDELINE 5.b: Commissioning and QA of treatment planning dose calculations-Megavoltage photon and electron beams

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines:

Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. While must is the term to be used in the guidelines, if an entity that adopts the guideline has shall as the preferred term, the AAPM considers that must and shall have the same meaning.

Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

Treatment planning system commissioning of the first clinical biology-guided radiotherapy machine

PURPOSE

The RefleXion X1 is a novel radiotherapy machine designed for image-guided radiotherapy (IGRT) and biology-guided radiotherapy (BgRT). Its treatment planning system (TPS) generates IMRT and SBRT plans for a 6MV-FFF beam delivered axially via 50 firing positions with the couch advancing every 2.1 mm. The purpose of this work is to report the TPS commissioning results for the first clinical installation of RefleXion™ X1.

METHODS

CT images of multiple phantoms were imported into the RefleXion TPS to evaluate the accuracy of data transfer, anatomical modeling, plan evaluation, and dose calculation. Comparisons were made between the X1, Eclipse™, and MIM™. Dosimetric parameters for open static fields were evaluated in water and heterogeneous slab phantoms. Representative clinical IMRT and SBRT cases were planned and verified with ion chamber, film, and ArcCHECK@ measurements. The agreement between TPS and measurements for various clinical plans was evaluated using Gamma analysis with a criterion of 3%/2 mm for ArcCHECK@ and film. End-to-end (E2E) testing was performed using anthropomorphic head and lung phantoms.

RESULTS

The average difference between the TPS-reported and known HU values was -1.4 ± 6.0 HU. For static fields, the agreements between the TPS-calculated and measured PDD10 , crossline profiles, and inline profiles (FWHM) were within 1.5%, 1.3%, and 0.5 mm, respectively. Measured output factors agreed with the TPS within 1.3%. Measured and calculated dose for static fields in heterogeneous phantoms agreed within 2.5%. The ArcCHECK@ mean absolute Gamma passing rate was 96.4% ± 3.4% for TG 119 and TG 244 plans and 97.8% ± 3.6% for the 21 clinical plans. E2E film analysis showed 0.8 mm total targeting error for isocentric and 1.1 mm for off-axis treatments.

CONCLUSIONS

The TPS commissioning results of the RefleXion X1 TPS were within the tolerances specified by AAPM TG 53, MPPG 5.a, TG 119, and TG 148. A subset of the commissioning tests has been identified as baseline data for an ongoing QA program.

[Effect of Differences in Insert Materials of Cutout Block on Dose Distribution in Electron Radiotherapy: Comparison between Measurements and Monte Carlo Simulation]

INTRODUCTION

In electron beam radiotherapy, an irradiation field is created with a cutout block using a low melting point lead alloy. The block can be replaced with a lead plate as a shield. Dose distribution is expected to be affected by differences in the material and thickness of the shield. Thus, this study aimed to investigate the cause of differences in dose distribution by reproducing the electron beam irradiation condition via Monte Carlo simulation, comparing dose distribution when each shield is used and analyzing energy fluence distribution.

MATERIALS AND METHODS

Radiation interaction in the treatment device manufactured by Varian was assessed using the general-purpose simulation code, and the dose distribution in the water was calculated. Electron energy fluence and incident angle of the electron fluence incident on the water surface were analyzed, and the effect of the difference in the shield was investigated in the irradiation field limited to 3 cm or less.

RESULTS

Regarding dose distribution, the deviation in the buildup area became larger when the lead plate was made thinner. A difference of 1.6-6.8% was observed on an average when comparing the buildup region of depth dose distributions except for 1×1 cm² field. In electron energy fluence, the lower the lead thickness, the higher the low energy component, which affected the buildup region. The effect was greater as the electron beam energy increased.

CONCLUSION

It was possible to evaluate the difference in scattered radiation between the low melting point lead alloy and the lead plate by MC simulation. Based on the study findings, the effect of scattered electrons generated from the block was strong as a factor.

Investigation on the impact of the leaf trailing effect using the Halcyon integrated platform system

PURPOSE

The double-stacked design of the Halcyon multileaf collimator (MLC) presents new challenges for treatment planning systems (TPSs). The leaf trailing effect has recently been described as the result of the interplay between the fluence transmitted through the leaf tip ends of each MLC layer. This effect makes the dosimetric leaf gap (DLG) dependent on the distance between the leaves of different layers (trailing distance) and is not adequately modeled by the Eclipse TPS. The purpose of our study was to investigate and report the dose discrepancies produced by these limitations in clinical plans and to explore how these discrepancies can be mitigated and avoided.

METHODS

The integrated platform with the Halcyon v2 system, Eclipse and Aria v15.6, was used. The dose discrepancies were obtained with EPID images and the portal dosimetry software and validated using radiochromic film dosimetry. The results for the AIDA commissioning test and for nine selected clinical beams with the sliding window intensity modulated radiotherapy (dIMRT) technique were thoroughly analyzed and presented. First, the DICOM RT plans were exported and the fluences were computed using different leaf tip models, and then were compared. Second, the detailed characteristics of the corresponding leaf sequences were investigated. Finally, modified DICOM RT plans were created in which the non-collimating (backup) leaves were retracted 2 mm to increase the leaf trailing distance, the modified plans were imported back into the TPS and the measurements were repeated. Dedicated in-house tools were developed in Python to carry out all analyses.

RESULTS

Dose discrepancies greater than 10% and regions of gamma failure were found in both the AIDA test and clinical beams using static-gantry dIMRT. Fluence analysis highlighted that the discrepancies were due to limitations in the MLC model implemented in the TPS. Analysis of leaf sequences indicated that regions of failure were associated with very low leaf speeds and virtually motionless leaves within the beam aperture. Some of these discrepancies were mitigated by increasing the trailing distance of the non-collimating leaves without affecting the beam aperture, but this strategy was not possible in regions where the leaves from both layers actively defined the beam aperture.

CONCLUSIONS

Current limitations of the MLC model in Eclipse produced discrepancies between calculated and delivered doses in clinical beams that caused plan-specific quality assurance failures and interruptions in the clinical workflow. Careful evaluation of the clinical plans produced by Eclipse for the Halcyon is recommended, especially for static gantry dIMRT treatments. Some characteristics of leaf sequences are problematic and should be avoided in clinical plans and, in general, a better leaf tip model is needed. This is particularly important in adaptive radiotherapy treatments, where the accuracy and reliability of TPS dose calculations are of the utmost importance.

Impact of systematic MLC positional uncertainties on the quality of single-isocenter multi-target VMAT-SRS treatment plans

Purpose

To study the impact of systematic MLC leaf positional uncertainties (stemming from mechanical inaccuracies or sub‐optimal MLC modeling) on the quality of intracranial single‐isocenter multi‐target VMAT‐SRS treatment plans. An estimation of appropriate tolerance levels is attempted.

Methods

Five patients, with three to four metastases and at least one target lying in close proximity to organs‐at‐risk (OARs) were included in this study. A single‐isocenter multi‐arc VMAT plan per patient was prepared, which served as the reference for dosimetric impact evaluation. A range of leaf offsets was introduced (±0.03 mm up to ±0.30 mm defined at the MLC plane) to both leaf banks, by varying the leaf offset MLC modeling parameter in Monaco for all the prepared plans, in order to simulate projected leaf offsets of ±0.09 mm up to ±0.94 mm at the isocenter plane, respectively. For all offsets simulated and cases studied, dose distributions were re‐calculated and compared with the corresponding reference ones. An experimental dosimetric procedure using the SRS mapCHECK diode array was also performed to support the simulation study results and investigate its suitability to detect small systematic leaf positional errors.

Results

Projected leaf offsets of ±0.09 mm were well‐tolerated with respect to both target dosimetry and OAR‐sparing. A linear relationship was found between D_95% percentage change and projected leaf offset (slope: 12%/mm). Impact of projected offset on target dosimetry was strongly associated with target volume. In two cases, plans that could be considered potentially clinically unacceptable (i.e., clinical dose constraint violation) were obtained even for projected offsets as small as 0.19 mm. The performed experimental dosimetry check can detect potential small systematic leaf errors.

Conclusions

Plan quality indices and dose–volume metrics are very sensitive to systematic sub‐millimeter leaf positional inaccuracies, projected at the isocenter plane. Acceptable and tolerance levels in systematic MLC uncertainties need to be tailored to VMAT‐SRS spatial and dosimetric accuracy requirements.

Validity of equivalent square field concept in small field dosimetry

PURPOSE

The equivalent Square (ES) concept has been used for traditional radiation fields defined by the machine collimating system. For small fields, the concept S_clin was introduced based on measuring dosimetric field width (full-width half maximum, FWHM) of the cardinal axis of the beam profiles. The pros and cons of this concept are evaluated in small fields and compared with the traditional ES using area and perimeter (4A/P) method based on geometric field size settings e.g. light field settings.

