The characterization of unflattened photon beams from a 6 MV linear accelerator

Fitting Monte Carlo simulation results with an empirical model of megavoltage x-ray beams for rapid depth dose calculations in water

Objective. The purpose of this study was to assess a method of accelerating Monte Carlo simulations for modeling depth dose distributions from megavoltage x-ray beams by fitting them to an empirically-derived function.Approach. Using Geant4, multiple simulations of a typical medical linear accelerator beam in water and in water with an air cavity were conducted with varying numbers of initial electrons. The resulting percent depth dose curves were compared to published data from actual linear accelerator measurements. Two methods were employed to reduce computation time for this modeling process. First, an empirical function derived from measurements at a particular linear accelerator energy, source-to-surface distance, and field size was used to directly fit the simulated data. Second, a linear regression was performed to predict the empirical function's parameters for simulations with more initial electrons.Main results. Fitting simulated depth dose curves with the empirical function yielded significant improvements in either accuracy or computation time, corresponding to the two methods described. When compared to published measurements, the maximum error for the largest simulation was 5.58%, which was reduced to 2.01% with the best fit of the function. Fitting the empirical function around the air cavity heterogeneity resulted in errors less than 2.5% at the interfaces. The linear regression prediction modestly improved the same simulation with a maximum error of 4.22%, while reducing the required computation time from 66.53 h to 43.75 h.Significance. This study demonstrates the effective use of empirical functions to expedite Monte Carlo simulations for a range of applications from radiation protection to food sterilization. These results are particularly impactful in radiation therapy treatment planning, where time and accuracy are especially valuable. Employing these methods may improve patient outcomes by ensuring that dose delivery more accurately matches the prescription or by shortening the preparation time before treatment in Monte Carlo-based treatment planning systems.

Comparison of skin dose in IMRT and VMAT with TrueBeam and Halcyon linear accelerator for whole breast irradiation

With the increasing use of flattening filter free (FFF) beams, it is important to evaluate the impact on the skin dose and target coverage of breast cancer treatments. This study aimed to compare skin doses of treatments using FFF and flattening filter (FF) beams for breast cancer. The study established treatment plans for left breast of an anthropomorphic phantom using Halcyon's 6-MV FFF beam and TrueBeam's 6-MV FF beam. Volumetric modulated arc therapy (VMAT) with varying numbers of arcs and intensity modulated radiation therapy (IMRT) were employed, and skin doses were measured at five points using Gafchromic EBT3 film. Each measurement was repeated three times, and averaged to reduce uncertainty. All plans were compared in terms of plan quality to ensure homogeneous target coverage. The study found that when using VMAT with two, four, and six arcs, in-field doses were 19%, 15%, and 6% higher, respectively, when using Halcyon compared to TrueBeam. Additionally, when using two arcs for VMAT, in-field doses were 10% and 15% higher compared to four and six arcs when using Halcyon. Finally, in-field dose from Halcyon using IMRT was about 1% higher than when using TrueBeam. Our research confirmed that when treating breast cancer with FFF beams, skin dose is higher than with traditional FF beams. Moreover, number of arcs used in VMAT treatment with FFF beams affects skin dose to the patient. To maintain a skin dose similar to that of FF beams when using Halcyon, it may be worth considering increasing the number of arcs.

Dosimetric Comparison of Commonly Used Volumetric Modulated Arc Therapy Field Arrangements Based on Flattening Filter-Free Beams for Synchronous Bilateral Breast Carcinoma Radiation Therapy

PURPOSE

Flattening filter-free (FFF)-based volumetric modulated arc therapy (VMAT) has been shown to be feasible and significantly improves treatment efficiency and lung protection for synchronous bilateral breast irradiation (SBBI). This research compared the commonly used VMAT field arrangements using FFF beams.

METHODS

Twenty-eight patients underwent SBBI were retrospectively enrolled to design irradiation plans using tangential arc VMAT (taVMAT), half arc VMAT (haVMAT), and large arc VMAT (laVMAT). Dosimetric and delivery parameters of all designed plans were recorded and compared.

RESULTS

Comparable target volume coverage was observed for all field arrangements. taVMAT significantly reduced the dose to spinal cord and the volume covered by 5 Gy (V5Gy) and V7Gy of the lungs while decreasing the conformity index of the target volume. It also increased the volume covered by 105% of the prescription dose (V105%) and V107% of the target volume. haVMAT considerably decreased V20 Gy and V30 Gy of the lungs, mean dose (Dmean) and V30 Gy of the heart and the liver. It also notably reduced Dmean and V40 Gy of the left anterior descending coronary artery while increasing the beam-on time. laVMAT significantly reduced the mean treatment time (range, 113-117 seconds) compared with the other field arrangements.

CONCLUSIONS

There were distinct differences in various dosimetric and delivery parameters for different field arrangements, highlighting the importance of selecting the appropriate field arrangement based on specific treatment goals and considerations. This study contributes valuable insights into the use of FFF-based VMAT techniques in SBBI.

Evaluation of Ion Recombination Correction for Indigenously Developed Farmer Ion Chamber in Flattening Filter-Free Photon Beams

Purpose and Aim

Modern generation linear accelerator (linac) either generates X-rays with a flattening filter (WFF beam) or without flattening filter free (FFF beam). The FFF beams are associated with a significantly higher dose per pulse compared to WFF beams due to the absence of a flattening filter and the corresponding attenuation caused by it. This results in increased ion recombination and a larger saturation correction factor (ks). In accordance with the IAEA TRS 398 dosimetry protocol, ks is necessary for the accurate measurement of absorbed dose at a point in water. The objective of this study was to evaluate the ks for the indigenous FAR 65-GB ion chamber (IC) for the FFF X-rays.

Materials and Methods

The study was carried out on TrueBeam linac (Varian, A Siemens Healthineers company) which offers 6 MV WFF, 6 MV FFF, 10 MV WFF and 10 MV FFF beams. The two-voltage method was employed to measure ks in a solid water phantom at a depth of 10 cm for a FAR 65-GB and SNC 600c and 0.6cc PTW 30013 Farmer chambers at 100 cm and 150 cm source-to-chamber distances for a 10 cm × 10 cm field size.

Results

The ks values for the FAR 65-GB, PTW 30,013, and SNC 600c were 1.0055 (1.0113), 1.0051 (1.0071), and 1.0033 (1.0066) for the 6 MV WFF (FFF) beams, respectively, and 1.0066 (1.0178), 1.0061 (1.0137), and 1.0035 (1.0119) for the 10MV WFF (FFF) beams, respectively. The ks values calculated by two-voltage method matches with ks values obtained from Jaffe's plot. The chamber exhibited a linear dose-response up to 3000 cGy, beyond which a saturation effect was observed.

Conclusions

Our study reveals that this chamber is suitable for the reference dosimetry for the FFF beams.

Stereotactic body radiotherapy with volumetric intensity-modulated arc therapy and flattening filter-free beams: dosimetric considerations

PURPOSE

The present study comparatively evaluates the impact of energy-matched flattening filter-free (FFF) photon beams with different energy levels on the physical-dosimetric quality of lung and liver stereotactic body radiotherapy (SBRT) treatment plans.

