Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218

A method for generating intensity-modulated radiation therapy fields for small animal irradiators utilizing 3D-printed compensator molds

PURPOSE

The purpose of this study was to investigate the feasibility of using fused deposition modeling (FDM) three-dimensional (3D) printer to generate radiation compensators for high-resolution (~1 mm) intensity-modulated radiation therapy (IMRT) for small animal radiation treatment. We propose a novel method incorporating 3D-printed compensator molds filled with NaI powder.

METHODS

The inverse planning module of the computational environment for radiotherapy research (CERR) software was adapted to simulate the XRAD-225Cx irradiator, both geometry and kV beam quality (the latter using a phase space file provided for XRAD-225Cx). A nine-field IMRT treatment was created for a scaled-down version of the imaging and radiation oncology core (IROC) Head and Neck IMRT credentialing test, recreated on a 2.2-cm-diameter cylindrical phantom. Optimized fluence maps comprising nine fields and a total of 2564 beamlets were calculated at resolution of 1.25 × 1.25 mm² . A hollow compensator mold was created (using in-house software and algorithm) for each field using 3D printing with polylactic acid (PLA) filaments. The molds were then packed with sodium iodide powder (NaI, measured density ρ_NaI  = 2.062 g/cm³ ). The mounted compensator mold thickness was limited to 13.8 mm due to clearance issues with couch collision. At treatment delivery, each compensator was manually mounted to a customized block tray attached to the reference 40 × 40 mm² collimator. Compensator reproducibility among three repeated 3D-printed molds was measured with Radiochromic EBT2 film. The two-dimensional (2D) dose distributions of the nine fields were compared to calculated 2D doses from CERR using gamma comparisons with distance-to-agreement criteria of 0.5-0.25 mm and dose difference criteria of 3-5%.

RESULTS

Good reproducibility of 3D-printed compensator manufacture was observed with mean error of ±0.024 Gy and relative dose error of ±4.2% within the modulated part of the beam. Within the limit of 13.8 mm compensator height, a maximum radiation blocking efficiency of 91.5% was achieved. Per field, about 45.5 g of NaI powder was used. Gamma analysis on each of the nine delivered IMRT fields using radiochromic films resulted in eight of nine treatment fields with >90% pass rate with 5%/0.5 mm tolerances. However, low gamma passing rate of 49-66% (3%/0.25 mm to 5%/0.5 mm) was noted in one field, attributed to fabrication errors resulting in over-filling the mold. The nine-field treatment plan was delivered in under 30 min with no mechanical or collisional issues.

CONCLUSIONS

We show the feasibility of high spatial resolution IMRT treatment on a small animal irradiator utilizing 3D-printed compensator shells packed with NaI powder. Using the PLA mold with NaI powder was attractive due to the ease of 3D printing a PLA mold at high geometric resolution and the well-balanced attenuation properties of NaI powders that prevented the mold from becoming too bulky. IMRT fields with 1.25-mm resolution are capable with significant fluence modulation with relative dose accuracy of ±4.2%.

In vivo dosimetry in external beam photon radiotherapy: Requirements and future directions for research, development, and clinical practice

External beam radiotherapy with photon beams is a highly accurate treatment modality, but requires extensive quality assurance programs to confirm that radiation therapy will be or was administered appropriately. In vivo dosimetry (IVD) is an essential element of modern radiation therapy because it provides the ability to catch treatment delivery errors, assist in treatment adaptation, and record the actual dose delivered to the patient. However, for various reasons, its clinical implementation has been slow and limited. The purpose of this report is to stimulate the wider use of IVD for external beam radiotherapy, and in particular of systems using electronic portal imaging devices (EPIDs). After documenting the current IVD methods, this report provides detailed software, hardware and system requirements for in vivo EPID dosimetry systems in order to help in bridging the current vendor-user gap. The report also outlines directions for further development and research. In vivo EPID dosimetry vendors, in collaboration with users across multiple institutions, are requested to improve the understanding and reduce the uncertainties of the system and to help in the determination of optimal action limits for error detection. Finally, the report recommends that automation of all aspects of IVD is needed to help facilitate clinical adoption, including automation of image acquisition, analysis, result interpretation, and reporting/documentation. With the guidance of this report, it is hoped that widespread clinical use of IVD will be significantly accelerated.

A delivery quality assurance tool based on the actual leaf open times in tomotherapy

PURPOSE

To validate a delivery quality assurance (DQA) protocol for tomotherapy based on the measurement of the leaf open times (LOTs). In addition, to show the correlation between the mean relative LOT discrepancy and the dose deviation in the planning target volume (PTV).

MATERIALS AND METHODS

We used a LOT measurement algorithm presented in a previous work on our two tomotherapy treatment units (TOMO1 and TOMO2). We generated TomoPhant plans with intentional random LOT discrepancies following Gaussian distributions of -6%, -4%, -2%, 2%, 4%, and 6%. We irradiated them on the Cheese Phantom with two ion chambers and collected the raw data on both our treatment units. Using the raw data, we measured the actual LOTs and verified that the induced discrepancies were highlightable. Then, we calculated the actual dose using Accuray's standalone dose calculator and verified that the calculated dose agreed with the ion chamber measurement. We randomly chose 60 clinical treatment plans, delivered them in air, and collected the raw detector data. We measured the actual LOTs from the raw data and calculated the corresponding dose distributions using Accuray's standalone dose calculator. We assessed the Pearson coefficient correlation of the deviation between expected and actual dose in the PTV (a) with the mean relative LOT discrepancy and (b) with the γ-index pass rate for different tolerances.

RESULTS

The mean relative discrepancy between actual (measured by the algorithm) and expected LOTs on the modified TomoPhant plans was 1.10 ± 0.05% on TOMO1 and 0.02 ± 0.03% on TOMO2, respectively. The agreement between measured and calculated dose was 0.2 ± 0.3% on TOMO1 and 0.1 ± 0.3% on TOMO2, respectively. On clinical plans, the means of the relative LOT discrepancies ranged from -3.0 % to 1.4%. The dose deviation in the PTVs ranged from -1.6% to 2.4%. The Pearson coefficient correlation between the mean relative LOT discrepancy and the dose deviation in the PTV was 0.76 (P ≈ 10^-15 ) on TOMO1 and 0.65 (P ≈ 10^-10 ) on TOMO2, respectively. There was no correlation between the γ-index pass rate and the dose deviation in the PTV.

CONCLUSION

The method made it possible to measure and to correctly highlight the LOT discrepancies on the TomoPhant plans. The dose subsequently calculated was accurate. On clinical plans, the mean LOT discrepancy correlated with the dose deviation in the PTV. This makes the mean LOT discrepancy a handy indicator of the plan quality.

Evaluation of three VMAT-TMI planning methods to find an appropriate balance between plan complexity and the resulting dose distribution

PURPOSE

Evaluation of different planning methods of treatment plan preparation for volumetric modulated arc therapy during total marrow irradiation (VMAT-TMI).

METHOD

Three different planning methods were evaluated to establish the most appropriate VMAT-TMI technique, based on organ at risk (OAR) dose reduction, conformity and plan simplicity. The methods were: (M1) the sub-plan method, (M2) use of eight arcs optimised simultaneously and (M3) M2 with monitor unit reduction. Friedman ANOVA comparison, with Nemenyi's procedures, was used in the statistical analysis of the results.

RESULTS

The dosimetric results obtained for the planning target volume and for most OARs do not differ statistically between methods. The M3 method was characterized by the lowest numbers of monitor units (3259 MU vs. 4450 MU for M1 and 4216 MU for M2) and, in general, the lowest complexity. The variability of the monitor units from control points was almost half for M3 than M1 and M2 (i.e. 0.33 MU vs. 0.61 MU for M1 and 0.58 for M2). Analysing the relationship between the dose distributions obtained for the plans and their complexity, the best result was observed for the M3 method.