METHODS

One hundred thirty-seven square and rectangular fields from 5-50 mm with every possible permutation (keeping one jaw fixed and varying other jaw from 5 mm to 50 mm) were utilized to measure FWHM for the validation of S_clin . Using a microSilicon detector and a scanning water tank, measurements were performed on an Elekta (Versa) machine with Agility head and a Varian TrueBeam with different MLC/Jaw design to evaluate the S_clin concept and to understand the effect of exchange factor in small fields. Field output factors were also measured for all 137 fields.

RESULTS

The data fitting for fields ranging from 5-50 mm between the traditional 4A/P method and S_clin shows differences and indicates a linear relationship with distinct separation of slope for Elekta and Varian machines. As Elekta does not have y jaws, the ES based on 4A/P < S_clin but for the Varian linac 4A/P > S_clin for square fields. Our measured data shows that both methods are equally valid but does vary by the machine design. The field output factor is dependent on the elongation factor as well as machine design. For fields with sides ≥10 mm, the exchange factor is nearly identical in both machines with magnitude up to 4% which is close to measurement uncertainty (±3%) but for small fields (<10 mm) the Elekta machine has higher exchange factors compared to the Varian machine.

CONCLUSION

The results demonstrate that the two concepts for defining equivalent field (S_clin and 4A/P) are equivalent and can be directly related through an empirical equation. This study confirms that 4A/P is still valid for small fields except for very small fields (≤10 mm) where source occlusion is a dominating factor. The S_clin method is potentially sensitive to measurement uncertainty due to measurement of FWHM which is machine, detector and user dependent, while the 4A/P method relies mainly on geometry of the machine and has less dependency on type of machine, detector and user. The exchange factors are comparable for both types of machines. The conclusion is based on data from an Elekta with Agility head and a Varian TrueBeam machine that may have potential for bias due to light field/collimator set up and alignment. Care should be taken in extrapolating these data to any other machine. This article is protected by copyright. All rights reserved.

Longitudinal assessment of quality assurance measurements in a 1.5 T MR-linac: Part II-Magnetic resonance imaging

PURPOSE

To describe and report longitudinal quality assurance (QA) measurements for the magnetic resonance imaging (MRI) component of the Elekta Unity MR-linac during the first year of clinical use in our institution.

MATERIALS AND METHODS

The performance of the MRI component of Unity was evaluated with daily, weekly, monthly, and annual QA testing. The measurements monitor image uniformity, signal-to-noise ratio (SNR), resolution/detectability, slice position/thickness, linearity, central frequency, and geometric accuracy. In anticipation of routine use of quantitative imaging (qMRI), we characterize B0/B1 uniformity and the bias/reproducibility of longitudinal/transverse relaxation times (T1/T2) and apparent diffusion coefficient (ADC). Tolerance levels for QA measurements of qMRI biomarkers are derived from weekly monitoring of T1, T2, and ADC.

RESULTS

The 1-year assessment of QA measurements shows that daily variations in each MR quality metric are well below the threshold for failure. Routine testing procedures can reproducibly identify machine issues. The longitudinal three-dimensional (3D) geometric analysis reveals that the maximum distortion in a diameter of spherical volume (DSV) of 20, 30, 40, and 50 cm is 0.4, 0.6, 1.0, and 3.1 mm, respectively. The main source of distortion is gradient nonlinearity. Maximum peak-to-peak B0 inhomogeneity is 3.05 ppm, with gantry induced B0 inhomogeneities an order of magnitude smaller. The average deviation from the nominal B1 is within 2%, with minimal dependence on gantry angle. Mean ADC, T1, and T2 values are measured with high reproducibility. The median coefficient of variation for ADC, T1, and T2 is 1.3%, 1.1%, and 0.5%, respectively. The median bias for ADC, T1, and T2 is -0.8%, -0.1%, and 3.9%, respectively.

CONCLUSION

The MRI component of Unity operates within the guidelines and recommendations for scanner performance and stability. Our findings support the recently published guidance in establishing clinically acceptable tolerance levels for image quality. Highly reproducible qMRI measurements are feasible in Unity.

Efficient dose-rate correction of silicon diode relative dose measurements

PURPOSE

Silicon diodes are often the detector of choice for relative dose measurements, particularly in the context of radiotherapy involving small photon beams. However, a major drawback lies in their dose-rate dependency. Although ionization chambers are often too large for small field output factor measurements, they are valuable instruments to provide reliable percent-depth dose curves in reference beams. The aim of this work is to propose a practical and accurate method for the characterization of silicon diode dose-rate dependence correction factors using ionization chamber measurements as a reference.

METHODS

The robustness of ionization chambers for percent-depth dose measurements is used to quantify the dose-rate dependency of a diode detector. A mathematical formalism, which exploits the error induced in percent-depth ionization curves for diodes by their dose-rate dependency, is developed to derive a dose-rate correction factor applicable to diode relative measurements. The method is based on the definition of the recombination correction factor given in the addendum to TG 51 and is applied to experimental measurements performed on a CyberKnife M6 radiotherapy unit using a PTW 60012 diode detector. A measurement-based validation is provided by comparing corrected percent-depth ionization curves to measurements performed with a PTW 60019 diamond detector which does not exhibit dose-rate dependence.

RESULTS

Results of dose-rate correction factors for percent-depth ionization curves, off-axis ratios, tissue-phantom ratios and small field output factors are coherent with the expected behavior of silicon diode detectors. For all considered setups and field sizes, the maximum correction and the maximum impact of the uncertainties induced by the correction are obtained for off-axis ratios for the 60 mm collimator, with a correction of 2.5% and an uncertainty of 0.34%. For output factors, corrections range from 0.33% to 0.82% for all field sizes considered, and increase with the reduction of the field size. Comparison of percent-depth ionization curves corrected for dose-rate and for in-depth beam quality variations illustrate excellent agreement with measurements performed using the diamond detector.

CONCLUSIONS

The proposed method allows the efficient and precise correction of the dose-rate dependence of silicon diode detectors in the context of clinical relative dosimetry. This article is protected by copyright. All rights reserved.

A multi-institutional comparison of dosimetric data for a 0.35 T MR-linac

Objective. A comparison of percent depth dose (PDD) curves, lateral beam profiles, output factors (OFs), multileaf collimator (MLC) leakage, and couch transmission factors was performed between ten institutes for a commercial 0.35 T MR-linac. Approach. The measured data was collected during acceptance testing of the MR-linac. The PDD curves were measured for the 3.32 × 3.32 cm2, 9.96 × 9.96 cm2, and 27.20 × 24.07 cm2 field sizes. The lateral beam profiles were acquired for a 27.20 × 24.07 cm2 field size using an ion chamber array and penumbra was defined as the distance between 80% of the maximum dose and 20% of the maximum dose after normalizing the profiles to the dose at the inflection points. The OFs were measured using solid-state dosimeters, whereas radiochromic films were utilized to measure radiation leakage through the MLC stacks. The relative couch transmission factors were measured for various gantry angles. The variation in the multi-institutional data was quantified using the percent standard deviation metric. Main results. Minimal variations (<1%) were found between the PDD data, except for the build-up region and the deeper regions of the PDD curve. The in-field region of the lateral beam profiles varied <1.5% between different institutions and a small variation (<0.7 mm) in penumbra was observed. A variation of <1% was observed in the OF data for field sizes above 1.66 × 1.66 cm2, whereas large variations were shown for small-field sizes. The average and maximum MLC leakage was calculated to be <0.3% and <0.6%, which was well below the international electrotechnical commission (IEC) leakage thresholds. The couch transmission was smallest for oblique beams and ranged from 0.83 to 0.87. Significance. The variation in the data was found to be relatively small and the different 0.35 T MR-linacs were concluded to have similar dosimetric characteristics.

Evaluating dosimetric accuracy of the 6 MV calibration on EBT3 film in the use of Ir-192 high dose rate brachytherapy

Purpose

To evaluate the dosimetric accuracy of EBT3 film calibrated with a 6 MV beam for high dose rate brachytherapy and propose a novel method for direct film calibration with an Ir‐192 source.

Methods

The 6 MV calibration was performed in water on a linear accelerator (linac). The Ir‐192 calibration was accomplished by irradiating the film wrapped around a cylinder applicator with an Ir‐192 source. All films were scanned 1‐day post‐irradiation to acquire calibration curves for all three (red, blue, and green) channels. The Ir‐192 calibration films were also used for single‐dose comparison. Moreover, an independent test film under a H.A.M. applicator was irradiated and the 2D dose distribution was obtained separately for each calibration using the red channel data. Gamma analysis and point‐by‐point profile comparison were performed to evaluate the performance of both calibrations. The uncertainty budget for each calibration system was analyzed.