METHODS

For this purpose, 54 different lung and liver lesions from 44 patients who had already received SBRT combined with volumetric modulated arc therapy (VMAT) were included in this retrospective planning study. Planning computed tomography scans already available were used for the renewed planning with 6 MV, 6 MV-FFF, 10 MV, and 10 MV-FFF under constant planning objectives. The treatment delivery data, dosimetric distributions, and dose-volume histograms as well as parameters such as the conformity index and gradient indices were the basis for the evaluation and comparison of treatment plans.

RESULTS

A significant reduction of beam-on time (BOT) was achieved due to the high dose rates of FFF beams. In addition, we showed that for FFF beams compared to flattened beams of the same energy level, smaller planning target volumes (PTV) require fewer monitor units (MU) than larger PTVs. An equal to slightly superior target volume coverage and sparing of healthy tissue as well as organs at risk in both lung and liver lesions were found. Significant differences were seen mainly in the medium to lower dose range.

CONCLUSION

We found that FFF beams together with VMAT represent an excellent combination for SBRT of lung or liver lesions with shortest BOT for 10 MV-FFF but significant dose savings for 6 MV-FFF in lung lesions.

Efficacy of flattening filter-free beams with the acuros XB algorithm in thoracic spine stereotactic body radiation therapy

This study aimed to determine the dosimetric value of flattening filter-free (FFF) beams compared to flattening filter (FF) beams using different algorithms in the treatment planning of thoracic spine stereotactic body radiation therapy (SBRT). A total of 120 plans were created for 15 patients using the Anisotropic Analytical Algorithm (AAA) and the Acuros External Beam (AXB) algorithm with FF and FFF beams at 6 MV and 10 MV energies. Various dosimetric parameters were evaluated, including target coverage, dose spillage, and organs-at-risk sparing of the spinal cord and esophagus. Treatment delivery parameters, such as the monitor units (MUs), modulation factors (MFs), beam-on time (BOT), and dose calculation time (DCT), were also collected. Significant differences were observed in the dosimetric parameters when AXB was used for all energies (P < 0.05). 6 XFFF energy was the best option for target coverage, dose spillage, and organs-at-risk sparing. In contrast, dosimetric parameters had no significant difference when using the AAA. The AAA and AXB calculations showed that the 6 XFFF beam had the shortest DCT. The treatment delivery parameters indicated that 10 XFF beam required the fewest MUs and MFs. In addition, the 10 XFFF beam demonstrated the shortest BOT. For effective treatment of the thoracic spine using SBRT, it is recommended to use the 10 XFFF beam because of the short BOT. Moreover, the AXB algorithm should be used because of its accurate dose calculation in regions with tissue heterogeneity.

Dosimetric Comparison of Unmatched Flattening Filter-free and Flattened Beams in Volumetric Arc Therapy Plans for Head-and-neck Cancer

Introduction

Conventionally, the fattened beam is being used in radiotherapy for routine clinical cases even after introduction of intensity-modulated radiotherapy with incorporation of multi-leaf collimator system. With the removal of the flattening filter (FF) from the beam's path average energy of the photon gets reduced resulting in reduced scatter, reduction in treatment time, and reduced neutron contamination for high-energy beam and ultimately resulting in treatment plan quality deviations. This study aims to investigate the usefulness of the FF-free (FFF) beam for routine head-and-neck cancer (HNC) cases treated with volumetric arc therapy (VMAT) and dosimetrically compares the result with the FF beam.

Materials and Methods

In this study, 20 patients treated on HalcyonTM (unmatched 6 megavoltage [6MV] FFF beam) medical linear accelerator with VMAT of different HNC selected and for comparison with 6MV FF beam, 20 equivalent treatment plans are created for TruebeamTM configuration and the plans have been evaluated for target coverage, doses to the organ at risk (OAR), and other dose quality indices.

Results

Comparable target coverage, doses to OARs except for rest right parotid (P = 0.02) between 6MV FFF beam and 6MV FF beam is observed. Insignificant differences in conformity index, homogeneity index, and gradient index have been observed. Higher monitor unit (MU) (P ≤ 0.001) and lesser beam on time (BOT) (P = 0.003) have been observed in 6MV FFF.

Conclusion

6MV FFF beam provides comparable target coverage and improved dose-sparing effect to most of the OARs. 6MV FFF beam has lesser BOT, but on the other hand number of MUs is higher as compared to 6MV-FF plans.

Dosimetric characteristics of 6MV flattening filter free and flattened beams among beam-matched linacs: a three-institutional study

BACKGROUND

Beam matching is a concept in radiotherapy applied to clinics where more than one linac is employed to harmonise beam characteristics across linacs for allowing patients interchange without replanning. In view of this, the current study analyzes and compares dosimetric characteristics of 6MV flattening filter free and flattened beams of three beam-matched linear accelerators (linacs) from three different clinics with the aim to evaluate the matching under tight criteria for gamma analysis.

METHODS

Three Elekta linacs from three different clinics were included. The linacs have the same collimator assembly, Elekta Agility. Beam data were collected during commissioning process using PTW dosimetry systems. Dose profiles and percentage depth doses (PDD) were analyzed using 1D gamma analysis (1 mm/1%) as well as the following parameters: depth of maximum dose, PDD10, flatness, unflattnes, symmetry, penumbra, output factors. Additionally, five stereotactic treatment plans were optimized in one clinic and calculated by all three planning systems (Monaco) for a dosimetric comparison.

RESULTS

Gamma analysis of dose profiles and PDDs showed clinically acceptable results of 96.3% passing rate for profiles and 100% passing rate for PDDs. All dosimetric parameters were in good agreement with the reference data. Furthermore, dosimetric comparisons between stereotactic treatment plans showed a maximum standard deviation of 0.48 Gy for the maximum dose to PTV, and a maximum standard deviation of 0.1 Gy for the dose to the organs at risk.

CONCLUSIONS

All three linacs showed a strong agreement between parameters and passed the gamma analysis using 1% DD/1mm DTA criteria. This study confirmed the matching between linacs, offering the possibility to interchange patients with no replanning.

A Simplified Approach for Determination of Inflection Points of Flattening Filter-Free Photon Beam Using In-House Developed Software and Derivation of Reference Levels

Aim

This article aimed to present the salient features of the in-house developed Java program for the determination of inflection point and dosimetric parameters of flattening filter-free (FFF) photon beam. Reference levels for the dosimetric parameters of the FFF photon beams were also presented.

Materials and Methods

Beam profiles of 6 MV FFF and 10 MV FFF photon beams for a collimator setting of 20 cm × 20 cm measured at 10 cm depth in an isocentric setup acquired from various institutions were analyzed using an in-house developed Java program and manual method. The values of reference dose value (RDV), field size, penumbra, and degree of un-flatness (defined as the lateral separation between 90% [X90%], 75% [X75%], and 60% [X60%] dose points on the profile) were calculated and compared. The reference values of field size, penumbra, and degree of un-flatness were also determined for Varian and Elekta medical electron linear accelerators (LINACs).

Results

The maximum differences for RDV determined using the Java method and manual method are 2.4% and 2.7% for 6 and 10 MV FFF photon beams, respectively. The maximum difference between the values of field size, penumbra, and degree of un-flatness determined using Java and manual methods is within 1.3 mm. The reference values of field size and penumbra for Varian LINACs are 19.94 ± 0.10 cm and 0.83 ± 0.08 cm (6 MV FFF) and 19.95 ± 0.10 cm and 0.83 ± 0.08 cm (10 MV FFF). Similarly, the reference values of field size and penumbra for Elekta LINACs are 20.02 ± 0.09 cm and 0.94 ± 0.12 cm (6 MV FFF) and 20.03 ± 0.11 cm and 0.97 ± 0.16 cm (10 MV FFF).