CONCLUSION

The use of eight simultaneously optimised arcs with MU reduction allows to obtain VMAT-TMI plans that are characterized by the lowest complexity, with dose distributions comparable to the plans generated by other methods.

Characteristics of inverse gamma histograms

This work explores the characteristics of the inverse gamma histogram and its potential use as part of the patient specific quality assurance (PSQA) program for volumetric modulated arc therapy (VMAT). ArcCheck measured dose files and TPS predicted dose files were imported and analysed using the in-house inverse gamma code developed in the Python package. Inverse gamma with fixed distance-to-agreement of 2 mm were calculated for 23 VMAT arcs. Dose difference histograms were plotted for six arbitrarily selected arcs with the 95th and 90th percentile values calculated. Dose difference histograms enabled visualisation of the dose difference distribution information. The 95th and 90th percentile values are equivalent to the dose difference criteria where the gamma pass rate is 95% and 90% respectively. These values can be used as a guide to assess plan acceptability, especially for plans that failed the initial gamma evaluation. The inverse gamma histograms are demonstrated to be a useful tool for plan evaluation in addition to the traditional gamma evaluation method. It contains dose difference or distance-to-agreement distribution information, which could be clinically useful for plan evaluation.

Commissioning of a 1.5T Elekta Unity MR-linac: A single institution experience

MR image-guided radiotherapy has the potential to improve patient care, but integration of an MRI scanner with a linear accelerator adds complexity to the commissioning process. This work describes a single institution experience of commissioning an Elekta Unity MR-linac, including mechanical testing, MRI scanner commissioning, and dosimetric validation. Mechanical testing included multileaf collimator (MLC) positional accuracy, measurement of radiation isocenter diameter, and MR-to-MV coincidence. Key MRI tests included magnetic field homogeneity, geometric accuracy, image quality, and the accuracy of navigator-triggered imaging for motion management. Dosimetric validation consisted of comparison between measured and calculated PDDs and profiles, IMRT measurements, and end-to-end testing. Multileaf collimator positional accuracy was within 1.0 mm, the measured radiation isocenter walkout was 0.20 mm, and the coincidence between MR and MV isocenter was 1.06 mm, which is accounted for in the treatment planning system (TPS). For a 350-mm-diameter spherical volume, the peak-to-peak deviation of the magnetic field homogeneity was 4.44 ppm and the geometric distortion was 0.8 mm. All image quality metrics were within ACR recommendations. Navigator-triggered images showed a maximum deviation of 0.42, 0.75, and 3.0 mm in the target centroid location compared to the stationary target for a 20 mm motion at 10, 15, and 20 breaths per minute, respectively. TPS-calculated PDDs and profiles showed excellent agreement with measurement. The gamma passing rate for IMRT plans was 98.4 ± 1.1% (3%/ 2 mm) and end-to-end testing of adapted plans showed agreement within 0.4% between ion-chamber measurement and TPS calculation. All credentialing criteria were satisfied in an independent end-to-end test using an IROC MRgRT phantom.

A comprehensive and clinical-oriented evaluation criteria based on DVH information and gamma passing rates analysis for IMRT plan 3D verification

Purpose

To accomplish the 3D dose verification to IMRT plan by incorporating DVH information and gamma passing rates (GPs) (DVH_GPs) so as to better correlate the patient‐specific quality assurance (QA) results with clinically relevant metrics.

Materials and methods

DVH_GPs analysis was performed to specific structures of 51 intensity‐modulated radiotherapy (IMRT) treatment plans (17 plans each for oropharyngeal neoplasm, esophageal neoplasm, and cervical neoplasm) with Delta4 3D dose verification system. Based on the DVH action levels of 5% and GPs action levels of 90% (3%/2 mm), the evaluation results of DVH_GPs analysis were categorized into four regions as follows: the true positive (TP) (%DE> 5%, GPs < 90%), the false positive (FP) (%DE ≤ 5%, GPs < 90%), the false negative (FN) (%DE> 5%, GPs ≥ 90%), and the true negative (TN) (%DE ≤ 5%, GPs ≥ 90%). Considering the actual situation, the final patient‐specific QA determination was made based on the DVH_GPs evaluation results. In order to exclude the impact of Delta4 phantom on the DVH_GPs evaluation results, 5 cm phantom shift verification was carried out to structures with abnormal results (femoral heads, lung, heart).

Results

In DVH_GPs evaluation, 58 cases with FN, 5 cases with FP, and 2 cases with TP were observed. After the phantom shift verification, the extremely abnormal FN of both lung (%DE = 21.52%±8.20%) and heart (%DE = 19.76%) in the oropharyngeal neoplasm plans and of the bilateral formal heads (%DE = 26.41%±13.45%) in cervical neoplasm plans disappeared dramatically. DVH_GPs analysis was performed to all evaluation results in combination with clinical treatment criteria. Finally, only one TP case from the oropharyngeal neoplasm plans and one FN case from the esophageal neoplasm plans did not meet the treatment requirements, so they needed to be replanned.

Conclusion

The proposed DVH_GPs evaluation method first make up the deficiency of conventional gamma analysis regarding intensity information and space information. Moreover, it improves the correlation between the patient‐specific QA results and clinically relevant metrics. Finally, it can distinguish the TP, TN, FP, and FN in the evaluation results. They are affected by many factors such as the action levels of DVH and GPs, the feature of the specific structure, the QA device, etc. Therefore, medical physicist should make final patient‐specific QA decision not only by taking into account the information of DVH and GPs, but also the practical situation.

Estimating dose delivery accuracy in stereotactic body radiation therapy: A review of in-vivo measurement methods

Stereotactic body radiation therapy (SBRT) has been recognized as a standard treatment option for many anatomical sites. Sophisticated radiation therapy techniques have been developed for carrying out these treatments and new quality assurance (QA) programs are therefore required to guarantee high geometrical and dosimetric accuracy. This paper focuses on recent advances on in-vivo measurements methods (IVM) for SBRT treatment. More specifically, all of the online QA methods for estimating the effective dose delivered to patients were compared. Determining the optimal IVM for performing SBRT treatments would reduce the risk of errors that could jeopardize treatment outcome. A total of 89 papers were included. The papers were subdivided into the following topics: point dosimeters (PD), transmission detectors (TD), log file analysis (LFA), electronic portal imaging device dosimetry (EPID), dose accumulation methods (DAM). The detectability capability of the main IVM detectors/devices were evaluated. All of the systems have some limitations: PD has no spatial data, EPID has limited sensitivity towards set-up errors and intra-fraction motion in some anatomical sites, TD is insensitive towards patient related errors, LFA is not an independent measure, DAMs are not always based on measures. In order to minimize errors in SBRT dose delivery, we recommend using synergic combinations of two or more of the systems described in our review: on-line tumor position and patient information should be combined with MLC position and linac output detection accuracy. In this way the effects of SBRT dose delivery errors will be reduced.