Results

The red channel had the best performance for both calibration systems in the single‐dose comparison. We found a significant 4.89% difference from the reference for doses <250 cGy using the 6 MV calibration, while the difference was only 0.87% for doses >600 cGy. Gamma analysis of the 2D dose distribution showed the Ir‐192 calibration had a higher passing rate of 91.9% for the 1 mm/2% criterion, compared to 83.5% for the 6 MV calibration. Most failing points were in the low‐dose region (<200 cGy). The point‐by‐point profile comparison reported a discrepancy of 2%–3.6% between the Ir‐192 and 6 MV calibrations in this low‐dose region. The linac‐ and Ir‐192‐based dosimetry systems had an uncertainty of 4.1% (k = 2) and 5.66% (k = 2), respectively.

Conclusions

Direct calibration of EBT3 films with an Ir‐192 source is feasible and reliable, while the dosimetric accuracy of 6 MV calibration depends on the dose range. The Ir‐192 calibration should be used when the measurement dose range is below 250 cGy.

Commissioning a secondary dose calculation software for a 0.35 T MR-linac

Secondary external dose calculations for a 0.35 T magnetic resonance image-guided radiation therapy (MRgRT) are needed within the radiation oncology community to follow safety standards set forth within the field. We evaluate the commercially available software, RadCalc, in its ability to accurately perform monitor unit dose calculations within a magnetic field. We also evaluate the potential effects of a 0.35 T magnetic field upon point dose calculations. Monitor unit calculations were evaluated with (wMag) and without (noMag) a magnetic field considerations in RadCalc for the ViewRay MRIdian. The magnetic field is indirectly accounted for by using asymmetric profiles for calculation. The introduction of double-stacked multi-leaf collimator leaves was also included in the monitor unit calculations and a single transmission value was determined. A suite of simple and complex geometries with a variety field arrangements were calculated for each method to demonstrate the effect of the 0.35 T magnetic field on monitor unit calculations. Finally, 25 patient-specific treatment plans were calculated using each method for comparison. All simple geometries calculated in RadCalc were within 2% of treatment planning system (TPS) values for both methods, except for a single noMag off-axis comparison. All complex muilt-leaf collimator (MLC) pattern calculations were within 5%. All complex phantom geometry calculations were within 5% except for a single field within a lung phantom at a distal point. For the patient calculations, the noMag method average percentage difference was 0.09 ± 2.5% and the wMag average percentage difference was 0.08 ± 2.5%. All results were within 5% for the wMag method. We performed monitor unit calculations for a 0.35 T MRgRT system using a commercially available secondary monitor unit dose calculation software and demonstrated minimal impact of the 0.35 T magnetic field on monitor unit dose calculations. This is the first investigation demonstrating successful calculations of dose using RadCalc in the low-field 0.35 T ViewRay MRIdian system.

Commissioning a multileaf collimator virtual cone for the stereotactic radiosurgery of trigeminal neuralgia

A multileaf collimator (MLC), virtual‐cone treatment technique has been commissioned for trigeminal neuralgia (TGN) at Tri‐Cities Cancer Center (TCCC). This novel technique was initially developed at the University of Alabama in Birmingham (UAB); it is designed to produce a spherical dose profile similar to a fixed, 5‐mm conical collimator distribution. Treatment is delivered with a 10‐MV flattening‐filter‐free (FFF) beam using a high‐definition MLC on a Varian Edge linear accelerator. Absolute dose output and profile measurements were performed in a 20 × 20 × 14 cm³ solid‐water phantom using an Exradin W2 scintillation detector and Gafchromic EBT3 film. Dose output constancy for the virtual cone was evaluated over 6 months using an Exradin A11 parallel plate chamber. The photo‐neutron dose generated by these treatments was assessed at distances of 50 and 100 cm from isocenter using a Ludlum Model 30–7 Series Neutron Meter. TGN treatments at TCCC have been previously delivered at 6‐MV FFF using a 5‐mm stereotactic cone. To assess the dosimetric impact of using a virtual cone, eight patients previously treated for TGN with a 5‐mm cone were re‐planned using a virtual cone. Seven patients have now been treated for TGN using a virtual cone at TCCC. Patient‐specific quality assurance was performed for each patient using Gafchromic EBT‐XD film inside a Standard Imaging Stereotactic Dose Verification Phantom. The commissioning results demonstrate that the virtual‐cone dosimetry, first described at UAB, is reproducible on a second Edge linear accelerator at an independent clinical site. The virtual cone is a credible alternative to a physical, stereotactic cone for the treatment of TGN at TCCC.

Transferability of patients for radiation treatment between unmatched machines

Purpose

The feasibility of transferring patients between unmatched machines for a limited number of treatment fractions was investigated for three‐dimensional conformal radiation therapy (3DCRT) and volumetric modulated arc therapy (VMAT) treatments.

Methods

Eighty patient‐plans were evaluated on two unmatched linacs: Elekta Versa HD and Elekta Infinity. Plans were equally divided into pelvis 3DCRT, prostate VMAT, brain VMAT, and lung VMAT plans. While maintaining the number of monitor units (MUs), plans were recalculated on the machine not originally used for treatment. Relative differences in dose were calculated between machines for the target volume and organs at risk (OARs). Differences in mean dose were assessed with paired t‐tests (p < 0.05). The number of interchangeable fractions allowable before surpassing a cumulative ±5% difference in dose was determined. Additionally, patient‐specific quality assurance (PSQA) measurements using ArcCHECK for both machines were compared with distributions calculated on the machine originally used for treatment using gradient compensation (GC) with 2%/2‐mm criteria.

Results

Interchanging the two machines for pelvic 3DCRT and VMAT (prostate, brain, and lung) plans resulted in an average change in target mean dose of 0.9%, −0.5%, 0.6%, 0.5%, respectively. Based on the differences in dose to the prescription point when changing machines, statistically, nearly one‐fourth of the prescribed fractions could be transferred between linacs for 3DCRT plans. While all of the prescribed fractions could typically be transferred among prostate VMAT plans, a rather large number of treatment fractions, 31% and 38%, could be transferred among brain and lung VMAT plans, respectively, without exceeding a ±5% change in the prescribed dose for two Elekta machines. Additionally, the OAR dosage was not affected within the given criterion with change of machine.

Conclusions

Despite small differences in calculated dose, transferring patients between two unmatched Elekta machines with similar multileaf collimator (MLC)‐head for target coverage and minimum changes in OAR dose is possible for a limited number of fractions (≤3) to improve clinical flexibility and institutional throughput along with patient satisfaction. A similar study could be carried out for other machines for operational throughput.

Personalized 3D-printed anthropomorphic whole-body phantom irradiated by protons, photons, and neutrons

The objective of this study was to confirm the feasibility of three-dimensionally-printed (3D-printed), personalized whole-body anthropomorphic phantoms for radiation dose measurements in a variety of charged and uncharged particle radiation fields. We 3D-printed a personalized whole-body phantom of an adult female with a height of 154.8 cm, mass of 90.7 kg, and body mass index of 37.8 kg/m². The phantom comprised of a hollow plastic shell filled with water and included a watertight access conduit for positioning dosimeters. It is compatible with a wide variety of radiation dosimeters, including ionization chambers that are suitable for uncharged and charged particles. Its mass was 6.8 kg empty and 98 kg when filled with water. Watertightness and mechanical robustness were confirmed after multiple experiments and transportations between institutions. The phantom was irradiated to the cranium with therapeutic beams of 170-MeV protons, 6-MV photons, and fast neutrons. Radiation absorbed dose was measured from the cranium to the pelvis along the longitudinal central axis of the phantom. The dose measurements were made using established dosimetry protocols and well-characterized instruments. For the therapeutic environments considered in this study, stray radiation from intracranial treatment beams was the lowest for proton therapy, intermediate for photon therapy, and highest for neutron therapy. An illustrative example set of measurements at the location of the thyroid for a square field of 5.3 cm per side resulted in 0.09, 0.59, and 1.93 cGy/Gy from proton, photon, and neutron beams, respectively. In this study, we found that 3D-printed personalized phantoms are feasible, inherently reproducible, and well-suited for therapeutic radiation measurements. The measurement methodologies we developed enabled the direct comparison of radiation exposures from neutron, proton, and photon beam irradiations.