Conclusions

A software program was developed in Java for analyzing the beam profiles of FFF photon beams. The results of Java-derived values of dosimetric parameters of FFF photon beams were found in good agreement with the values determined using the manual method. The reference values of these parameters were also derived and quoted using a large cohort of the data.

Investigation of low and high dose rate X-ray effects on histopathological changes and prognostic importance of Ki-67 in laryngeal cancer radiotherapy

The aim of this study is to analyze the effect on histopathological changes and Ki-67 expression levels of Flattening Filter (FF) and Flattening Filter Free (FFF) beams to investigate the radiobiological mechanisms underlying laryngeal cancer (LCa) post-radiotherapy (RT) on mice models. Forty adult NOD SCID gamma (NSG) mice models were randomly divided into four groups; the sham, LCa, FF-RT and FFF-RT groups. The head and neck region of mice in FF-RT and FFF-RT groups (LCa plus RT groups) were irradiated with a single dose of 18 Gy at 400 MU/min and 1400 MU/min. The NSG mice received radiotherapy 30 days after tumor transplantation and sacrificed 2 days after radiotherapy for analysis of histopathology parameters and K-67 expression levels. Comparing the LCa, FF-RT and FFF-RT groups with the sham group, statistically significant differences were observed in histopathological parameters depending on tumor tissue and dose rate (p < 0.05). When the histopathological effects of FF-RT beam on LCa tissue were compared with FFF-RT beam, it was observed that statistically significant differences occurred (p < 0.05). Comparing the LCa group with the sham group, it was observed that the Ki-67 level affected significantly depending on the development of cancer (p < 0.01). It was concluded that FF and FFF beams caused significant changes in the histopathological parameters and Ki-67 expression levels. When the effects of FFF beam on Ki-67 levels, cell nucleus and cytoplasmic findings were compared with FF beam, significant radiobiological differences were observed.

The Study of Dosimetric Characteristics of the XHA600D Medical Linear Accelerator Based on a Monte Carlo Code

By investigating the influence of initial electrons on dosimetric characteristics, reasonable incident electron parameters for the nominal 6 MV photon beam of the XHA600D accelerator are finally established, i.e., a 6 MeV monoenergetic electron beam with a radial intensity FWHM of 2.5 mm and an angular divergency of 0.15°. Based on reasonable initial parameters, Percentage Depth Doses (PDDs), Off-Axis Ratios (OARs), total scatter factors, beam qualities, and penumbra widths of both flatteningfilter (FF) and flattening-filter-free (FFF) beams for fields ranging from 4 × 4 to 30 × 30 cm2 are simulated systematically with EGSnrc codes. Not only the simulated dosimetric properties are in excellent agreement with the measurements, but also the dosimetric discrepancies between FF and FFF beams are consistent with the laws of previous studies on other accelerators. Therefore, reasonable incident electron parameters are able to accurately verify the performance of the XHA600D accelerator and can be used for further dosimetry research.

Flattening filter free Stereotactic radiosurgery for brain metastases using dynamic conformal arcs: 6 MV or 10 MV?

Introduction:

Stereotactic radiosurgery (SRS) has proven itself as an effective tool in the treatment of intracranial lesions. Image-guided high dose single fraction treatments have the potential to deliver ablative doses to tumours; however, treatment times can be long. Flattening filter free (FFF) beams are available on most modern linacs and offer a higher dose rate compared to conventional flattened beams which should reduce treatment times. This study aimed to compare 6 MV FFF and 10 MV FFF to a 6 MV flattened beam for single fraction dynamic conformal arc SRS for a Varian Truebeam linac.

Materials and methods:

In total, 21 individual clinical treatment plans for 21 brain metastases treated with 6 MV were retrospectively replanned using both 6 MV FFF and 10 MV FFF. Plan quality and efficiency metrics were evaluated by analysing dose coverage, dose conformity, dose gradients, dose to normal brain, beam-on-time (BOT), treatment time and monitor units.

Results:

FFF resulted in a significant reduction in median BOT for both 6 MV FFF (57·9%; p < 0·001) and 10 MV FFF (76·3%; p < 0·001) which led to reductions in treatment times of 16·8 and 21·5% respectively. However, 6 MV FFF showed superior normal brain dose sparing (p < 0·001) and dose gradient (p < 0·001) compared to 10 MV FFF. No differences were observed for conformity.

Conclusion:

6 MV FFF offers a significant reduction in average treatment time compared to 6 MV (3·7 minutes; p = 0·002) while maintaining plan quality.

Empirical modeling of the percent depth dose for megavoltage photon beams

Introduction

This study presents an empirical method to model the high-energy photon beam percent depth dose (PDD) curve by using the home-generated buildup function and tail function (buildup-tail function) in radiation therapy. The modeling parameters n and μ of buildup-tail function can be used to characterize the Collimator Scatter Factor (S_c) either in a square field or in the different individual upper jaw and lower jaw setting separately for individual monitor unit check.

Methods and materials

The PDD curves for four high-energy photon beams were modeled by the buildup and tail function in this study. The buildup function was a quadratic function in the form of dd2+n with the main parameter of d (depth in water) and n, while the tail function was in the form of e^−μd and was composed by an exponential function with the main parameter of d and μ. The PDD was the product of buildup and tail function, PDD = dd2+n·e−μd. The PDD of four-photon energies was characterized by the buildup-tail function by adjusting the parameters n and μ. The S_c of 6 MV and 10 MV can then be expressed simply by the modeling parameters n and μ.

Results

The main parameters n increases in buildup-tail function when photon energy increased. The physical meaning of the parameter n expresses the beam hardening of photon energy in PDD. The fitting results of parameters n in the buildup function are 0.17, 0.208, 0.495, 1.2 of four-photon energies, 4 MV, 6 MV, 10 MV, 18 MV, respectively. The parameter μ can be treated as attenuation coefficient in tail function and decreases when photon energy increased. The fitting results of parameters μ in the tail function are 0.065, 0.0515, 0.0458, 0.0422 of four-photon energies, 4 MV, 6 MV, 10 MV, 18 MV, respectively. The values of n and μ obtained from the fitted buildup-tail function were applied into an analytical formula of S_c = n_E(S)^0.63μ_E to get the collimator to scatter factor S_c for 6 and 10 MV photon beam, while n_E, ^μ_E, S denotes n, μ at photon energy E of field size S, respectively. The calculated S_c were compared with the measured data and showed agreement at different field sizes to within ±1.5%.

Conclusions

We proposed a model incorporating a two-parameter formula which can improve the fitting accuracy to be better than 1.5% maximum error for describing the PDD in different photon energies used in clinical setting. This model can be used to parameterize the S_c factors for some clinical requirements. The modeling parameters n and μ can be used to predict the S_c in either square field or individual jaws opening asymmetrically for treatment monitor unit double-check in dose calculation. The technique developed in this study can also be used for systematic or random errors in the QA program, thus improves the clinical dose computation accuracy for patient treatment.