Independent calculation-based verification of volumetric-modulated arc therapy-stereotactic body radiotherapy plans for lung cancer

This study aimed to investigate the feasibility of independent calculation‐based verification of volumetric‐modulated arc therapy (VMAT)–stereotactic body radiotherapy (SBRT) for patients with lung cancer using a secondary treatment planning system (sTPS). In all, 50 patients with lung cancer who underwent VMAT‐SBRT between April 2018 and May 2019 were included in this study. VMAT‐SBRT plans were devised using the Collapsed‐Cone Convolution in RayStation (primary TPS: pTPS). DICOM files were transferred to Eclipse software (sTPS), which utilized the Eclipse software, and the dose distribution was then recalculated using Acuros XB. For the verification of dose distribution in homogeneous phantoms, the differences among pTPS, sTPS, and measurements were evaluated using passing rates of a dose difference of 5% (DD5%) and gamma index of 3%/2 mm (γ3%/2 mm). The ArcCHECK cylindrical diode array was used for measurements. For independent verification of dose‐volume parameters per the patient’s geometry, dose‐volume indices for the planning target volume (PTV) including D_95% and the isocenter dose were evaluated. The mean differences (± standard deviations) between the pTPS and sTPS were then calculated. The gamma passing rates of DD5% and γ3%/2 mm criteria were 99.2 ± 2.4% and 98.6 ± 3.2% for pTPS vs. sTPS, 92.9 ± 4.0% and 94.1 ± 3.3% for pTPS vs. measurement, and 93.0 ± 4.4% and 94.3 ± 4.1% for sTPS vs. measurement, respectively. The differences between pTPS and sTPS for the PTVs of D_95% and the isocenter dose were −3.1 ± 2.0% and −2.3 ± 1.8%, respectively. Our investigation of VMAT‐SBRT plans for lung cancer revealed that independent calculation‐based verification is a time‐efficient method for patient‐specific quality assurance.

Technical Note: Comprehensive performance tests of the first clinical real-time motion tracking and compensation system using MLC and jaws

Purpose

To evaluate the performance of the first clinical real‐time motion tracking and compensation system using multileaf collimator (MLC) and jaws during helical tomotherapy delivery.

Methods

Appropriate mechanical and dosimetry tests were performed on the first clinical real‐time motion tracking system (Synchrony on Radixact, Accuray Inc) recently installed in our institution. kV radiography dose was measured by CTDIw using a pencil chamber. Changes of beam characteristics with jaw offset and MLC leaf shift were evaluated. Various dosimeters and phantoms including A1SL ion chamber (Standard Imaging), Gafchromic EBT3 films (Ashland), TomoPhantom (Med Cal), ArcCheck (Sun Nuclear), Delta4 (ScandiDos), with fiducial or high contrast inserts, placed on two dynamical motion platforms (CIRS dynamic motion‐CIRS, Hexamotion‐ScandiDos), were used to assess the dosimetric accuracy of the available Synchrony modalities: fiducial tracking with nonrespiratory motion (FNR), fiducial tracking with respiratory modeling (FR), and fiducial free (e.g., lung tumor tracking) with respiratory modeling (FFR). Motion detection accuracy of a tracking target, defined as the difference between the predicted and instructed target positions, was evaluated with the root mean square (RMS). The dose accuracy of motion compensation was evaluated by verifying the dose output constancy and by comparing measured and planned (predicted) three‐dimensional (3D) dose distributions based on gamma analysis.

Results

The measured CTDIw for a single radiograph with a 120 kVp and 1.6 mAs protocol was 0.084 mGy, implying a low imaging dose of 8.4 mGy for a typical Synchrony motion tracking fraction with 100 radiographs. The dosimetric effect of the jaw swing or MLC leaf shift was minimal on depth dose (<0.5%) and was <2% on both beam profile width and output for typical motions. The motion detection accuracies, that is, RMS, were 0.84, 1.13, and 0.48 mm for FNR, FR, and FFR, respectively, well within the 1.5 mm recommended tolerance. Dose constancy with Synchrony was found to be within 2%. The gamma passing rates of 3D dose measurements for a variety of Synchrony plans were well within the acceptable level.

Conclusions

The motion tracking and compensation using kV radiography, MLC shifting, and jaw swing during helical tomotherapy delivery was tested to be mechanically and dosimetrically accurate for clinical use.

Technical Note: A step-by-step guide to Temporally Feathered Radiation Therapy planning for head and neck cancer

Purpose

Prior in silico simulations propose that Temporally Feathered Radiation Therapy (TFRT) may reduce toxicity related to head and neck radiation therapy. In this study we demonstrate a step‐by‐step guide to TFRT planning with modern treatment planning systems.

Methods

One patient with oropharyngeal cancer planned for definitive radiation therapy using intensity‐modulated radiation therapy (IMRT) techniques was replanned using the TFRT technique. Five organs at risk (OAR) were identified to be feathered. A “base plan” was first created based on desired planning target volumes (PTV) coverage, plan conformality, and OAR constraints. The base plan was then re‐optimized by modifying planning objectives, to generate five subplans. All beams from each subplan were imported onto one trial to create the composite TFRT plan. The composite TFRT plan was directly compared with the non‐TFRT IMRT plan. During plan assessment, the composite TFRT was first evaluated followed by each subplan to meet preset compliance criteria.

Results

The following organs were feathered: oral cavity, right submandibular gland, left submandibular gland, supraglottis, and OAR Pharynx. Prescription dose PTV coverage (>95%) was met in each subplan and the composite TFRT plan. Expected small variations in dose were observed among the plans. The percent variation between the high fractional dose and average low fractional dose was 29%, 28%, 24%, 19%, and 10% for the oral cavity, right submandibular, left submandibular, supraglottis, and OAR pharynx nonoverlapping with the PTV.

Conclusions

Temporally Feathered Radiation Therapy planning is possible with modern treatment planning systems. Modest dosimetric changes are observed with TFRT planning compared with non‐TFRT IMRT planning. We await the results of the current prospective trial to seeking to demonstrate the feasibility of TFRT in the modern clinical workflow (NCT03768856). Further studies will be required to demonstrate the potential benefit of TFRT over non‐TFRT IMRT Planning.

Dosimetric impact of tracheostomy devices in head and neck cancer patients

INTRODUCTION

The tracheostomy site and adjacent skin is at risk for recurrence in head/neck squamous cell cancer patients. The tracheostomy tube is an in situ device located directly over the tracheostomy site and may have clinical implications on the radiation dose delivered to the peristomal region. This study aimed to investigate this effect by comparing the prescribed treatment planning dose with the actual dose in vivo to the peristomal clinical target region. A retrospective, dosimetric study was performed with approval of the institutional research ethics board.

METHODS

Fifteen patients who had received high-dose radiotherapy to the tracheostomy region with in vivo dose measurements were included. The radiation dose at the skin surface underneath the tracheostomy device was measured using an optically stimulated luminescent dosimeter (OSLD) and was compared with the prescribed dose from the radiation planning system. The effect of the tracheostomy flange and/or soft tissue equivalent bolus on the peristomal dose was calculated.

RESULTS AND DISCUSSION

Patients with tracheostomy equipment in situ were found to have a 3.7% difference between their prescribed and actual dose. With a tissue equivalent bolus there was a 2.0% difference between predicted and actual. The mean prescribed single fraction dose (mean = 191.8 cGy, SD = 40.18) and OSLD measured dose (mean = 194.02 cGy, SD = 44.3) were found to have no significant difference. However, with the flange excluded from the planning simulation (density = air) target skin dose deviated from predicted by an average of 55.3% (range = 12.4-72.9, SD = 22.5) and volume coverage was not achieved.

CONCLUSION

In summary, the tracheostomy flange acts like bolus with a twofold increase in the skin surface dose. Changes in the peristomal apparatus from simulation to treatment needs to be considered to ensure that the simulated dose and coverage is achieved.

Comprehensive validation of halcyon 2.0 plans and the implementation of patient specific QA with multiple detector platforms

Purpose

To perform a comprehensive validation of plans generated on a preconfigured Halcyon 2.0 with preloaded beam model, including evaluations of new features and implementing the patient specific quality assurance (PSQA) process with multiple detectors.