Effect of inaccurate small field output factors on brain SRS plans

External beam radiotherapy often includes the use of field sizes 3 × 3 cm²or less, which can be defined as small fields. Dosimetry is a difficult, yet important part of the radiotherapy process. The dosimetry of small fields has additional challenges, which can lead to treatment inconsistencies if not done properly. Most important is the use of an appropriate detector, as well as the application of the necessary corrections. The International Atomic Energy Agency and the American Association of Physicists in Medicine provide the International Code of Practice (CoP) TRS-483 for the dosimetry of small static fields used in external MV photon beams. It gives guidelines on how to apply small-field correction factors for small field dosimetry. The purpose of this study was to evaluate the impact of inaccurate small-field output factors on clinical brain stereotactic radiosurgery plans with and without applying the small-field correction factors as suggested in the CoP. Small-field correction factors for a Varian TrueBeam linear accelerator were applied to uncorrected relative dose factors. Uncorrected and corrected clinical plans were created with two different beam configurations, 6 MV with a flattening filter (6 WFF) and 6 MV without a flattening filter (6 FFF). For the corrected plans, the planning target volume mean dose was 1.6 ± 0.9% lower with p < 0.001 for 6 WFF and 1.8 ± 1.5% lower with p < 0.001 for 6 FFF. For brainstem, a major organ at risk, the corrected plans had a dose that was 1.6 ± 0.9% lower with p = 0.03 for 6 WFF and 1.8 ± 1.5% lower with p = 0.10 for 6 FFF. This represents a systematic error that should and can be corrected.

Technical Note: Bremsstrahlung dose in the electron beam at extended distances in total skin electron therapy

PURPOSE

Electron beam from a linear accelerator is commonly used in total skin electron Therapy (TSET) at extended distances. Since Das et al (Med Phys 21, p.1733, 1994) reported 5% bremsstrahlung dose for a 6 MeV electron beam at extended distance of 500 cm it has been accepted as common knowledge. However, measurements by Chen et al (Int J. Rad Onc Biol Phys 59 p.872, 2004) and Monte Carlo simulations by Ding et al (Phys. Med. Biol. 66, 075010, 2021) were unable to reproduce such high bremsstrahlung dose. As bremsstrahlung dose contributes to whole-body dose which could produce bone marrow toxicity with serious complications for the outcome of the TSET, it is important to re-evaluate the magnitude of bremsstrahlung dose accurately.

METHODS

The EGSnrc Monte Carlo system is used to investigate bremsstrahlung doses from 6 MeV high dose rate total skin electron (HDTSe) beams from Varian TrueBeam and Clinac Accelerators. The measurements were carried out at depth of d_max and 5 cm in solid water and Acrylic phantoms at extended distances using a parallel-plate chamber and a cylindrical ion chamber.

RESULTS

We were able to reproduce previously reported high bremsstrahlung dose at extended distances by using a parallel plate ionization chamber. However, both the measurements by using a cylindrical chamber and Monte Carlo simulations showed an insignificant bremsstrahlung dose (∼1%) even at SSD = 500 cm.

CONCLUSION

The bremsstrahlung doses of a 6 MeV electron beam are 0.5% to 1% for SSD from 100 to 700 cm, although it increases with the increasing extended distance. The common belief of up to 5% bremsstrahlung dose at large extended distances is incorrect. Previously reported high bremsstrahlung doses might be due to poor signal-to-noise ratio of using parallel plate chamber for measuring very low dose or particular set-up. This article is protected by copyright. All rights reserved.

Dosimetric validation of Acuros® XB algorithm for RapidArc™ treatment technique: A post software upgrade analysis

Aim

To validate the Acuros^® XB (AXB) algorithm in Eclipse treatment planning system (TPS) for RapidArc™ (RA) technique following the software upgrades.

Materials and Methods

A Clinac-iX (2300CD) linear accelerator and Eclipse TPS (Varian Medical System, Inc., Palo Alto, USA) was used for commissioning of AXB algorithm using a 6 megavolts photon beam. Percentage depth dose (PDD) and profiles for field size 2 cm × 2 cm, 4 cm × 4 cm, 6 cm × 6 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm to 40 cm × 40 cm were taken. AXB calculated PDDs and profiles were evaluated against the measured and analytical anisotropic algorithm (AAA)-calculated PDDs and profiles. Test sites recommended by American Association of Physicists in Medicine task group (AAPM TG)-119 recommendation were used for RA planning and delivery verification using AXB algorithm.

Results

Dosimetric analysis of AXB calculated data showed that difference between calculated and measured data for PDD curves were maximum <1% beyond the depth of dose maximum and computed profiles in central region matches with maximum <1% for all considered field sizes. Ion-chamber measurements showed that the average confidence limit (CLs) was 0.034 and 0.020 in high-gradient and 0.047 and 0.042 in low-gradient regions, respectively, for AAA and AXB calculated RA plans. Portal measurements show the average CLs were 2.48 and 2.58 for AAA and AXB-calculated RA plans, with gamma passing criteria of 3%/3 mm.

Conclusions

AXB shows excellent agreement with measurements and AAA calculated data. The CLs were consistent with the baseline values published by TG-119. AXB algorithm has the potential to perform photon dose calculation with comparable fast calculation speed without negotiating the accuracy. AAPM TG-119 was successfully implemented to access the proper configuration of AXB algorithm following the TPS upgrade.

Beam modeling and commissioning for Monte Carlo photon beam on an Elekta Versa HD LINAC

PURPOSE

This study aims at analyzing beam data commissioning along with verifying Monte Carlo-based treatment planning system on the basis of the manufacturer guidelines for Elekta Versa HD Linear Accelerator. Moreover, evaluating the beam match process in terms of quality index, photon profile and multi leaf collimator (MLC) offset is aimed as well.

MATERIALS AND METHODS

The process of collecting beam data for Monaco 5.51 Treatment Planning System commissioning was done based on the instructions provided by the manufacturer as well as AAPM TG-106. Monte Carlo analysis was done for output factors in water, percent depth dose and beam profiles. A set of eight static and intensity modulated radiation therapy fields were used to verify the MLC parameters.

RESULTS

The difference between the measured and modeled penetrative quality (D₁₀) was achieved to be 0.54%. The output factors for 6 MV photon energy were measured and the difference between the measured and Monte Carlo output results was smaller than 1% for all the fields. The average percentage of passing the gamma criteria for commissioning test fields was (95+-4)%, however, the minimum agreement was 87.5% belonging to "7SEGA". Additionally, the agreement between both LINAC is 96%, however, the second LINAC reveals a positive offset in the point of approximately -4 cm on the x-axis at the crossplane.

CONCLUSION

Test commissioning was successfully verified using a homogeneous phantom for point dose measurements, post modelling MLC parameters and patient QA plans. All plan parameters pass the gamma criteria. 6 MV photon beam was successfully commissioned for Elekta VersaHD LINAC and is ready for clinical implementation.

Assessment of Delivery Quality Assurance for Stereotactic Radiosurgery With Cyberknife

Purpose

The purpose of this study is to establish and assess a practical delivery quality assurance method for stereotactic radiosurgery with Cyberknife by analyzing the geometric and dosimetric accuracies obtained using a PTW31016 PinPoint ionization chamber and EBT3 films. Moreover, this study also explores the relationship between the parameters of plan complexity, target volume, and deliverability parameters and provides a valuable reference for improving plan optimization and validation.

Methods

One hundred fifty cases of delivery quality assurance plans were performed on Cyberknife to assess point dose and planar dose distribution, respectively, using a PTW31016 PinPoint ionization chamber and Gafchromic EBT3 films. The measured chamber doses were compared with the planned mean doses in the sensitive volume of the chamber, and the measured planar doses were compared with the calculated dose distribution using gamma index analysis. The gamma passing rates were evaluated using the criteria of 3%/1 mm and 2%/2 mm. The statistical significance of the correlations between the complexity metrics, target volume, and the gamma passing rate were analyzed using Spearman’s rank correlation coefficient.

Results

For point dose comparison, the averaged dose differences (± standard deviations) were 1.6 ± 0.73% for all the cases. For planar dose distribution, the mean gamma passing rate for 3%/1 mm, and 2%/2 mm evaluation criteria were 94.26% ± 1.89%, and 93.86% ± 2.16%, respectively. The gamma passing rates were higher than 90% for all the delivery quality assurance plans with the criteria of 3%/1 mm and 2%/2 mm. The difference in point dose was lowly correlated with volume of PTV, number of beams, and treatment time for 150 DQA plans, and highly correlated with volume of PTV for 18 DQA plans of small target. DQA gamma passing rate (2%/2 mm) was a moderate significant correlation for the number of nodes, number of beams and treatment time, and a low correlation with MU.

Conclusion

PTW31016 PinPoint ionization chamber and EBT3 film can be used for routine Cyberknife delivery quality assurance. The point dose difference should be within 3%. The gamma passing rate should be higher than 90% for the criteria of 3%/1 mm and 2%/2 mm. In addition, the plan complexity and PTV volume were found to have some influence on the plan deliverability.