Flattening Filter-Free Volumetric-Modulated Arc Radiotherapy for Left-Sided Whole-Breast, Partial-Breast, and Postmastectomy Irradiations

Purpose:

Unflattened photon beams exhibit many benefits over traditional flattened beams for radiotherapy (RT), but comprehensive evaluations of dosimetric results and beam-on time using flattening filter-free (FFF) beams for all types of breast irradiations are still lacking. The purpose of this study was to investigate if FFF RT can maintain equal or better dose coverage than standard flattened-beam RT while reducing doses to organs at risk (OARs) and beam-on time for various types of breast cancer irradiations.

Methods and Materials:

FFF volumetric-modulated arc therapy (FFF-VMAT) and standard VMAT (STD-VMAT) treatment plans were created for 15 whole-breast irradiation (WBI) patients with 50 Gy/25 fractions, 13 partial-breast irradiation (PBI) patients with 38.5 Gy/10 fractions, and 9 postmastectomy irradiation (PMI) patients with 50 Gy/25 fractions. Planning target volume (PTV) coverage and dose to OARs were evaluated.

Results:

Both techniques produced clinically acceptable plans for all three types of irradiations. For WBI, FFF-VMAT plans exhibited similar PTV and OARs evaluation metrics as STD-VMAT. For PBI, FFF-VMAT plans showed significantly lower mean and maximum doses for ipsilateral and contralateral lungs, contralateral breast, and heart. For PMI, FFF-VMAT plans showed significantly lower mean dose and V₅ for contralateral breast but significantly higher D_mean, D_max, and V₂₀ for ipsilateral lung and significantly higher D_mean, V_22.5, and V₃₀ for heart. FFF-VMAT techniques significantly reduced beam-on time than STD-VMAT for all cases.

Conclusion:

This work has shown that FFF beams are most beneficial for small-field irradiation such as PBI, and breast cancer patients could potentially benefit from the shortened beam-on time.

Calculated relative biological effectiveness (RBE) for initial DNA double-strand breaks (DSB) from flattening filter and flattening filter-free 6 MV X-ray fields

Objectives

We evaluated the radiobiological effectiveness based on the yields of DNA double-strand breaks (DSBs) of field induction with flattening filter (FF) and FF-free (FFF) photon beams.

Methods

We used the particle and heavy ion transport system (PHITS) and a water equivalent phantom (30 × 30 × 30 cm³) to calculate the physical qualities of the dose-mean lineal energy (y_D) with 6 MV FF and FFF. The relative biological effectiveness based on the yields of DNA-DSBs (RBE_DSB) was calculated for standard radiation such as 220 kVp X-rays by using the estimating yields of SSBs and DSBs. The measurement points used to calculate the in-field y_D and RBE_DSB were located at a depth of 3, 5, and 10 cm in the water equivalent phantom on the central axis. Measurement points at 6, 8, and 10 cm in the lateral direction of each of the three depths from the central axis were set to calculate the out-of-field y_D and RBE_DSB.

Results

The RBE_DSB of FFF in-field was 1.7% higher than FF at each measurement depth. The RBE_DSB of FFF out-of-field was 1.9 to 6.4% higher than FF at each depth measurement point. As the distance to out-of-field increased, the RBE_DSB of FFF rose higher than those of FF. FFF has a larger RBE_DSB than FF based on the yields of DNA-DSBs as the distance to out-of-field increased.

Conclusions

The out-of-field radiobiological effect of FFF could thus be greater than that of FF since the spreading of the radiation dose out-of-field with FFF could be a concern compared to the FF.

Advances in knowledge

The RBE_DSB of FFF of out-of-field might be larger than FF.

Commissioning and quality assurance of HalcyonTM 2.0 linear accelerator

BACKGROUND

Varian Medical Systems has introduced a new medical linear accelerator called HalcyonTM 2.0, which is based on the ring delivery system (RDS). It is a true IGRT machine having 6MV FFF photon energy. In addition to the planar and MV-CBCT imaging techniques it also has an option of ultra-fast kV-iCBCT which enhances the image reconstruction and improves the visualization of soft tissue. The field portals are shaped by a unique dual layer MLC with special stacked and staggered design which enables high modulation with low radiation leakage. Recently, we have commissioned our first Halcyon 2.0 machine. The aim of this work was to systematically investigate various parameters of a newly installed HalcyonTM 2.0 linear accelerator.

MATERIALS AND METHODS

Detailed measurements were conducted as per various guidelines. Also, the measurements were performed to fulfil the national regulatory requirements. Commissioning data of Halcyon 6 MV-FFF beam was performed in a water tank. For absolute measurements, a 0.6-cc waterproof Farmer chamber and electrometer were used. All relative measurements (PDDs, in-line, cross-line and angular profiles) were performed with 0.0125 cc point chamber.

RESULTS

All the tests were within the acceptable limit. Measured data were compared with factory data as well as the existing medical linear accelerator of the same category. The obtained results were quite satisfactory.

CONCLUSIONS

This study summarizes the commissioning experience with Halcyon linear accelerator. Evaluation of mechanical, radiation safety and dosimetric parameters were performed. The obtained parameters were well below the specified tolerance limits.

Dosimetry of a 6 MV flattening filter-free small photon beam using various detectors

The present study aimed to dosimetrically evaluate the small-fields of a 6 MV flattening filter-free (FFF) photon beam using different detectors.The 6 MV FFF photon beam was used for measurement of output factor, depth dose, and beam profile of small-fields of sizes 0.6 cm × 0.6 cm to 6.0 cm × 6.0 cm. The five detectors used were SNC125c, PinPoint, EDGE, EBT3, and TLD-100. All measurements were performed as per the International Atomic Energy Agency TRS 483 protocol. Output factors measured using different detectors as direct reading ratios showed significant variation for the smallest fields, whereas after correcting them according to TRS 483, all sets of output factors were nearly compatible with each other when measurement uncertainty was also considered. The beam profile measured using SNC125c showed the largest penumbra for all field sizes, whereas the smallest was recorded with EDGE. Compared with that of EBT3, the surface dose was found to be much higher for all the other detectors. PinPoint, EBT3, TLD-100, and EDGE were found to be the detector of choice for small-field output factor measurements; however, PinPoint needs special attention when used for the smallest field size (0.6 cm × 0.6 cm). EDGE and EBT3 are optimal for measuring beam profiles. EBT3, PinPoint, and EDGE can be selected for depth dose measurements, and EBT3 is suitable for surface dose estimation.

Factors affecting post-treatment radiation-induced lung disease in patients receiving stereotactic body radiotherapy to lung

The aim of the study is to investigate factors that may cause radiation-induced lung disease (RILD) in patients undergoing stereotactic body radiotherapy (SBRT) for lung tumors. Medical records of patients treated between May 2018 and June 2019 with SBRT were retrospectively evaluated. All patients should have a diagnosis of either primary non-small cell lung cancer (NSCLC) or less than three metastases to lung from another primary. The median treatment dose was 50 Gy in 4-5 fractions. Tumor response and RILD were evaluated in thoracic computer tomography (CT) using RECIST criteria. 82 patients with 97 lung lesions were treated. The median age was 68 years (IQR = 62-76). With a median follow-up of 7.2 months (3-18 months), three patients had grade 3 radiation pneumonitis (RP). RILD was observed in 52% of cases. Patients who had RILD had a higher risk of symptomatic RP (p = 0.007). In multivariate analyses older age, previous lung radiotherapy history, and median planning treatment volume (PTV) D95 value of ≥ 48 Gy were associated with RILD. Local recurrence (LR) was observed in 5.1% of cases. There was no difference in overall survival and LR with the presence of RILD. Older age, previous lung radiotherapy history, and median PTV D95 value of ≥ 48 Gy seems to be associated with post-SBRT RILD.