Methods

A total of 56 plans were generated in Eclipse V15.6 (Varian Medical System) with a preconfigured Halcyon treatment machine. Ten plans were developed via the AAPM TG‐119 test suite with both IMRT and VMAT techniques. 34 clinically treated plans using C‐arm LINAC from 24 patients were replanned on Halcyon using IMRT or VMAT techniques for a variety of sites including: brain, head and neck, lung, breast, abdomen, and pelvis. Six of those plans were breast VMAT plans utilizing the extended treatment field technique available with Halcyon 2.0. The dynamically flattened beam (DFB), another new feature on Halcyon 2.0, was also used for an AP/PA spine and four field box pelvis, as well as ten 3D breast plans. All 56 plans were measured with an ion chamber (IC), film, portal dosimetry (PD), ArcCHECK, and Delta4. Tolerance and action limits were calculated and compared to the recommendations of TG‐218.

Results

TG‐119 IC and film confidence limits met those set by the task group, except for IMRT target point dose. Forty‐four of 46 clinical plans were within 3% for IC measurements. Average gamma passing rates with 3% dose difference and 2mm distance‐to‐agreement for IMRT/VMAT plans were: Film – 96.8%, PD – 99.9%, ArcCHECK – 99.1%, and Delta4 – 99.2%. Calculated action limits were: Film – 86.3%, PD – 98.4%, ArcCHECK – 96.1%, and Delta4 – 95.7%. Extended treatment field technique was fully validated and 3D plans with DFB had similar results to IMRT/VMAT plans.

Conclusion

Halcyon plan deliveries were verified with multiple measurement devices. New features of Halcyon 2.0 were also validated. Traditional PSQA techniques and process specific tolerance and action limits were successfully implemented.

Automatic genetic planning for volumetric modulated arc therapy: A large multi-centre validation for prostate cancer

PURPOSE

The first clinical genetic autoplanning algorithm (Genetic Planning Solution, GPS) was validated in ten radiotherapy centres for prostate cancer VMAT by comparison with manual planning (Manual).

METHODS

Although there were large differences among centres in planning protocol, GPS was tuned with the data of a single centre and then applied everywhere without any centre-specific fine-tuning. For each centre, ten Manual plans were compared with autoGPS plans, considering dosimetric plan parameters and the Clinical Blind Score (CBS) resulting from blind clinician plan comparisons. AutoGPS plans were used as is, i.e. there was no patient-specific fine-tuning.

RESULTS

For nine centres, all ten plans were clinically acceptable. In the remaining centre, only one plan was acceptable. For the 91% acceptable plans, differences between Manual and AutoGPS in target coverage were negligible. OAR doses were significantly lower in AutoGPS plans (p < 0.05); rectum D_15% and D_mean were reduced by 8.1% and 17.9%, bladder D_25% and D_mean by 5.9% and 10.3%. According to clinicians, 69% of the acceptable AutoGPS plans were superior to the corresponding Manual plan. In case of preferred Manual plans (31%), perceived advantages compared to autoGPS were minor. QA measurements demonstrated that autoGPS plans were deliverable. A quick configuration adjustment in the centre with unacceptable plans rendered 100% of plans acceptable.

CONCLUSION

A novel, clinically applied genetic autoplanning algorithm was validated in 10 centres for in total 100 prostate cancer patients. High quality plans could be generated at different centres without centre-specific algorithm tuning.

The influence of errors in small field dosimetry on the dosimetric accuracy of treatment plans

Background: Dosimetric effects of inaccuracies of output factors (OFs) implemented in treatment planning systems (TPSs) were investigated.Materials and methods: Modified beam models (MBM) for which the OFs of small fields (down to 1 × 1 cm²) were increased by up to 12% compared to the original beam models (OBM) were created for two TPSs. These beam models were used to recalculate treatment plans of different complexity. Treatment plans using stereotactic 3D-conformal (s3D-CRT) for brain metastasis as well as VMAT plans for head and neck and prostate cancer patients were generated. Dose distributions calculated with the MBM and the OBM were compared to measured dose distributions acquired using film dosimetry and a 2D-detector-array. For the s3D-CRT plans the calculated and measured dose at the isocenter was evaluated. For VMAT, gamma pass rates (GPRs) were calculated using global gamma index with 3%/3 mm, 2%/3 mm, 1%/3 mm and 2%/2 mm with a 20% threshold. Contribution of small fields to the total fluence was expressed as the ratio (F) of fluence trough leaf openings smaller than 2 cm to the total fluence.Results: Using film dosimetry for the s3D-CRT plans, the average of the ratio of calculated dose to measured dose at the isocenter was 1.01 and 1.06 for the OBM and MBM model, respectively. A significantly lower GPR of the MBM compared to the OBM was only found for the localized prostate cases (F = 12.4%) measured with the 2D-detector-array and an acceptance criterion of 1%/3 mm.Conclusion: The effects of uncertainties in small field OFs implemented in TPSs are most pronounced for s3D-CRT cases and can be clearly identified using patient specific quality assurance. For VMAT these effects mainly remain undetected using standard patient specific quality assurance. Using tighter acceptance criteria combined with an analysis of the fluence generated by small fields can help identifying inaccuracies of OFs implemented in TPSs.

Survey results of 3D-CRT and IMRT quality assurance practice

PURPOSE

To create a snapshot of common practices for 3D-CRT and intensity-modulated radiation therapy (IMRT) QA through a large-scale survey and compare to TG-218 recommendations.

METHODS

A survey of 3D-CRT and IMRT QA was constructed at and distributed by the IROC-Houston QA center to all institutions monitored by IROC (n = 2,861). The first part of the survey asked about methods to check dose delivery for 3D-CRT. The bulk of the survey focused on IMRT QA, inquiring about treatment modalities, standard tools used to verify planned dose, how assessment of agreement is calculated and the comparison criteria used, and the strategies taken if QA fails.

RESULTS

The most common tools for dose verification were a 2D diode array (52.8%), point(s) measurement (39.0%), EPID (27.4%), and 2D ion chamber array (23.9%). When IMRT QA failed, the highest average rank strategy utilized was to remeasure with the same setup, which had an average position ranking of 1.1 with 90.4% of facilities employing this strategy. The second highest average ranked strategy was to move to a new calculation point and remeasure (54.9%); this had an average ranking of 2.1.

CONCLUSION

The survey provided a snapshot of the current state of dose verification for IMRT radiotherapy. The results showed variability in approaches and that work is still needed to unify and tighten criteria in the medical physics community, especially in reference to TG-218's recommendations.

Technical Note: Integrating an open source Monte Carlo code "MCsquare" for clinical use in intensity-modulated proton therapy

PURPOSE

To commission an open source Monte Carlo (MC) dose engine, "MCsquare" for a synchrotron-based proton machine, integrate it into our in-house C++-based I/O user interface and our web-based software platform, expand its functionalities, and improve calculation efficiency for intensity-modulated proton therapy (IMPT).

METHODS

We commissioned MCsquare using a double Gaussian beam model based on in-air lateral profiles, integrated depth dose of 97 beam energies, and measurements of various spread-out Bragg peaks (SOBPs). Then we integrated MCsquare into our C++-based dose calculation code and web-based second check platform "DOSeCHECK." We validated the commissioned MCsquare based on 12 different patient geometries and compared the dose calculation with a well-benchmarked GPU-accelerated MC (gMC) dose engine. We further improved the MCsquare efficiency by employing the computed tomography (CT) resampling approach. We also expanded its functionality by adding a linear energy transfer (LET)-related model-dependent biological dose calculation.