Electron Beam Measurements Employing Electron Montecarlo Algorithm on TrueBeam STx® and Clinac iX® Linear Accelerators

Electron beam measurement comparison between TrueBeam STx® and Clinac iX® established. Data evaluation of eMC-calculated and measured for TrueBeam STx® performed. Dosimetric parameters measured including depth dose curves for each applicator, percentage depth dose (PDDs) curves without applicator, the profile in-air for a large field size 40×40 cm2, and the Absolute Dose (cGy/MU) for each applicator using a large water phantom (PTW, Freiburg, Germany), employing Roos and Markus plane-parallel ionization chambers. The data were examined for five electron beams of Varian’s TrueBeam STx® and Clinac iX® machines. A comparison between measurement PDDs and calculated by the Eclipse electron Monte Carlo (eMC) algorithm was performed to validate Truebeam STx® commissioning. The measured data indicated that electron beam PDDs from the TrueBeam STx® machine are well matched to those from Clinac iX® machine. The quality index R50 for applicator 15×15 cm2 was in the tolerance intervals. However, Surface dose (Ds) increases with increasing energy for both accelerators. Comparisons between the measured and eMC-calculated values revealed that the R100, R90, R80, and R50 values mostly agree within 5 mm. Measured and calculated bremsstrahlung tail Rp correlates well statistically. Ds agrees mostly within 2%. Electron beams were successfully validated for TrueBeam STx®, a good agreement between modeled and measured data was observed.

Comparing the dosimeter-specific corrections for influence quantities of some parallel-plate ionization chambers in conventional electron beam dosimetry

The performance characteristics of some widely employed parallel-plate ionization chambers in dosimetry of conventional high energy electron beams were evaluated and compared in the present study following the recommendations of the IAEA TRS-398 reference dosimetry protocol. Three different types of PTW-made parallel-plate ionization chambers including Roos (TM34001), Markus (TM23343), and Advanced Markus (TM34045) were employed, and correction factors for polarity (k_pol), recombination (k_s), and quality conversion factor ( [Formula: see text] ) were determined at different nominal electron energies of 4, 6, 9, 12, 16, and 20 MeV produced by a Varian Trilogy clinical Linac. All measurements were performed inside a MP3-M automatic water phantom in the reference condition of 100 cm SSD (source to surface distance), reference measurement depth (z_ref), and 10 × 10 cm² field size at the phantom surface. The maximum and minimum range of k_pol deviations from unity were respectively found for Markus and Roos ionization chambers. The maximum k_s values also belonged to the Markus ionization chamber, while the minimum k_s values were observed for the Advanced Markus chamber. The measured k_s values through recommendations of the TRS-398 dosimetry protocol were in good accordance with those obtained by Jaffe-plot analysis for all considered ionization chambers. The type of employed ionization chamber can minimally affect the measured electron beam quality index (R₅₀), while it can have a more considerable impact on [Formula: see text] value, especially in the case of the Markus chamber. From the results, it can be concluded that the Roos and Advanced Markus ionization chambers have a superior performance in the case of electron beam dosimetry, although all considered ionization chambers fulfilled the criteria requested by relevant reference dosimetry protocols.

CytoRADx: A High-Throughput, Standardized Biodosimetry Diagnostic System Based on the Cytokinesis-Block Micronucleus Assay

In a large-scale catastrophe, such as a nuclear detonation in a major city, it will be crucial to accurately diagnose large numbers of people to direct scarce medical resources to those in greatest need. Currently no FDA-cleared tests are available to diagnose radiation exposures, which can lead to complex, life-threatening injuries. To address this gap, we have achieved substantial advancements in radiation biodosimetry through refinement and adaptation of the cytokinesis-block micronucleus (CBMN) assay as a high throughput, quantitative diagnostic test. The classical CBMN approach, which quantifies micronuclei (MN) resulting from DNA damage, suffers from considerable time and expert labor requirements, in addition to a lack of universal methodology across laboratories. We have developed the CytoRADx™ System to address these drawbacks by implementing a standardized reagent kit, optimized assay protocol, fully automated microscopy and image analysis, and integrated dose prediction. These enhancements allow the CytoRADx System to obtain high-throughput, standardized results without specialized labor or laboratory-specific calibration curves. The CytoRADx System has been optimized for use with both humans and non-human primates (NHP) to quantify radiation dose-dependent formation of micronuclei in lymphocytes, observed using whole blood samples. Cell nuclei and resulting MN are fluorescently stained and preserved on durable microscope slides using materials provided in the kit. Up to 1,000 slides per day are subsequently scanned using the commercially based RADxScan™ Imager with customized software, which automatically quantifies the cellular features and calculates the radiation dose. Using less than 1 mL of blood, irradiated ex vivo, our system has demonstrated accurate and precise measurement of exposures from 0 to 8 Gy (90% of results within 1 Gy of delivered dose). These results were obtained from 636 human samples (24 distinct donors) and 445 NHP samples (30 distinct subjects). The system demonstrated comparable results during in vivo studies, including an investigation of 43 NHPs receiving single-dose total-body irradiation. System performance is repeatable across laboratories, operators, and instruments. Results are also statistically similar across diverse populations, considering various demographics, common medications, medical conditions, and acute injuries associated with radiological disasters. Dose calculations are stable over time as well, providing reproducible results for at least 28 days postirradiation, and for blood specimens collected and stored at room temperature for at least 72 h. The CytoRADx System provides significant advancements in the field of biodosimetry that will enable accurate diagnoses across diverse populations in large-scale emergency scenarios. In addition, our technological enhancements to the well-established CBMN assay provide a pathway for future diagnostic applications, such as toxicology and oncology.

Longitudinal assessment of quality assurance measurements in a 1.5T MR-linac: Part I-Linear accelerator

Purpose

To describe and report longitudinal quality assurance (QA) measurements for the mechanical and dosimetric performance of an Elekta Unity MR‐linac during the first year of clinical use in our institution.

Materials and methods

The mechanical and dosimetric performance of the MR‐linac was evaluated with daily, weekly, monthly, and annual QA testing. The measurements monitor the size of the radiation isocenter, the MR‐to‐MV isocenter concordance, MLC and jaw position, the accuracy and reproducibility of step‐and‐shoot delivery, radiation output and beam profile constancy, and patient‐specific QA for the first 50 treatments in our institution. Results from end‐to‐end QA using anthropomorphic phantoms are also included as a reference for baseline comparisons. Measurements were performed in water or water‐equivalent plastic using ion chambers of various sizes, an ion chamber array, MR‐compatible 2D/3D diode array, portal imager, MRI, and radiochromic film.

Results

The diameter of the radiation isocenter and the distance between the MR/MV isocenters was (μ ± σ) 0.39 ± 0.01 mm and 0.89 ± 0.05 mm, respectively. Trend analysis shows both measurements to be well within the tolerance of 1.0 mm. MLC and jaw positional accuracy was within 1.0 mm while the dosimetric performance of step‐and‐shoot delivery was within 2.0%, irrespective of gantry angle. Radiation output and beam profile constancy were within 2.0% and 1.0%, respectively. End‐to‐end testing performed with ion‐chamber and radiochromic film showed excellent agreement with treatment plan. Patient‐specific QA using a 3D diode array identified gantry angles with low‐pass rates allowing for improvements in plan quality after necessary adjustments.

Conclusion

The MR‐linac operates within the guidelines of current recommendations for linear accelerator performance, stability, and safety. The analysis of the data supports the recently published guidance in establishing clinically acceptable tolerance levels for relative and absolute measurements.

Reconstruction of volume averaging effect-free continuous photon beam profiles from discrete ionization chamber array measurements using a machine learning technique

PURPOSE

The use of the ionization chamber array ICProfiler (ICP) is limited by its relatively poor detector spatial resolution and the inherent volume averaging effect (VAE). The purpose of this work is to study the feasibility of reconstructing VAE-free continuous photon beam profiles from ICP measurements with a machine learning technique.

METHODS

In- and cross-plane photon beam profiles of a 6 MV beam from an Elekta linear accelerator, ranging from 2 × 2 to 10 × 10 cm² at 1.5 cm, 5 cm, and 10 cm depth, were measured with an ICP. The discrete measurements were interpolated with a Makima method to obtain continuous beam profiles. Artificial neural networks (ANNs) were trained to restore the penumbra of the beam profiles. Plane-specific (in- and cr-plane) ANNs and a combined ANN were separately trained. The performance of the ANNs was evaluated using the penumbra width difference (PWD, the difference between the penumbra widths of the reconstructed and the reference profile). The plane-specific and the combined ANNs were compared to study the feasibility of using a single ANN for both in- and cross-plane.

RESULTS

The profiles reconstructed with all the ANNs had excellent agreement with the reference. For in-plane, the ANNs reduced the PWD from 1.6 ± 0.7 mm at 1.5 cm depth to 0.1 ± 0.1 mm, from 1.8 ± 0.6 mm at 5.0 cm depth to 0.1 ± 0.1 mm, and from 2.4 ± 0.1 mm at 10.0 cm depth to 0.0 ± 0.0 mm; for cross-plane, the ANNs reduced the PWD from 1.2 ± 0.4 mm at 1.5 cm depth, 1.2 ± 0.3 mm at 5.0 cm depth, and 1.6 ± 0.1 mm at 10.0 cm depth, to 0.1 ± 0.1 mm.