Dose accuracy improvement on head and neck VMAT treatments by using the Acuros algorithm and accurate FFF beam calibration

Background

The purpose of this study was to assess dose accuracy improvement and dosimetric impact of switching from the anisotropic analytical algorithm (AA) to the Acuros XB algorithm (AXB) when performing an accurate beam calibration in head and neck (H&N) FFF-VMAT treatments.

Materials and methods

Twenty H&N cancer patients treated with FFF-VMAT techniques were included. Calculations were performed with the AA and AXB algorithm (dose-to-water - AXB_w- and dose-to-medium - AXB_m-). An accurate beam calibration was used for AXB calculations. Dose prescription to the tumour (PTV70) and at-risk-nodal region (PTV58.1) were 70 Gy and 58.1 Gy, respectively. A PTV70_{_bone} including bony structures in PTV70 was contoured. Dose-volume parameters were compared between the algorithms. Statistical tests were used to analyze the differences in mean values and the correlation between compliance with the D95 > 95% requirement and occurrence of local recurrence.

Results

AA systematically overestimated the dose compared to AXB algorithm with mean dose differences within 1.3 Gy/2%, except for the PTV70_{_bone} (2.2 Gy/3.2%). Dose differences were significantly higher for AXB_m calculations when including accurate beam calibration (maximum dose differences up to 2.8 Gy/4.1% and 4.2 Gy/6.3% for PTV70 and PTV70_{_bone}, respectively). 80% of AA-calculated plans did not meet the D95 > 95% requirement after recalculation with AXB_m and accurate beam calibration. The reduction in D95 coverage in the tumour was not clinically relevant.

Conclusions

Using the AXB_m algorithm and carefully reviewing the beam calibration procedure in H&N FFF-VMAT treatments ensures (1) dose accuracy increase by approximately 3%; (2) a consequent dose increase in targets; and (3) a dose reporting mode that is consistent with the trend of current algorithms.

Transition From Manual to Automated Planning and Delivery of Volumetric Modulated Arc Therapy Stereotactic Radiosurgery: Clinical, Dosimetric, and Quality Assurance Results

PURPOSE

Properly planned single isocenter volumetric modulated arc therapy (VMAT) radiosurgery plans exhibit high quality and efficiency. We report here the largest clinical experience to date, to our knowledge, comparing manual planning with a new automated platform designed to standardize and simplify radiosurgery planning and delivery processes.

METHODS

We treated 693 patients with single isocenter VMAT radiosurgical plans generated by either our conventional manual (mVMAT) or a recently implemented automated (HyperArc^TM) technique. All plans targeted the gross tumor volume without margin. Radiochromic film was used for patient-specific quality assurance (PSQA). We evaluated local control and toxicity data for a subgroup of 107 patients having 377 metastatic tumors that were treated with HyperArc.

RESULTS

The median Radiation Therapy Oncology Group (RTOG) conformity index was 1.14 and was not different between the 2 techniques. The median Paddick gradient index was 5.42 for HyperArc versus 7.09 for mVMAT (P < .001). The median mean brain doses were 4.6% and 5.1% for HyperArc and mVMAT, respectively (P = .04). The PSQA for both techniques met clinical criteria, but 97% of the HyperArc plans satisfied the gamma tolerance limit recommended by the American Association of Physicists in Medicine Task Group No. 218, compared with 94% of the mVMAT plans (P = .02). The median treatment-planning times were not significantly different. The median treatment times were 10.5 and 11.4 minutes for HyperArc and mVMAT, respectively (P < .001). The Kaplan-Meier estimate of local control was 90.1% at 1 year.

CONCLUSIONS

HyperArc produces high-quality radiosurgical plans that are at least as good as mVMAT plans created by an expert manual planner with easier planning and more efficient delivery workflow. A less experienced planner can produce very high-quality radiosurgical plans even for patients with more than 10 targets. The use of a single-isocenter technique for multiple targets with no PTV margin did not compromise clinical outcomes, and 1-year local control for treated targets remained congruent with historical series.

Surface and build‐up dose comparison between Elekta 6 MV flattening filter and flattening‐filter‐free beams using an advanced Markus ionization chamber and a solid water‐equivalent phantom

Using a plane‐parallel advanced Markus ionization chamber and a stack of water‐equivalent solid phantom blocks, percentage surface and build‐up doses of Elekta 6 MV flattening filter (FF) and flattening‐filter‐free (FFF) beams were measured as a function of the phantom depth for field sizes ranging from 2 × 2 to 10 × 10 cm². It was found that the dose difference between the FF and the FFF beams was relatively small. The maximum dose difference between the FF and the FFF beams was 4.4% at a depth of 1 mm for a field size of 2 × 2 cm². The dose difference was gradually decreased while the field size was increased up to 10 × 10 cm². The measured data were also compared to published Varian FF and FFF data, suggesting that the percentage surface and build‐up doses as well as the percentage dose difference between FF and FFF beams by our Elekta linac were smaller than those by the Varian linac.

Initial Clinical Experience Treating Patients With Lung Cancer on a 6MV-Flattening-Filter-Free O-Ring Linear Accelerator

Introduction Modern technologies, like intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT), have improved the therapeutic ratio of thoracic radiotherapy (TRT) for lung cancer (LC). Halcyon™ (Varian Medical Systems, Palo Alto, CA, USA), a novel 6MV-flattening-filter-free O-ring linear accelerator (6X-FFF ORL), was designed to deliver IMRT and VMAT with greater speed than a C-arm linac. Herein, we report our initial clinical experience treating patients with LC on this linac. Methods All patients who received TRT for LC on the 6X-FFF ORL at our institution were retrospectively identified. Patients' clinicopathologic data, radiotherapy details, early disease-control and toxicity outcomes, dosimetric data, couch corrections, and treatment times are reported. Results Between 10/2018-12/2019, 30 consecutive patients (median age 66 years, range 54-94 years) received definitive or post-operative TRT for LC (median 66 Gy/33 fractions; range 5-70 Gy/2-37 fractions) following four-dimensional computed tomography (CT) simulation (97%) using daily kilovoltage KV cone-beam CT (CBCT) (100%) on a 6X-FFF ORL for non-small cell LC (84%) or small cell LC (16%), with 53% receiving VMAT, 43% receiving static-field IMRT, and 77% receiving concurrent systemic therapy. All plans were approved through institutional peer review. The average three-dimensional vector couch correction based on CBCT guidance was 0.90 ± 0.50 cm. The average beam-on and beam on plus CBCT times were 1.7 ± 1.1 min, and 5.0 ± 3.2 min, respectively. Grade 3 dyspnea and fatigue occurred in 3% and 3% of patients, respectively. There were no grade ≥4 toxicities. Conclusion In this first clinical report of TRT for LC on a 6X-FFF ORL, daily CBCT-guided treatment was fast and safe with respect to dosimetry and clinical outcomes. Thus, use of this linac for TRT may increase LC patient throughput without a detriment in radiotherapy quality.