RESULTS

Differences between MCsquare calculations and SOBP measurements were <2.5% (<1.5% for ~85% of measurements) in water. The dose distributions calculated using MCsquare agreed well with the results calculated using gMC in patient geometries. The average 3D gamma analysis (2%/2 mm) passing rates comparing MCsquare and gMC calculations in the 12 patient geometries were 98.0 ± 1.0%. The computation time to calculate one IMPT plan in patients' geometries using an inexpensive CPU workstation (Intel Xeon E5-2680 2.50 GHz) was 2.3 ± 1.8 min after the variable resolution technique was adopted. All calculations except for one craniospinal patient were finished within 3.5 min.

CONCLUSIONS

MCsquare was successfully commissioned for a synchrotron-based proton beam therapy delivery system and integrated into our web-based second check platform. After adopting CT resampling and implementing LET model-dependent biological dose calculation capabilities, MCsquare will be sufficiently efficient and powerful to achieve Monte Carlo-based and LET-guided robust optimization in IMPT, which will be done in the future studies.

Ocular and periocular radiation toxicity in dogs treated for sinonasal tumors: A critical review

Visual impairment from radiation‐induced damage can be painful, disabling, and reduces the patient's quality of life. Ocular tissue damage can result from the proximity of ocular organs at risk to irradiated sinonasal target volumes. As toxicity depends on the radiation dose delivered to a certain volume, dose‐volume constraints for organs at risk should ideally be known during treatment planning in order to reduce toxicity. Herein, we summarize published ocular toxicity data of dogs irradiated for sinonasal tumors from 36 publications (1976‐2018). In particular, we tried to extract a dose guideline for a clinically acceptable rate of ocular toxicity. The side effects to ocular and periocular tissues were reported in 26/36 studies (72%) and graded according to scoring systems (10/26; 39%). With most scoring systems, however, toxicities of different ocular and periocular tissues are summed into one score. Further, the scores were mostly applied in retrospect and lack volume‐ and dose‐data. This incomplete information reflects the crux of the matter for radiation dose tolerance in canine ocular tissues: The published information of the last three decades does not allow formulating dose‐volume guidelines. As a start, we can only state that a mean dose of 39 Gy (given in 10 x 4.2 Gy fractions) will lead to loss of vision by one or both eyes, while mean doses of <30 Gy seem to preserve functionality. With a future goal to define tolerated doses and volumes of ocular and periocular tissues at risk, we propose the use of combined ocular toxicity scoring systems.

Three-dimensional dose reconstruction-based pretreatment dosimetric verification in volumetric modulated arc therapy for prostate cancer

Purpose

We performed three-dimensional (3D) dose reconstruction-based pretreatment verification to evaluate gamma analysis acceptance criteria in volumetric modulated arc therapy (VMAT) for prostate cancer.

Materials and Methods

Pretreatment verification for 28 VMAT plans for prostate cancer was performed using the COMPASS system with a dolphin detector. The 3D reconstructed dose distribution of the treatment planning system calculation (TC) was compared with that of COMPASS independent calculation (CC) and COMPASS reconstruction from the dolphin detector measurement (CR). Gamma results (gamma failure rate and average gamma value [GFR and γAvg]) and dose-volume histogram (DVH) deviations, 98%, 2% and mean dose-volume difference (DD_98%, DD_2% and DD_mean), were evaluated. Gamma analyses were performed with two acceptance criteria, 2%/2 mm and 3%/3 mm.

Results

The GFR in 2%/2 mm criteria were less than 8%, and those in 3%/3 mm criteria were less than 1% for all structures in comparisons between TC, CC, and CR. In the comparison between TC and CR, GFR and γAvg in 2%/2 mm criteria were significantly higher than those in 3%/3 mm criteria. The DVH deviations were within 2%, except for DD_mean (%) for rectum and bladder.

Conclusions

The 3%/3 mm criteria were not strict enough to identify any discrepancies between planned and measured doses, and DVH deviations were less than 2% in most parameters. Therefore, gamma criteria of 2%/2 mm and DVH related parameters could be a useful tool for pretreatment verification for VMAT in prostate cancer.

The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets-Quantification and possible corrections

Purpose

To assess the impact of isocenter shifts due to linac gantry and table rotation during cranial stereotactic radiosurgery on D₉₈, target volume coverage (TVC), conformity (CI), and gradient index (GI).

Methods

Winston‐Lutz (WL) checks were performed on two Elekta Synergy linacs. A stereotactic quality assurance (QA) plan was applied to the ArcCHECK phantom to assess the impact of isocenter shift corrections on Gamma pass rates. These corrections included gantry sag, distance of collimator and couch axes to the gantry axis, and distance between cone‐beam computed tomography (CBCT) isocenter and treatment beam (MV) isocenter. We applied the shifts via script to the treatment plan in Pinnacle 16.2. In a planning study, isocenter and mechanical rotation axis shifts of 0.25 to 2 mm were applied to stereotactic plans of spherical planning target volumes (PTVs) of various volumes. The shifts determined via WL measurements were applied to 16 patient plans with PTV sizes between 0.22 and 10.4 cm³.

Results

ArcCHECK measurements of a stereotactic treatment showed significant increases in Gamma pass rate for all three measurements (up to 3.8 percentage points) after correction of measured isocenter deviations. For spherical targets of 1 cm³, CI was most severely affected by increasing the distance of the CBCT isocenter (1.22 to 1.62). Gradient index increased with an isocenter‐collimator axis distance of 1.5 mm (3.84 vs 4.62). D₉₈ (normalized to reference) dropped to 0.85 (CBCT), 0.92 (table axis), 0.95 (collimator axis), and 0.98 (gantry sag), with similar but smaller changes for larger targets. Applying measured shifts to patient plans lead to relevant drops in D₉₈ and TVC (7%) for targets below 2 cm³ treated on linac 1.

Conclusion

Mechanical deviations during gantry, collimator, and table rotation may adversely affect the treatment of small stereotactic lesions. Adjustments of beam isocenters in the treatment planning system (TPS) can be used to both quantify their impact and for prospective correction of treatment plans.

End-to-end delivery quality assurance of computed tomography-based high-dose-rate brachytherapy using a gel dosimeter

PURPOSE

The purpose of this study was to develop a novel quality assurance (QA) program to check the entire treatment chain of image-guided brachytherapy with dose distribution evaluation in a single setup and irradiation using a gel dosimeter.

METHODS AND MATERIALS

A polymer gel was used, and the readout was performed by magnetic resonance scanning. A CT-based treatment plan was generated using the Oncentra planning system (Elekta, Sweden), and irradiation was performed three times using an afterloading device with an Ir-192 source. The dose-response curve of the gel was created using 6-MV X-ray, which is independent of the source beams. Planar gamma images on a coronal plane along the source transport axis were calculated using the measured dose as a reference, and the calculated doses were used in several error simulations (no error; 2.0 or 2.5 mm systematic and random source dwell mispositioning; and dose error of 2%, 5%, 10%, and 20%).

RESULTS

The dose-R₂ (spin-spin relaxation rate) conversion table revealed that the uncertainty and dose resolution of 6-MV X-ray were better than those of Ir-192 and also constant between the three measurements. With the 3%/1 mm criteria, there were statistically significant differences between each pair of settings except dose error of 2% and 5%.

CONCLUSION

This work depicts a simple and efficient end-to-end test that can provide a clinically useful tool for QA of image-guided brachytherapy. In this QA program, air kerma strength and dwell position setting could also be verified. This test can also distinguish between different types of error.

Retrospective analysis of portal dosimetry pre-treatment quality assurance of hybrid IMRT breast treatment plans

Background:

The purpose of this study is to evaluate the effectiveness and sensitivity of the Varian portal dosimetry (PD) system as a quality assurance (QA) tool for breast intensity-modulated radiation therapy (IMRT) treatment plans.