CONCLUSIONS

This study demonstrated the feasibility of using simple ANNs to reconstruct VAE-free continuous photon beam profiles from discrete ICP measurements. A combined ANN can restore the penumbra of in- and cross-plane beam profiles of various fields at different depths.

Acceptance procedure for the linear accelerator component of the 1.5 T MRI-linac

Purpose

To develop and implement an acceptance procedure for the new Elekta Unity 1.5 T MRI‐linac.

Methods

Tests were adopted and, where necessary adapted, from AAPM TG106 and TG142, IEC 60976 and NCS 9 and NCS 22 guidelines. Adaptations were necessary because of the atypical maximum field size (57.4 × 22 cm), FFF beam, the non‐rotating collimator, the absence of a light field, the presence of the 1.5 T magnetic field, restricted access to equipment within the bore, fixed vertical and lateral table position, and the need for MR image to MV treatment alignment. The performance specifications were set for stereotactic body radiotherapy (SBRT).

Results

The new procedure was performed similarly to that of a conventional kilovoltage x‐ray (kV) image guided radiation therapy (IGRT) linac. Results were acquired for the first Unity system.

Conclusions

A comprehensive set of tests was developed, described and implemented for the MRI‐linac. The MRI‐linac met safety requirements for patients and operators. The system delivered radiation very accurately with, for example a gantry rotation locus of isocenter of radius 0.38 mm and an average MLC absolute positional error of 0.29 mm, consistent with use for SBRT. Specifications for clinical introduction were met.

Current Evidence for Stereotactic Body Radiotherapy in Lung Metastases

Lung metastases are the second most common malignant neoplasms of the lung. It is estimated that 20–54% of cancer patients have lung metastases at some point during their disease course, and at least 50% of cancer-related deaths occur at this stage. Lung metastases are widely accepted to be oligometastatic when five lesions or less occur separately in up to three organs. Stereotactic body radiation therapy (SBRT) is a noninvasive, safe, and effective treatment for metastatic lung disease in carefully selected patients. There is no current consensus on the ideal dose and fractionation for SBRT in lung metastases, and it is the subject of study in ongoing clinical trials, which examines different locations in the lung (central and peripheral). This review discusses current indications, fractionations, challenges, and technical requirements for lung SBRT.

Stereotactic radiosurgery commissioning and QA test cases-A TG-119 approach for Stereotactic radiosurgery

PURPOSE

To develop a standardized set of representative clinical treatment cases that pose a range of optimization problems for evaluating the plan quality and dosimetric accuracy within the commissioning process for linac-based stereotactic radiosurgery (SRS).

METHODS

Five test cases with increasing complexity were created to validate delivery accuracy in SRS commissioning similar to the approach used by AAPM TG-119 in developing a test suite for IMRT commissioning. Standardized structure sets, planning goals, and delivery requirements were specified for each case including a small sphere target, irregular target, irregular target placed off-axis, multi-target, and abutting organs-at-risk (OARs). Various VMAT field arrangements including a single arc, two coplanar arcs, full arc and vertex half arc, and four noncoplanar arcs were tested to generate clinically appropriate treatment plans.

RESULTS

The small spherical target was 1.0 cm in diameter. The irregular target was a clinical cavity (2.3 × 2.2 × 1.4 cm³) and was shifted 4.5 cm for the irregular target off-axis case. The multi-target case used the irregular target and four spherical targets representing metastases ranging 0.9 to 1.6 cm in diameter, placed up to 7.5 cm off-axis. The abutting OARs case included an acoustic neuroma and target placed near the optic nerve. All spherical targets received 24 Gy and the cavity received 18 Gy. The abutting OAR cases included a 3.74 cc lesion adjacent to the brainstem receiving 13 Gy and a 1.11 cc lesion adjacent to the optic nerve receiving 12 Gy. All plans used a single-isocenter placed at the target center or geometric center of multiple targets. Planning goals for all cases included constraints for the target and brain minus PTV, along with brainstem and optic nerve where applicable. Deliverability was assessed through ion chamber measurements, in addition to composite and per-field planar measurements on Gafchromic film and small-field diode array. A mean and SD for measured versus planned doses of 101.0% ± 2.9% was observed over the 14 ion chamber measurements. Mean and SD for gamma pass rates were 98.5% ± 2.2% and 97.1% ± 4.9% for film and diode array, respectively, for gamma criteria of 2% and 1 mm.

CONCLUSION

These cases could provide the preliminary groundwork for a novel benchmark for institutions to evaluate linac-based SRS commissioning and delivery accuracy prior to clinical implementation. The rapid widespread implementation of linac-based SRS, the complexity associated with dosimetry and delivery, and high-profile treatment deviations that have already resulted from its use, highlight the importance of such a benchmark test suite. Comprehensive dosimetric measurements from this standardized set of SRS optimization problems were used to fine-tune and understand the limitations of our SRS planning and delivery system and establish a set of baseline data for comparison with other delivery platforms.

Surface dose and build-up region depth dose measurements in non-standard beams of Cyberknife and tomotherapy systems

The purpose of this study was to measure the surface dose and build-up region depth dose characteristics of 6 MV photon beams in Cyberknife and helical tomotherapy (HT) systems for non-standard small fields using parallel plate chambers (Roos and Markus), Gafchromic EBT3 films, and nanoDot optically stimulated luminescence dosimeters (OSLDs), as well as to investigate the effect of oblique incidence on the surface dose of the beam. All measurements were conducted in a virtual water phantom under machine-specific reference conditions. The Roos and OSLDs overestimated the surface dose when compared with the Markus chamber and EBT3 films by 20%. We applied water equivalent thickness (WET) correction to account for the intrinsic build-up thickness of the detectors from their effective point of measurement (EPOM). With WET correction, a reasonably close surface dose estimate was obtained for all detectors, within 1.9% agreement for the 60 mm collimator of Cyberknife and 3.1% agreement for the HT system, with a 5 × 10 cm² field size. The surface dose increased from the normally incident Cyberknife and HT fields with increasing angle of incidence. The surface dose increased to twice its value at normal incidence for highly oblique angles of incidence above 55°. For the tested fields, a reasonable surface dose estimate could be measured with the detectors if the correction for intrinsic buildup thickness was applied. Nevertheless, the use of Roos chambers with large dimensions and nanoDot OSLDs is not recommended for estimating the surface dose for small fields.

Medical X-band linear accelerator for high-precision radiotherapy

PURPOSE

Recently, high-precision radiotherapy systems have been developed by integrating computerized tomography or magnetic resonance imaging to enhance the precision of radiotherapy. For integration with additional imaging systems in a limited space, miniaturization and weight reduction of the linear accelerator (linac) system have become important. The aim of this work is to develop a compact medical linac based on 9.3 GHz X-band RF technology instead of the S-band RF technology typically used in the radiotherapy field.

METHODS

The accelerating tube was designed by 3D finite-difference time-domain and particle-in-cell simulations because the frequency variation resulting from the structural parameters and processing errors is relatively sensitive to the operating performance of the X-band linac. Through the 3D simulation of the electric field distribution and beam dynamics process, we designed an accelerating tube to efficiently accelerate the electron beam and used a magnetron as the RF source to miniaturize the entire linac. In addition, a side-coupled structure was adopted to design a compact linac to reduce the RF power loss. To verify the performance of the linac, we developed a beam diagnostic system to analyze the electron beam characteristics and a quality assurance (QA) experimental environment including 3D lateral water phantoms to analyze the primary performance parameters (energy, dose rate, flatness, symmetry, and penumbra) The QA process was based on the standard protocols AAPM TG-51, 106, 142 and IAEA TRS-398.

RESULTS

The X-band linac has high shunt impedance and electric field strength. Therefore, even though the length of the accelerating tube is 37 cm, the linac could accelerate an electron beam to more than 6 MeV and produce a beam current of more than 90 mA. The transmission ratio is measured to be approximately 30% ~ 40% when the electron gun operates in the constant emission region. The percent depth dose ratio at the measured depths of 10 and 20 cm was approximately 0.572, so we verified that the photon beam energy was matched to approximately 6 MV. The maximum dose rate was measured as 820 cGy/min when the source-to-skin distance was 80 cm. The symmetry was smaller than the QA standard and the flatness had a higher than standard value due to the flattening filter-free beam characteristics. In the case of the penumbra, it was not sufficiently steep compared to commercial equipment, but it could be compensated by improving additional devices such as multileaf collimator and jaw.

CONCLUSIONS

A 9.3 GHz X-band medical linac was developed for high-precision radiotherapy. Since a more precise design and machining process are required for X-band RF technology, this linac was developed by performing a 3D simulation and ultraprecision machining. The X-band linac has a short length and a compact volume, but it can generate a validated therapeutic beam. Therefore, it has more flexibility to be coupled with imaging systems such as CT or MRI and can reduce the bore size of the gantry. In addition, the weight reduction can improve the mechanical stiffness of the unit and reduce the mechanical load.