Initial Clinical Experience Treating Patients With Gynecologic Cancers on a 6MV Flattening Filter Free O-Ring Linear Accelerator

PURPOSE

Radiation therapy (RT) is commonly used in the treatment of gynecologic cancers. Intensity-modulated RT (IMRT) has been shown to reduce gastrointestinal toxicity compared with 2-dimensional and 3-dimensional RT modalities. We report the initial clinical experience using IMRT for gynecologic cancers with a novel 6MV flattening filter free O-ring linear accelerator (6X-FFF ORL).

METHODS AND MATERIALS

We retrospectively identified consecutive women with uterine or cervical cancer who received pelvic RT on Halcyon (Varian Medical Systems, Palo Alto, CA), a novel 6X-FFF ORL. We report their clinicopathologic data, RT details, early disease-control outcomes, acute toxicities, dose-volume histogram data, couch corrections, and treatment times.

RESULTS

Seventeen women received RT on a 6X-FFF ORL for uterine cancer (76%) or cervical cancer (24%) between January 2017 and September 2019. RT was delivered postoperatively (82%) or to intact disease (18%), to a median dose of 50.4 Gy (range, 19.8-55.0 Gy) in 25 fractions (range, 11-28), with 12% receiving extended-field RT and 65% receiving chemotherapy. Target and organ-at-risk constraints were met in all plans. The 3-dimensional vector couch correction average was 0.90 ± 0.37 cm. The mean beam-on time was 2.9 ± 0.4 min and mean treatment time, from imaging start to beam-off, was 3.6 ± 0.4 min. Grade 2 fatigue, anorexia, diarrhea, bloating, and nausea occurred in 41%, 12%, 12%, 6%, and 6% of patients, respectively. There were no grade ≥3 toxicities.

CONCLUSIONS

In the initial clinical report of pelvic RT for gynecologic cancers using a 6X-FFF ORL, the linac showed versatility in treatment; comparability to flattening-filtered IMRT for early disease-control, toxicity, and dosimetry; and treatment speed that compared favorably to IMRT on a C-arm gantry. Accordingly, a 6X-FFF ORL may increase throughput or reduce day length in departments with high gynecologic cancer volumes, without compromising clinical outcomes.

Comparison of radiotherapy techniques with flattening filter and flattening filter-free in lung radiotherapy according to the treatment volume size

In external radiotherapy (RT), the use of flattening filter-free (FFF) radiation beams obtained by removing the flattening filter (FF) in standard linear accelerators is rapidly increasing, and the benefits of clinical use are the issue of research. Advanced treatment techniques have increased the interest in the operation of linear accelerators in FFF mode. The differences of the beams with non-uniform dose distribution created by removing FF compared to the beams with uniform dose distribution used as a standard were examined. These differences were compared in the treatment plans of lung patients who have different planning target volumes (PTV). Clinac IX linear accelerator units were used. Twenty patients with previously completed treatment were divided into two groups depending on the size of the target volume. All patients underwent two different intensity-modulated RT (IMRT) plans using FF and FFF beams. The Wilcoxon Signed-Rank test was used to compare two different techniques (Significance p < 0.05). There was no statistically significant difference between the two techniques when looking at the D2%(Gy), D98%(Gy), D50%(Gy), homogeneity (HI), and conformity index (CI) data for both groups. When the critical organ doses were evaluated, there was a statistically significant difference only in the V20(%) values of the lungs, but these differences were not very large. Monitor unit (MU) data were found to be lower in FF planning, and treatment time was lower in FFF planning. Except for shorter treatment times, and of the lungs V20(%) value, in standard fractionated RT of lung cancer, there was no significant difference between the use of FFF and FF techniques for large and small target volumes.

Technical Note: Flattening filter free beam from Halcyon linac: Evaluation of the profile parameters for quality assurance

INTRODUCTION

The use of flattening filter free (FFF) beams generated by standard linear accelerators is increasing in the clinical practice. The radiation intensity peaked toward the beam central axis is properly managed in the optimization process of treatment planning through intensity modulation. Specific FFF parameters for profile analysis, as unflatness and slope for FFF beams, based on the renormalization factor concept has been introduced for quality assurance purposes. Recently, Halcyon, an O-ring based linear accelerator equipped with a 6 MV FFF beam only has been introduced by Varian.

METHODS

Renormalization factors and related fit parameters according to Fogliata et al. ["Definition of parameters for quality assurance of FFF photon beams in radiation therapy," Med. Phys. 39, 6455-6464 (2012)] have been evaluated for the 6 MV FFF beam generated by Halcyon units. The Halcyon representative beam data provided by Varian were used. Dose fall-off at the field edges was matched with an unflattened beam generated by a 6 MV from a TrueBeam linac. Consistency of the results was evaluated against measurements on a clinical Halcyon unit, as well as a TrueBeam 6 MV FFF for comparison.

RESULTS

The five parameters in the analytical equation for estimating the renormalization factor were determined with an R² of 0.997. The comparison of the unflatness parameters between the Halcyon representative and hospital beam data was consistent within a range of 0.6%. Consistently with the computed parameters, the Halcyon profiles resulted in a less pronounced peak than TrueBeam.

CONCLUSION

Renormalization factors and related fit parameters from the 6 MV FFF beam generated by the Varian Halcyon unit are provided.

Beam flatness modulation for a flattening filter free photon beam utilizing a novel direct leaf trajectory optimization model

Flattening filter free (FFF) linear accelerators produce a fluence distribution that is forward peaked. Various dosimetric benefits, such as increased dose rate, reduced leakage and out of field dose has led to the growth of FFF technology in the clinic. The literature has suggested the idea of vendors offering dedicated FFF units where the flattening filter (FF) is removed completely and manipulating the beam to deliver conventional flat radiotherapy treatments. This work aims to develop an effective way to deliver modulated flat beam treatments, rather than utilizing a physical FF. This novel optimization model is an extension of the direct leaf trajectory optimization (DLTO) previously developed for volumetric modulated radiation therapy (VMAT) and is capable of accounting for all machine and multileaf collimator (MLC) dynamic delivery constraints, using a combination of linear constraints and a convex objective function. Furthermore, the tongue and groove (T&G) effect was also incorporated directly into our model without introducing nonlinearity to the constraints, nor nonconvexity to the objective function. The overall beam flatness, machine deliverability, and treatment time efficiency were assessed. Regular square fields, including field sizes of 10 × 10 cm² to 40 × 40 cm² were analyzed, as well as three clinical fields, and three arbitrary contours with "concave" features. Quantitative flatness was measured for all modulated FFF fields, and the results were comparable or better than their open FF counterparts, with the majority having a quantitative flatness of less than 3.0%. The modulated FFF beams, due to the included efficiency constraint, were able to achieve acceptable delivery time compared to their open FF counterpart. The results indicated that the dose uniformity and flatness for the modulated FFF beams optimized with the DLTO model can successfully match the uniformity and flatness of their conventional FF counterparts, and may even provide further benefit by taking advantage of the unique FFF beam characteristics.