Materials and methods:

Four hundred portal dose images from 200 breast cancer patient IMRT treatment plans were analysed. The images were obtained using Varian PortalVision electronic portal imaging devices (EPIDs) on Varian TrueBeam Linacs. Three patient plans were selected, and the multi-leaf collimator (MLC) positions were randomly altered by a mean of 0·5, 1, 1·5 and 2 mm with a standard deviation of 0·1 mm on 50, 75 and 100% of control points. Using the improved/global gamma calculation algorithm with a low-dose threshold of 10% in the EPID, the change in gamma passing rates for 3%/3 mm, 2%/2 mm and 1%/1 mm criterion was analysed as a function of the introduced error. The changes in the dose distributions of clinical target volume and organ at risk due to MLC positioning errors were also analysed.

Results:

Symmetric and asymmetric breast or chest wall plan fields are different in delivery as well as in the QA. An average gamma passing rate of 99·8 ± 0·5 is presented for 3%/3 mm symmetric plans and 96·9 ± 4·5 is presented for 3%/3 mm asymmetric plans. An average gamma passing rate of 98·4 ± 4·3 is presented for 2%/2 mm symmetric plans and 89·7 ± 9·5 is presented for 2%/2 mm asymmetric plans. A large-induced error in MLC positioning (2·0 mm, 100% of control points) results in an insignificant change in dose that would be delivered to the patient. However, EPID portal dosimetry is sensitive enough to detect even the slightest change in MLC positioning error (0·5 mm, 50% of control points).

Conclusions:

Stricter pre-treatment QA action levels can be established for breast IMRT plans utilising EPID. For improved sensitivity, a multigamma criteria approach is recommended. The PD tool is sensitive enough to detect MLC positioning errors that contribute to even insignificant dose changes.

Simulation of x-ray-induced acoustic imaging for absolute dosimetry: Accuracy of image reconstruction methods

PURPOSE

Three-dimensional in-vivo dose verification is one of the standing challenges in radiation therapy. X-ray-induced acoustic tomography has recently been proposed as an imaging method for use in in-vivo dosimetry. The aim of this study was to investigate the accuracy of reconstructing three-dimensional (3D) absolute dose using x-ray-induced acoustic tomography. We performed this investigation using two different tomographic dose reconstruction techniques.

METHODS

Two examples of 3D dose reconstruction techniques for x-ray acoustic imaging are investigated. Dose distributions are calculated for varying field sizes using a clinical treatment planning system. The induced acoustic pressure waves which are generated by the increase in temperature due to the absorption of pulsed MV x-rays are simulated using an advanced numerical modeling package for acoustic wave propagation in the time domain. Two imaging techniques, back projection and iterative time reversal, are used to reconstruct the 3D dose distribution in a water phantom with open fields. Image analysis is performed and reconstructed depth dose curves from x-ray acoustic imaging are compared to the depth dose curves calculated from the treatment planning system. Calculated field sizes from the reconstructed dose profiles by back projection and time reversal are compared to the planned field size to determine their accuracy. The iterative time reversal imaging technique is also used to reconstruct dose in an example clinical dose distribution. Image analysis of this clinical test case is performed using the gamma passing rate. In addition, gamma passing rates are used to validate the stopping criteria in the iterative time reversal method.

RESULTS

Water phantom simulations showed that back projection does not adequately reconstruct the shape and intensity of the depth dose. When compared to the depth of maximum dose calculated by a treatment planning system, the maximum dose depth by back projection is shifted deeper by 55 and 75 mm for 4 × 4 cm and 10 × 10 cm field sizes, respectively. The reconstructed depth dose by iterative time reversal accurately agrees with the planned depth dose for a 4 × 4 cm field size and is shifted deeper by 12 mm for the 10 × 10 cm field size. When reconstructing field sizes, the back projection method leads to 18% and 35% larger sizes for the 4 × 4 cm and 10 × 10 cm fields, respectively, whereas the iterative time reversal method reconstructs both field sizes with < 2% error. For the clinical dose distribution, we were able to reconstruct the dose delivered by a 1 degree sub-arc with a good accuracy. The reconstructed and planned doses were compared using gamma analysis, with> 96% gamma passing rate at 3%/2 mm.

CONCLUSIONS

Our results show that the 3D x-ray acoustic reconstructed dose by iterative time reversal is considerably more accurate than the dose reconstructed by back projection. Iterative time reversal imaging has a potential for use in 3D absolute dosimetry.

IAEA methodology for on-site end-to-end IMRT/VMAT audits: an international pilot study

Background: The IAEA has developed and tested an on-site, end-to-end IMRT/VMAT dosimetry audit methodology for head and neck cases using an anthropomorphic phantom. The audit methodology is described, and the results of the international pilot testing are presented.Material and methods: The audit utilizes a specially designed, commercially available anthropomorphic phantom capable of accommodating a small volume ion chamber (IC) in four locations (three in planning target volumes (PTVs) and one in an organ at risk (OAR)) and a Gafchromic film in a coronal plane for the absorbed dose to water and two-dimensional dose distribution measurements, respectively. The audit consists of a pre-visit and on-site phases. The pre-visit phase is carried out remotely and includes a treatment planning task and a set of computational exercises. The on-site phase aims at comparing the treatment planning system (TPS) calculations with measurements in the anthropomorphic phantom following an end-to-end approach. Two main aspects were tested in the pilot study: feasibility of the planning constraints and the accuracy of IC and film results in comparison with TPS calculations. Treatment plan quality was scored from 0 to 100.Results: Forty-two treatment plans were submitted by 14 institutions from 10 countries, with 79% of them having a plan quality score over 90. Seventeen sets of IC measurement results were collected, and the average measured to calculated dose ratio was 0.988 ± 0.016 for PTVs and 1.020 ± 0.029 for OAR. For 13 film measurement results, the average gamma passing rate was 94.1% using criteria of 3%/3 mm, 20% threshold and global gamma.Conclusions: The audit methodology was proved to be feasible and ready to be adopted by national dosimetry audit networks for local implementation.

Impact of the MLC leaf-tip model in a commercial TPS: Dose calculation limitations and IROC-H phantom failures

Purpose

Treatment planning system (TPS) dose calculation is sensitive to multileaf collimator (MLC) modeling, especially when treating with intensity‐modulated radiation therapy (IMRT) or VMAT. This study investigates the dosimetric impact of the MLC leaf‐tip model in a commercial TPS (RayStation v.6.1). The detectability of modeling errors was assessed through both measurements with an anthropomorphic head‐and‐neck phantom and patient‐specific IMRT QA using a 3D diode array.

Methods and Materials

An Agility MLC (Elekta Inc.) was commissioned in RayStation. Nine IMRT and VMAT plans were optimized to treat the head‐and‐neck phantom from the Imaging and Radiation Oncology Core Houston branch (IROC‐H). Dose distributions for each plan were re‐calculated on 27 beam models, varying leaf‐tip width (2.0, 4.5, and 6.5 mm) and leaf‐tip offset (−2.0 to +2.0 mm) values. Doses were compared to phantom TLD measurements. Patient‐specific IMRT QA was performed, and receiver‐operating characteristic (ROC) analysis was performed to determine the detectability of modeling errors.

Results

Dose calculations were very sensitive to leaf‐tip offset values. Offsets of ±1.0 mm resulted in dose differences up to 10% and 15% in the PTV and spinal cord TLDs respectively. Offsets of ±2.0 mm caused dose deviations up to 50% in the spinal cord TLD. Patient‐specific IMRT QA could not reliably detect these deviations, with an ROC area under the curve (AUC) value of 0.537 for a ±1.0 mm change in leaf‐tip offset, corresponding to >7% dose deviation. Leaf‐tip width had a modest dosimetric impact with <2% and 5.6% differences in the PTV and spinal cord TLDs respectively.