Commissioning of and preliminary experience with a new fully integrated computed tomography linac

Purpose

A new medical linear accelerator (linac) platform integrated with helical computed tomography (CT), the uRT‐linac 506c, was introduced into clinical application in 2019 by United Imaging Healthcare (UIH) Co., Ltd. (Shanghai, China). It combines a Carm linac with a diagnostic‐quality 16‐slice CT imager, providing seamless workflow from simulation to treatment. The aim of this report is to assess the technical characteristics, commissioning results and preliminary experiences stemming from clinical usage.

Methods

The mechanical and imaging test procedures, commissioning data collection and TPS validation were summarized. CTIGRT accuracy was investigated with different loads and couch extensions. A series of end‐to‐end cases for different treatment sites and delivery techniques were tested preclinically to estimate the overall accuracy for the entire treatment scheme. The results of patient‐specific QA and machine stability during a one‐year operation are also reported.

Results

Gantry/couch/collimator isocentricity was measured as 0.63 mm in radius. The TPS models were in agreement with the beam commissioning data within a deviation of 2%. An overall submillimeter accuracy was demonstrated for the CT‐IGRT process under all conditions. The absolute point dose difference for all the preclinical end‐to‐end tests was within 3%, and the gamma passing rate of the 2D dose distribution measured by EBT3 film was better than 90% (3% DD, 3 mm DTA and 10% threshold). Pretreatment QA of clinical cases resulted with better than 3% point dose difference and more than 99% gamma passing rate (3% DD/2 mm DTA/10% threshold) tested with Delta4. The output of the linac was mostly within 1% of variation in a one‐year operation.

Conclusion

The commissioning results and clinical QA results show that the uRT‐linac 506c platform exhibits good and stable performance in mechanical and dosimetric accuracy. The integrated CT system provides an efficient workflow for image guidance with submillimeter localization precision, and will be a good starting point to proceed advanced adaptive radiotherapy.

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions

Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

Evaluation of candidate template beam models for a matched TrueBeam treatment delivery system

PURPOSE

To explore candidate RayStation beam models to serve as a class-specific template for a TrueBeam treatment delivery system.

METHODS

Established validation techniques were used to evaluate three photon beam models: a clinically optimized model from the authors' institution, the built-in RayStation template, and a hybrid consisting of the RayStation template except substituting average MLC parameter values from a recent IROC survey. Comparisons were made for output factors, dose profiles from open fields, as well as representative VMAT test plans.

RESULTS

For jaw-defined output factors, each beam model was within 1.6% of expected published values. Similarly, the majority (57-66%) of jaw-defined dose curves from each model had a gamma pass rate >95% (2% / 3 mm, 20% threshold) when compared to TrueBeam representative beam data. For dose curves from MPPG 5.a MLC-defined fields, average gamma pass rates (1% / 1 mm, 20% threshold) were 92.9%, 85.1%, and 86.0% for the clinical, template, and hybrid models, respectively. For VMAT test plans measured with a diode array detector, median dose differences were 0.6%, 1.3%, and 1.1% for the clinical, template, and hybrid models, respectively. For in-phantom ionization chamber measurements with the same VMAT test plans, the average percent difference was -0.3%, -1.4%, and -1.0% for the clinical, template, and hybrid models, respectively.

CONCLUSION

Beam model templates taken from the vendor and aggregate results within the community were both reasonable starting points, but neither approach was as optimal as a clinically tuned model, the latter producing better agreement with all validation measurements. Given these results, the clinically optimized model represents a better candidate as a consensus template. This can benefit the community by reducing commissioning time and improving dose calculation accuracy for matched TrueBeam treatment delivery systems.

AAPM Task Group 198 Report: An implementation guide for TG 142 quality assurance of medical accelerators

The charges on this task group (TG) were as follows: (a) provide specific procedural guidelines for performing the tests recommended in TG 142; (b) provide estimate of the range of time, appropriate personnel, and qualifications necessary to complete the tests in TG 142; and (c) provide sample daily, weekly, monthly, or annual quality assurance (QA) forms. Many of the guidelines in this report are drawn from the literature and are included in the references. When literature was not available, specific test methods reflect the experiences of the TG members (e.g., a test method for door interlock is self-evident with no literature necessary). In other cases, the technology is so new that no literature for test methods was available. Given broad clinical adaptation of volumetric modulated arc therapy (VMAT), which is not a specific topic of TG 142, several tests and criteria specific to VMAT were added.

Dosimetric leaf gap and leaf trailing effect in a double-stacked multileaf collimator

PURPOSE

To investigate (i) the dosimetric leaf gap (DLG) and the effect of the "trailing distance" between leaves from different multileaf collimator (MLC) layers in Halcyon systems and (ii) the ability of the currently available treatment planning systems (TPSs) to approximate this effect.

METHODS

DICOM plans with transmission beams and sweeping gap tests were created in Python for measuring the DLG for each MLC layer independently and for both layers combined. In clinical Halcyon plans both MLC layers are interchangeably used and leaves from different layers are offset, thus forming a trailing pattern. To characterize the impact of such configuration, new tests called "trailing sweeping gaps" were designed and created where the leaves from one layer follow the leaves from the other layer at a fixed "trailing distance" t between the tips. Measurements were carried out on five Halcyons SX2 from different institutions and calculations from both the Eclipse and RayStation TPSs were compared with measurements.

RESULTS

The dose accumulated during a sweeping gap delivery progressively increased with the trailing distance t . We call this "the trailing effect." It is most pronounced for t between 0 and 5 mm, although some changes were obtained up to 20 mm. The dose variation was independent of the gap size. The measured DLG values also increased with t up to 20 mm, again with the steepest variation between 0 and 5 mm. Measured DLG values were negative at t  = 0 (the leaves from both layers at the same position) but changed sign for t  ≥ 1 mm, in line with the positive DLG sign usually observed with single-layer rounded-end MLCs. The Eclipse TPS does not explicitly model the leaf tip and, as a consequence, could not predict the dose reduction due to the trailing effect. This resulted in dose discrepancies up to +10% and -8% for the 5 mm sweeping gap and up to ±5% for the 10 mm one depending on the distance t . RayStation implements a simple model of the leaf tip that was able to approximate the trailing effect and improved the agreement with measured doses. In particular, with a prototype version of RayStation that assigned a higher transmission at the leaf tip the agreement with measured doses was within ±3% even for the 5 mm gap. The five Halcyon systems behaved very similarly but differences in the DLG around 0.2 mm were found across different treatment units and between MLC layers from the same system. The DLG for the proximal layer was consistently higher than for the distal layer, with differences ranging between 0.10 mm and 0.24 mm.

CONCLUSIONS

The trailing distance between the leaves from different layers substantially affected the doses delivered by sweeping gaps and the measured DLG values. Stacked MLCs introduce a new level of complexity in TPSs, which ideally need to implement an explicit model of the leaf tip in order to reproduce the trailing effect. Dynamic tests called "trailing sweeping gaps" were designed that are useful for characterizing and commissioning dual-layer MLC systems.

PENELOPE simulations and experiment for 6 MV clinac iX accelerator for standard and small static fields

The goal of this work was to produce accurate data for use as a 'gold standard' and a valid tool for measurements in reference dosimetry for standard/small static field sizes from 0.5 × 0.5 to 10 × 10 cm². It is based on the accuracy of the phase space files (PSFs) as a key quantity. Because the IAEA general public database provides few PSFs for the Varian iX, we simulated the head through Monte Carlo (MC) simulations and calculated validated PSFs for 12 square field sizes including seven for small static fields. The resulting dosimetric calculations allowed us to reach a good level of agreement in comparison to our relative and absolute dose measurements performed on a Varian iX in water phantom. Measured and MC calculated output factors were investigated for different detectors. Based on the TRS 483 formalism and MC (PENELOPE/penEasy), we calculated output correction factors for the unshielded Diode-E (T60017) and the PinPoint-3D (T31016) micro-chamber according to manufacturers' blueprints. Our MC results were in agreement with the recommended data; they compete with recent measurements and MC simulations and in particular the TRS 483 MC data obtained from similar simulations. Moreover, our MC results provide supplemental data in comparison to TRS 483 data in particular for the PinPoint-3D (T31016). We suggest our MC output correction factors as new datasets for future TRS compilations. The work was substantial, used different robust MC strategies depending on the scoring regions, and led in most cases to uncertainties of less than 1%.

Monte Carlo simulation of conical collimators for stereotactic radiosurgery with a 6 MV flattening-filter-free photon beam

PURPOSE

Conical collimators, or cones, are tertiary collimators that attach to a radiotherapy linac and are suited for the stereotactic radiosurgery treatment of small brain lesions. The small diameter of the most used cones makes difficult the acquisition of the dosimetry data needed for the commissioning of treatment planning systems. Although many publications report dosimetric data of conical collimators for stereotactic radiosurgery, most of the works use different setups, which complicates comparisons. In other cases, the cone output factors reported do not take into account the effect of the small cone diameter on the detector response. Finally, few data exist on the dosimetry of cones with flattening-filter-free (FFF) beams from modern linac models. This work aims at obtaining a dosimetric characterization of the conical collimators manufactured by Brainlab AG (Munich, Germany) in a 6 MV FFF beam from a TrueBeam STx linac (Varian Medical Systems).