10-MV SBRT FFF IRRADIATION TECHNIQUE IS ASSOCIATED TO THE LOWEST PERIPHERAL DOSE: THE OUTCOME OF 142 TREATMENT PLANS FOR THE 10 MOST COMMON TUMOUR LOCATIONS

There is a growing interest in the combined use of Stereotactic Body Radiation Therapy (SBRT) with Flattening Filter Free (FFF) due to the high local control rates and reduced treatment times, compared to conventionally fractionated treatments. It has been suggested that they may also provide a better radiation protection to radiotherapy patients as a consequence of the expected decrease in peripheral doses. This work aims to determine this reduction in unattended out-of-field regions, where no CT information is available but an important percentage of second primary cancers occur. For that purpose, ten different cases suitable for SBRT were chosen. Thus, 142 different treatment plans including SBRT, as well as 3D-CRT, IMRT and VMAT (with standard fractionation) in low and high energies for Varian (FF and FFF), Siemens and Elekta machines were created. Then, photon and neutron peripheral dose in 14 organs were assessed and compared using two analytical models. For the prostate case, uncomplicated and cancer free control probability estimation was also carried out. As a general behavior, SBRT plans led to the lowest peripheral doses followed by 3D-CRT, VMAT and IMRT, in this order. Unflattened beams proved to be the most effective in reducing peripheral doses, especially for 10 MV. The obtained results suggest that FFF beams for SBRT with 10 MV represent the best compromise between dose delivery efficiency and peripheral dose reduction.

Study of Dosimetric Properties of Flattening Filter Free Photon Beam Passing through Cadmium Free Compensator Alloy

BACKGROUND

This study aims to investigate radiation beam geometry of Cyberknife beam and change in dosimetric characteristics of six megavoltage (6MV) flattening filter free (FFF) beam after passing through high density cadmium free compensator alloy.

MATERIAL AND METHODS

In this experimental study, changes in FFF beam dosimetric characteristics after passing through compensator alloy was measured. Transmitted intensity of FFF beam was measured in air by an ion chamber at a source to detector distance (SDD) of 800mm. Extended SDD measurement also has been performed at a distance of 1270mm to analyze scattering due to compensator. Linear attenuation coefficient (µ_eff) was measured for cadmium free compensator alloy using simple exponential attenuation model. Percentage depth doses (PDDs) have been measured by a radiation field analyzer with compensator material to observe the beam hardening and change in surface doses and depth doses.

RESULTS

Linear attenuation coefficient of compensator alloy was measured 0.042 (Standard Deviation ±0.00099) mm^-1 and it was found that there is no change with increase in collimator size. Even after increasing distance source from detector, µ_eff has no change. PDDs were found to increase with thickness of compensator. PDD from a 60mm collimator size increased by 5% and 6% at a depth of 100mm and 200mm, respectively in water. PDD also increased with collimator size less significantly. Surface dose was found to decrease with increase in compensator thickness.

CONCLUSION

Cyberknife beam has been found to be narrow beam geometry. FFF beam contains lesser scattered photons. Presence of high density compensator filters out the soft x-ray photon causes significant dosimetric changes.

Impact of detector selections on inter-institutional variability of flattening filter-free beam data for TrueBeam™ linear accelerators

This study evaluates the type of detector influencing the inter-institutional variability in flattening filter-free (FFF) beam-specific parameters for TrueBeam™ linear accelerators (Varian Medical Systems,Palo Alto, CA, USA). Twenty-four beam data sets, including the percent depth dose (PDD), off-center ratio (OCR), and output factor (OPF) for modeling within the Eclipse (Varian Medical Systems) treatment planning system, were collected from 19 institutions. Although many institutions collected the data using CC13 (IBA Dosimetry, Schwarzenbruck, Germany) or PTW31010 semiflex (PTW Freiburg, Freiburg, Germany) ionization chambers, some institutions used diode detectors, diamond detectors, and ionization chambers with smaller cavities. The OCR data included penumbra width, full width at half maximum (FWHM), and FFF beam-specific parameters, including unflatness and slope. The data measured by CC13/PTW31010 ionization chambers were compared with those measured by all other detectors. PDD data demonstrated the variations within ±1% at the dose fall-off region deeper than peak depth. The penumbra widths of the OCR measured with the CC13/PTW31010 detectors were significantly larger than those measured with all other detectors (P < 0.05). Especially the EDGE detector (Sun Nuclear Corp., Melbourne, FL, USA) and the microDiamond detectors (model 60019; PTW Freiburg) demonstrated much smaller penumbra values compared to those of the CC13/PTW31010 detectors for the 30 × 30 mm² field. There was no difference in the FWHM, unflatness, and slope parameters between the values for the CC13/PTW31010 detectors and all other detectors. OPF curves demonstrated small variations, and the relative difference from the mean value of each data point was almost within 1% for all field sizes. Although the penumbra region exhibited detector-dependent variations, all other parameters showed tiny interunit variations regardless of the detector type.

Increase in Superficial Dose in Whole-Breast Irradiation With Halcyon Straight-Through Linac Compared With Traditional C-arm Linac With Flattening Filter: In vivo Dosimetry and Planning Study

Purpose

Superficial dose is an important parameter in breast cancer radiation therapy. When treated with conventional linacs, bolus is commonly applied to improve target coverage near the surface while also managing the risk of severe skin reactions and negative cosmesis. With the introduction of modern linacs with 6X flattening filter free (FFF) photon beams, the effect on superficial dose and the need for bolus must be evaluated.

Methods and Materials

In vivo measurements of superficial dose were made with optically stimulated luminescence dosimeters on 11 breast cancer patients treated with the Halcyon 6X FFF linac (Varian Medical Systems, Palo Alto, CA). Additionally, measurements were made with the Halycon 6X FFF beam and a 6X beam with flattening filter (FF) delivered to an anthropomorphic phantom. A planning study was carried out in which 14 patients treated on the Halcyon were replanned with a conventional linac to determine the difference in superficial dose predicted by the treatment planning system. Measures were taken to increase the accuracy of the treatment planning system superficial dose.

Results

The use of the Halcyon 6X FFF beam led to higher superficial dose compared with 6X FF beams. The in vivo measurements show an average superficial dose of 83.8% ± 0.6%, which is an increase of approximately 10% compared with published measurements for a 6X FF linac. Comparison of superficial dose for 6X FF and 6X FFF beams in the phantom measurements show an increase from 70% ± 1.3% to 84% ± 1.3%, which is consistent with the in vivo measurements. The planning comparison shows an increase in V70%Rx from 62% ± 4.4% to 81% ± 2.2% for the superficial breast tissue for the Halcyon 6X FFF beam compared with a standard C-arm linac with FF.

Conclusions

The use of the Halcyon 6X FFF beam was associated with higher superficial dose which may obviate the use of bolus.

Studies on Fundamental Interaction Parameters for Stainless Steel and Titanium Biomaterials Using Flattened and Un-Flattened Megavoltage X-Ray Beams

Purpose: This work presents the measure of fundamental interaction parameters like mass attenuation coefficient (μ/ρ), mean energy, total atomic (σa) and electronic (σe) cross section, effective atomic number (Zeff), electron density (Nel) and mean free path (mfp) using FF and UF megavoltage x-ray beam for high Z implants. Methods: Narrow beam geometry is used to find out mass attenuation coefficient (μ/ρ) (MAC) which is then used to calculate mean energy (using NIST data), total atomic (σa) and electronic cross section (σe) for different energies. The effective atomic number (Zeff), Electron density (Nel), mean free path (mfp) for both flattened and unflattened x-ray beams for high Z material stainless steel (SS316) and titanium alloy (Grade 5) are studied. Results: The mean energies calculated from NIST data against mass attenuation coefficient were in good agreement with Monte Carlo value. It shows that spectral weighted effective atomic number is independent of megavoltage energies in the Compton region. Effective electron density calculated using Zeff and MAC method is lesser compared to direct method for both high Z materials. The mean free path (mfp) is higher along the central axis than off-axis for flattened beam in comparison to unflattened beam for both of the high Z materials studied because of the variation in energy spectrum for both FF and UF x-ray beams. Conclusion: This study elaborated the fundamental interaction parameters of different energies of flattened and unflattened x-ray beam interactions with high Z materials such as Stainless Steel (SS316) and Titanium (Grade5) relevant in a clinical scenario.