Conclusions

Small changes in the MLC leaf‐tip offset in this TPS model can cause large changes in the calculated dose for IMRT and VMAT plans that are difficult to identify through either dose curves or standard patient‐specific IMRT QA. These results may, in part, explain the reported high failure rate of IROC‐H phantom tests.

Template-based automation of treatment planning in advanced radiotherapy: a comprehensive dosimetric and clinical evaluation

Despite the recent advanced developments in radiation therapy planning, treatment planning for head-neck and pelvic cancers remains challenging due to large concave target volumes, multiple dose prescriptions and numerous organs at risk close to targets. Inter-institutional studies highlighted that plan quality strongly depends on planner experience and skills. Automated optimization of planning procedure may improve plan quality and best practice. We performed a comprehensive dosimetric and clinical evaluation of the Pinnacle3 AutoPlanning engine, comparing automatically generated plans (AP) with the historically clinically accepted manually-generated ones (MP). Thirty-six patients (12 for each of the following anatomical sites: head-neck, high-risk prostate and endometrial cancer) were re-planned with the AutoPlanning engine. Planning and optimization workflow was developed to automatically generate "dual-arc" VMAT plans with simultaneously integrated boost. Various dose and dose-volume parameters were used to build three metrics able to supply a global Plan Quality Index evaluation in terms of dose conformity indexes, targets coverage and sparing of critical organs. All plans were scored in a blinded clinical evaluation by two senior radiation oncologists. Dose accuracy was validated using the PTW Octavius-4D phantom together with the 1500 2D-array. Autoplanning was able to produce high-quality clinically acceptable plans in all cases. The main benefit of Autoplanning strategy was the improvement of overall treatment quality due to significant increased dose conformity and reduction of integral dose by 6-10%, keeping similar targets coverage. Overall planning time was reduced to 60-80 minutes, about a third of time needed for manual planning. In 94% of clinical evaluations, the AP plans scored equal or better to MP plans. Despite the increased fluence modulation, dose measurements reported an optimal agreement with dose calculations with a γ-pass-rate greater than 95% for 3%(global)-2 mm criteria. Autoplanning engine is an effective device enabling the generation of VMAT high quality treatment plans according to institutional specific planning protocols.

Evaluation of complexity and deliverability of prostate cancer treatment plans designed with a knowledge-based VMAT planning technique

PURPOSE

Knowledge-based planning (KBP) techniques have been reported to improve plan quality, efficiency, and consistency in radiation therapy. However, plan complexity and deliverability have not been addressed previously for treatment plans guided by an established in-house KBP system. The purpose of this work was to assess dosimetric, mechanical, and delivery properties of plans designed with a common KBP method for prostate cases treated via volumetric modulated arc therapy (VMAT).

METHODS

Thirty-one prostate patients previously treated with VMAT were replanned with an in-house KBP method based on the overlap volume histogram. VMAT plan complexities of the KBP plans and the reference clinical plans were quantified via monitor units, modulation complexity scores, the edge metric, and average leaf motion per degree of gantry rotation. Each set of plans was delivered to the same diode array and agreement between computed and measured dose distributions was evaluated using the gamma index. Varying percent dose-difference (1-3%) and distance-to-agreement (1 mm to 3 mm) thresholds were assessed for gamma analyses.

RESULTS

Knowledge-based planning (KBP) plans achieved average reductions of 6.4 Gy (P < 0.001) and 8.2 Gy (P < 0.001) in mean bladder and rectum dose compared to reference plans, while maintaining clinically acceptable target dose. However, KBP plans were significantly more complex than reference plans in each evaluated metric (P < 0.001). KBP plans also showed significant reductions (P < 0.05) in gamma passing rates at each evaluated criterion compared to reference plans.

CONCLUSIONS

While KBP plans had significantly reduced bladder and rectum dose, they were significantly more complex and had significantly worse quality assurance outcomes than reference plans. These results suggest caution should be taken when implementing an in-house KBP technique.

Technical Note: Dosimetric characterization of the dynamic beam flattening MLC sequence on a ring shaped, Jawless Linear Accelerator with double stacked MLC

PURPOSE

To characterize the dosimetric features and limitations of the dynamic beam flattening (DBF) on the Halcyon 2.0 linear accelerator (Varian Medical Systems).

METHODS

A predefined multi-leaf collimator (MLC) sequence was introduced and used to flatten the 6 MV flattening filter free (FFF) beam on the Halcyon 2.0. Dosimetric characterizations of the flattened beams, including beam flatness, symmetry, percent depth dose (PDD), output factor and MU linearity, were investigated. Flatness and symmetry were obtained from profile measurements with both radiographic films (EDR2) and a two dimensional ion-chamber array (IC Profiler, Sun Nuclear Corporation). MU linearity, output factors, and PDDs were measured in a water tank with a CC13 ion chamber (Scanditronix Wellhöfer, Nuremburg, Germany). In addition, the effect of the DBF sequence on 3D plan quality was evaluated by creating DBF plans for a 4-field box rectum and an AP/PA spine plan. Patient specific QA was performed on these plans.

RESULTS

At 100 cm SSD and 10 cm depth, a flatness of <3% was observed on both transversal and radial profiles for all square field sizes ≥10 cm with DBF. For both larger and smaller field sizes the flatness showed a tendency to increase as the fields got bigger or smaller, respectively. Similar trends in flatness were observed at all depths measured. All measured output factors for square field sizes ≥5 cm were within 1% of the TPS prediction. Linearity was ≤2.02% for all measurements. For both treatment sites, the MD judged the plans created for the Halcyon without the use of DBF not to be clinically acceptable, however considered both the TrueBeam plan and the Halcyon plan with the DBF sequence to be clinically acceptable.

CONCLUSIONS

The DBF sequence on the Halcyon and its characteristics were investigated. The analysis indicates that the DBF sequence can be used on the Halcyon to generate clinically acceptable 3D treatment plans.

Technical note: A modified gamma evaluation method for dose distribution comparisons

Purpose

In this work we have developed a novel method of dose distribution comparison, the inverse gamma (IG) evaluation, by modifying the commonly used gamma evaluation method.

Methods

The IG evaluation calculates the gamma criteria (dose difference criterion, ΔD, or distance‐to‐agreement criterion, Δd) that are needed to achieve a predefined pass rate or gamma agreement index (GAI). In‐house code for evaluating IG with a fixed ΔD of 3% was developed using Python (v3.5.2) and investigated using treatment plans and measurement data from 25 retrospective patient specific quality assurance tests (53 individual arcs).

Results

It was found that when the desired GAI was set to 95%, approximately three quarters of the arcs tested were able to achieve Δd within 1 mm (mean Δd: 0.7 ± 0.5 mm). The mean Δd required in order for all points to pass the gamma evaluation (i.e., GAI = 100%) was 4.5 ± 3.1 mm. The possibility of evaluating IG by fixing the Δd or ΔD/Δd, instead of fixing the ΔD at 3%, was also investigated.

Conclusion

The IG method and its indices have the potential to be implemented clinically to quantify the minimum dose and distance criteria based on a specified GAI. This method provides additional information to augment standard gamma evaluation results during patient specific quality assurance testing of individual treatment plans. The IG method also has the potential to be used in retrospective audits to determine an appropriate set of local gamma criteria and action levels based on a cohort of patient specific quality assurance plans.