METHODS

Percentage depth dose curves, lateral dose profiles and cone output factors were obtained using Monte Carlo simulations for the cones with diameters of 4, 5, 6, 7.5, 8, 10, 12.5, 15, 17.5, 20, 25, and 30 mm. The simulation of the linac head was carried out with the PRIMO Monte Carlo software, and the simulations of the cones and the water phantom were run with the general-purpose Monte Carlo code PENELOPE. The Monte Carlo model was validated by comparing the simulation results with measurements performed for the cones of 4, 5, and 7.5 mm of diameter using a stereotactic field diode, a microDiamond detector and EBT3 radiochromic film. In addition, for those cones, simulations and measurements were done for comparison purposes, by reproducing the experimental setups from the available publications.

RESULTS

The experimental data acquired for the cones of 4, 5, and 7.5 mm validated the developed Monte Carlo model. The simulations accurately reproduced the experimental depths of maximum dose and the dose ratio at 20- and 10-cm depth (PDD_20/10 ). A good agreement was obtained between simulated and experimental lateral dose profiles: The differences in the full-width at half-maximum were smaller than 0.2 mm, and the differences in the penumbra 80%-20% were smaller than 0.25 mm. The difference between the simulated and the average of the experimental output factors for the cones of 4, 5, and 7.5 mm of diameter was 0.0%, 0.0%, and 3.0%, respectively, well within the statistical uncertainty of the simulations (4.4% with coverage factor k = 2). It was also found that the simulated cone output factors agreed within 2% with the average of output factors reported in the literature for a variety of setup conditions, detectors, beam qualities, and cone manufacturers.

CONCLUSION

A Monte Carlo model of cones for stereotactic radiosurgery has been developed and validated. The cone dosimetry dataset obtained in this work, consisting of percentage depth doses, lateral dose profiles and output factors, is useful to benchmark data acquired for the commissioning of cone-based radiosurgery treatment planning systems.

Treatment planning system and beam data validation for the ZAP-X: A novel self-shielded stereotactic radiosurgery system

PURPOSE

To evaluate the treatment planning system (TPS) performance of the ZAP-X stereotactic radiosurgery (SRS) system through nondosimetric, dosimetric, and end-to-end (E2E) tests.

METHODS

A comprehensive set of TPS commissioning and validation tests was developed using published guidelines. Nondosimetric validation tests included information transfer, computed tomography-magnetic resonance (CT-MR) image registration, structure/contouring, geometry, dose tools, and CT density. Dosimetric validation included comparisons between TPS and water tank/Solid Water measurements for various geometries and beam arrangements and end-to-end (E2E) tests. Patient-specific quality assurance was performed with an ion chamber in the Lucy phantom and with Gafchromic EBT3 film in the CyberKnife head phantom. RadCalc was used for independent verification of monitor units. Additional E2E tests were performed using the RPC Gamma Knife thermoluminescent dosimeter (TLD) phantom, MD Anderson SRS head phantom, and PseudoPatient gel phantom for independent absolute dose verification.

RESULTS

CT-MR image registrations with known translational and rotational offsets were within tolerance (<0.5 × maximum voxel dimension). Slice thickness and distance accuracy were within 0.1 mm, and volume accuracy was within 0 to 0.11 cm³ . Treatment planning system volume measurement uncertainty was within 0.1 to 0.4 cm³ . Ion chamber point-dose measurements for a single beam in a water phantom agreed to TPS-calculated values within ±4% for collimator diameters 10 to 25 mm, and ±6% for 7.5 mm, for all measured depths (7, 50, 100, 150, and 200 mm). In homogeneous Solid Water, point-dose measurements agreed to within ±4% for cones sizes 7.5 to 25 mm. With 1-cm high/low density inserts, measurements were within ±4.2% for cone sizes 10 to 25 mm. Film-based E2E using 4/5-mm cones resulted in a gamma passing rate (%GP) of 99.8% (2%/1.5 mm). Point-dose measurements in a Lucy phantom with an ion chamber using 36 beams distributed along three noncoplanar arcs agreed to within ±4% for cone sizes 10 to 25 mm. The RPC Gamma Knife TLD phantom yielded passing results with a measured-to-expected TLD dose ratio of 1.02. The MD Anderson SRS head phantom yielded passing results, with 4% TLD agreement and %GP of 95%/93% (5%/3 mm) for coronal/sagittal film planes. The RTsafe gel phantom gave %GP of >95% (5%/2 mm) for all four targets. For our first 58 patients, film-based patient-specific quality assurance has resulted in an average %GP of 98.7% (range, 94-100%) at 2%/2 mm.

CONCLUSIONS

Core ZAP-X features were found to be functional. On the basis of our results, point-dose and planar measurements were in agreement with TPS calculations using multiple phantoms and setup geometries, validating the ZAP-X TPS beam model for clinical use.

Monte Carlo as quality control tool of stereotactic body radiation therapy treatment plans

PURPOSE/OBJECTIVE

The objective of this study was to verify the accuracy of treatment plans of stereotactic body radiation therapy (SBRT) and to verify the feasibility of the use of Monte Carlo (MC) as quality control (QC) on a daily basis.

MATERIAL/METHODS

Using EGSnrc, a MC model of Agility™ linear accelerator was created. Various measurements (Percentage depth dose (PDD), Profiles and Output factors) were done for different fields sizes from 1x1 up to 40x40 (cm²). An iterative model optimization was performed to achieve adequate parameters of MC simulation. 40 SBRT patient's dosimetry plans were calculated by Monaco™ 3.1.1. CT images, RT-STRUCT and RT-PLAN files from Monaco™ being used as input for Moderato MC code. Finally, dose volume histogram (DVH) and paired t-tests for each contour were used for dosimetry comparison of the Monaco™ and MC.

RESULTS

Validation of MC model was successful, as <2% difference comparing to measurements for all field's sizes. The main energy of electron source incident on the target was 5.8 MeV, and the full width at half maximum (FWHM) of Gaussian electron source were 0.09 and 0.2 (cm) in X and Y directions, respectively. For 40 treatment plan comparisons, the minimum absolute difference of mean dose of planning treatment planning (PTV) was 0.1% while the maximum was 6.3%. The minimum absolute difference of Max dose of PTV was 0.2% while the maximum was 8.1%.

CONCLUSION

SBRT treatment plans of Monaco agreed with MC results. It possible to use MC for treatment plans verifications as independent QC tool.

Commissioning cranial single-isocenter multi-target radiosurgery for the Versa HD

PURPOSE

Brainlab's Elements Multiple Brain Mets SRS (MBMS) is a dedicated treatment planning system for single-isocenter multi-target (SIMT) cranial stereotactic radiosurgery (SRS) treatments. The purpose of this study is to present the commissioning experience of MBMS on an Elekta Versa HD.

METHODS

MBMS was commissioned for 6 X, 6 FFF, and 10 FFF. Beam data collected included: output factors, percent depth doses (PDDs), diagonal profiles, collimator transmission, and penumbra. Beam data were processed by Brainlab and resulting parameters were entered into the planning system to generate the beam model. Beam model accuracy was verified for simple fields. MBMS plans were created on previously treated cranial SRS patient data sets. Plans were evaluated using Paddick inverse conformity (ICI), gradient indices (GI), and cumulative volume of brain receiving 12 Gy. Dosimetric accuracy of the MBMS plans was verified using microDiamond, Gafchromic film, and SRS Mapcheck measurements of absolute dose and dose profiles for individual targets. Finally, an end-to-end (E2E) test was performed with a MR-CT compatible phantom to validate the accuracy of the simulation-to-delivery process.

RESULTS

For square fields, calculated scatter factors were within 1.0% of measured, PDDs were within 0.5% past dmax, and diagonal profiles were within 0.5% for clinically relevant off-axis distances (<10 cm). MBMS produced plans with ICIs < 1.5 and GIs < 5.0 for targets > 10 mm. Average point doses of the MBMS plans, measured by microDiamond, were within 0.31% of calculated (max 2.84%). Average per-field planar pass rates were 98.0% (95.5% minimum) using a 2%/1 mm/10% threshold relative gamma analysis. E2E point dose measurements were within 1.5% of calculated and Gafchromic film pass rates were 99.6% using a 5%/1 mm/10% threshold gamma analysis.

CONCLUSION

The experience presented can be used to aid the commissioning of the Versa HD in the Brainlab MBMS treatment planning system, to produce safe and accurate SIMT cranial SRS treatments.