Hybrid 3D analytical linear energy transfer calculation algorithm based on precalculated data from Monte Carlo simulations

PURPOSE

The dose-averaged linear energy transfer (LET_d ) for intensity-modulated proton therapy (IMPT) calculated by one-dimensional (1D) analytical models deviates from more accurate but time-consuming Monte Carlo (MC) simulations. We developed a fast hybrid three-dimensional (3D) analytical LET_d calculation that is more accurate than 1D analytical model.

METHODS

We used the Geant4 MC code to generate 3D LET_d distributions of monoenergetic proton beams in water for all energies and used a customized error function to fit the LET_d lateral profiles at various depths to the MC simulation. The 3D LET_d calculation kernel was a lookup table of these fitted coefficients, and LET_d was determined directly from spot energies and voxel coordinates during analytical dose calculations. We validated our new method by comparing the calculated LET_d distributions to MC results using 3D Gamma index analysis with 3%/2 mm criteria in 12 patient geometries. The significance of the improvement in Gamma index analysis passing rates over the 1D analytical model was determined using the Wilcoxon rank-sum test.

RESULTS

The passing rate of 3D Gamma analysis comparing LET_d distributions from the hybrid 3D method and the 1D method to MC simulations was significantly improved from 94.0% ± 2.5% to 98.0% ± 1.0% (P = 0.0003). The typical time to calculate dose and LET_d simultaneously using an Intel Xeon E5-2680 2.50 GHz workstation was approximately 2.5 min.

CONCLUSIONS

Our new method significantly improved the LET_d calculation accuracy compared to the 1D method while maintaining significantly shorter calculation time even comparing with the GPU-based fast MC code.

Transitioning from measurement-based to combined patient-specific quality assurance for intensity-modulated proton therapy

OBJECTIVE

This study is part of ongoing efforts aiming to transit from measurement-based to combined patient-specific quality assurance (PSQA) in intensity-modulated proton therapy (IMPT). A Monte Carlo (MC) dose-calculation algorithm is used to improve the independent dose calculation and to reveal the beam modeling deficiency of the analytical pencil beam (PB) algorithm.

METHODS

A set of representative clinical IMPT plans with suboptimal PSQA results were reviewed. Verification plans were recalculated using an MC algorithm developed in-house. Agreements of PB and MC calculations with measurements that quantified by the γ passing rate were compared.

RESULTS

The percentage of dose planes that met the clinical criteria for PSQA (>90% γ passing rate using 3%/3 mm criteria) increased from 71.40% in the original PB calculation to 95.14% in the MC recalculation. For fields without beam modifiers, nearly 100% of the dose planes exceeded the 95% γ passing rate threshold using the MC algorithm. The model deficiencies of the PB algorithm were found in the proximal and distal regions of the SOBP, where MC recalculation improved the γ passing rate by 11.27% (p < 0.001) and 16.80% (p < 0.001), respectively.

CONCLUSIONS

The MC algorithm substantially improved the γ passing rate for IMPT PSQA. Improved modeling of beam modifiers would enable the use of the MC algorithm for independent dose calculation, completely replacing additional depth measurements in IMPT PSQA program. For current users of the PB algorithm, further improving the long-tail modeling or using MC simulation to generate the dose correction factor is necessary.

ADVANCES IN KNOWLEDGE

We justified a change in clinical practice to achieve efficient combined PSQA in IMPT by using the MC algorithm that was experimentally validated in almost all the clinical scenarios in our center. Deficiencies in beam modeling of the current PB algorithm were identified and solutions to improve its dose-calculation accuracy were provided.

MLC parameters from static fields to VMAT plans: an evaluation in a RT-dedicated MC environment (PRIMO)

Background

PRIMO is a graphical environment based on PENELOPE Monte Carlo (MC) simulation of radiotherapy beams able to compute dose distribution in patients, from plans with different techniques. The dosimetric characteristics of an HD-120 MLC (Varian), simulated using PRIMO, were here compared with measurements, and also with Acuros calculations (in the Eclipse treatment planning system, Varian).

Materials and methods

A 10 MV FFF beam from a Varian EDGE linac equipped with the HD-120 MLC was used for this work. Initially, the linac head was simulated inside PRIMO, and validated against measurements in a water phantom. Then, a series of different MLC patterns were established to assess the MLC dosimetric characteristics. Those tests included: i) static fields: output factors from MLC shaped fields (2 × 2 to 10 × 10 cm²), alternate open and closed leaf pattern, MLC transmitted dose; ii) dynamic fields: dosimetric leaf gap (DLG) evaluated with sweeping gaps, tongue and groove (TG) effect assessed with profiles across alternate open and closed leaves moving across the field. The doses in the different tests were simulated in PRIMO and then compared with EBT3 film measurements in solid water phantom, as well as with Acuros calculations. Finally, MC in PRIMO and Acuros were compared in some clinical cases, summarizing the clinical complexity in view of a possible use of PRIMO as an independent dose calculation check.

Results

Static output factor MLC tests showed an agreement between MC calculated and measured OF of 0.5%. The dynamic tests presented DLG values of 0.033 ± 0.003 cm and 0.032 ± 0.006 cm for MC and measurements, respectively. Regarding the TG tests, a general agreement between the dose distributions of 1–2% was achieved, except for the extreme patterns (very small gaps/field sizes and high TG effect) were the agreement was about 4–5%. The analysis of the clinical cases, the Gamma agreement between MC in PRIMO and Acuros dose calculation in Eclipse was of 99.5 ± 0.2% for 3%/2 mm criteria of dose difference/distance to agreement.

Conclusions

MC simulations in the PRIMO environment were in agreement with measurements for the HD-120 MLC in a 10 MV FFF beam from a Varian EDGE linac. This result allowed to consistently compare clinical cases, showing the possible use of PRIMO as an independent dose calculation check tool.

Testing the methodology for a dosimetric end-to-end audit of IMRT/VMAT: results of IAEA multicentre and national studies

Introduction: Within an International Atomic Energy Agency (IAEA) co-ordinated research project (CRP), a remote end-to-end dosimetric quality audit for intensity modulated radiation therapy (IMRT)/ volumetric arc therapy (VMAT) was developed to verify the radiotherapy chain including imaging, treatment planning and dose delivery. The methodology as well as the results obtained in a multicentre pilot study and national trial runs conducted in close cooperation with dosimetry audit networks (DANs) of IAEA Member States are presented.Material and methods: A solid polystyrene phantom containing a dosimetry insert with an irregular solid water planning target volume (PTV) and organ at risk (OAR) was designed for this audit. The insert can be preloaded with radiochromic film and four thermoluminescent dosimeters (TLDs). For the audit, radiotherapy centres were asked to scan the phantom, contour the structures, create an IMRT/VMAT treatment plan and irradiate the phantom. The dose prescription was to deliver 4 Gy to the PTV in two fractions and to limit the OAR dose to a maximum of 2.8 Gy. The TLD measured doses and film measured dose distributions were compared with the TPS calculations.Results: Sixteen hospitals from 13 countries and 64 hospitals from 6 countries participated in the multicenter pilot study and in the national runs, respectively. The TLD results for the PTV were all within ±5% acceptance limit for the multicentre pilot study, whereas for national runs, 17 participants failed to meet this criterion. All measured doses in the OAR were below the treatment planning constraint. The film analysis identified seven plans in national runs below the 90% passing rate gamma criteria.Conclusion: The results proved that the methodology of the IMRT/VMAT dosimetric end-to-end audit was feasible for its intended purpose, i.e., the phantom design and materials were suitable; the phantom was easy to use and it was robust enough for shipment. Most importantly the audit methodology was capable of identifying suboptimal IMRT/VMAT delivery.