Determination of an inflection point for a dosimetric analysis of unflattened beam using the first principle of derivatives by python code programming

Background

Practice of Unflattened or Flattening filter free (FFF) beam has become the high dose standard in radiotherapy (RT), such as stereotactic radio-surgery (SRS) and stereotactic radiotherapy (SRT). The removal of a flattening filter (FF) from the path of a photon beam alters the characteristics of FFF beam. Since the conventional route for dosimetric analysis of FF beam cannot be applied to FFF beam, the procedure of analyzing beam characteristics for FFF beam based on inflection points (IPs) is used. IP is a point where the concavity change observed corresponds to its change in sign (±) of the second derivative.

Aim

The objective of the study is to determine IPs for dosimetric analysis of the FFF beam profile.

Methods and materials

In this study, IPs are determined through the python code programming based on the mathematical first principle of the derivative. They are compared with IPs estimated by the conventional graphical manual method using Microsoft Excel (MS). IPs and their dependent dosimetric parameters determined by both mathematical and graphical manual methods are compared.

Result

Percentage differences between the IPs determined by both methods, for 6MVFFF inline and crossline beam profile are found to be 2.7% and 0.8% respectively. Similarly, the average penumbra differences for 6MVFFF inline and crossline beam profile are found to be 0.15 mm and 0.9 mm, respectively. However, differences in the field width between both methods are found insignificant.

Conclusion

Graphical manual method is very time-consuming, tedious and user dependent. However, the mathematical method through python code programming is more precise, faster and independent of individual users.

Initial Clinical Experience Treating Patients with Breast Cancer on a 6-MV Flattening-Filter-Free O-Ring Linear Accelerator

Purpose

Three-dimensional (3D) conformal radiation therapy is the standard technique used for adjuvant breast radiation. We report the clinical use of a novel 6-MV flattening-filter-free O-ring linear accelerator (6X-FFF ORL) for breast cancer that may improve upon 3D conformal radiation therapy with its higher dose rate and faster rotation and leaf speed than traditional C-arm gantries.

Methods and Materials

We retrospectively identified consecutive women with breast cancer who underwent surgery followed by radiation therapy to the breast or chest wall on Halcyon (Varian Medical Systems, Palo Alto, CA), a novel 6X-FFF ORL. We report their clinicopathologic information, radiation therapy details, acute toxicities, dose-volume histogram data, couch corrections, and treatment times.

Results

Thirty-four women were treated for breast cancer on a 6X-FFF ORL between February 2018 and September 2018. All patients underwent lumpectomy (92%) or mastectomy (8%). Tumors were left sided in 44% and bilateral in 9%, and 9% included comprehensive nodal radiation therapy. Twelve percent of patients were treated prone and 29% with deep-inspiration breath hold. Standard target and normal-tissue constraints were met in nearly all plans. The 3D vector couch correction average was 0.77 ± 0.05 cm. The mean beam-on time was 2.0 ± 0.3 minutes, and mean treatment time from start of imaging to beam-off was 4.4 ± 0.4 minutes. Grade 2 dermatitis, fatigue, and breast pain occurred in 18%, 9%, and 3% of patients, respectively.

Conclusions

In this first clinical report of breast radiation therapy with a 6X-FFF ORL, treatment was versatile and fast for complex setups and techniques, with acceptable toxicity and organ-at-risk doses. Thus, a 6X-FFF ORL can increase throughput or reduce length of day compared with a conventional C-arm linear accelerator in departments with a busy breast service.

Comparison of detector performance in small 6 MV and 6 MV FFF beams using a Versa HD accelerator

1. BACKGROUND & PURPOSE

Investigate the applicability of a series of detectors in small field dosimetry and the possible differences between their responses to FF and FFF beams. This work extends upon the series of detectors used by other authors to also include metal-oxide-semiconductor field-effect transistors (MOSFETs) detectors and radiochromic film. We also included a later correction of output factors (OFs) recommended by the recently published IAEA´s code of practice TRS 483 on dosimetry of small static fields used in external beam radiotherapy.

2. MATERIALS & METHODS

The OFs, profiles, and PDDs of 6 MV and 6 MV FFF beams were measured with 11 different detectors using field sizes between 0.6 × 0.6 cm2 and 10 × 10 cm2.

3. RESULTS

The OFs of the FFF beams were lower than those of the FF beams for field sizes larger than 3 × 3 cm2 but higher for field sizes smaller than 3 × 3 cm2. After applying the IAEA´s TRS 483 corrections, the final OFs were compatible with our initial results when considering uncertainties involved. Small-volume detectors are preferable for measuring the penumbra of these small fields where this attribute is higher in the crossline direction than in the inline direction. The R100 of equivalent-quality FFF beams was higher compared to the corresponding flattened beams.

4. CONCLUSIONS

We observed no difference for the dose responses between 6 MV and 6 MV FFF beams for any of the detectors. OF results, profiles and PDDs were clearly consistent with the previously published literature regarding the Versa HD linac. Correcting our first OFs, taken as ratio of detector charges, with the IAEA´s TRS 483 corrections to obtain the final OFs, did not make the former significantly different.

Three-dimensional dose comparison of flattening filter (FF) and flattening filter-free (FFF) radiation therapy by using NIPAM gel dosimetry

Intensity-modulated radiotherapy and volumetric modulated arc therapy are modern radiation therapy technologies that can create the desired dose distribution by multileaf collimator movement and dose-rate control. However, the homogeneous dose delivery of small-field irradiation techniques shows disagreement with that of treatment planning system. The removal of the flattening filter by flattening filter free (FFF) beam irradiation increases dose conformity and further reduces treatment delivery time in radiosurgery. This study aims to investigate the dose distribution of FFF and flattened beams for small-field irradiation by using the 3D gel dosimeter. The N-isopropylacrylamide (NIPAM) polymer gel dosimeter was employed to record the 3D dose distribution. In addition, flattened and FFF beams were compared using the gamma evaluation technique. The use of an FFF accelerator resulted in excellent radiation treatments with short delivery times and low doses to normal tissues and organs. Results also showed that the passing rate increased with the decrease of field size (30 × 30, 20 × 20, and 10 × 10 mm²) at post-irradiation times of 24, 48, 72, and 96 h. The passing rates for each field size were retained at the same level when gamma criteria, namely, distance-to-agreement (DTA) = 3 mm/dose difference (DD) = 3%, were used. Nevertheless, the passing rates for each field size decreased slowly after 48 h when DTA = 2 mm/DD = 2%. The Wilcoxon signed-rank test was used to determine statistical difference with a significant level of p < 0.05. The passing rates of flattened and FFF beams showed no significant difference. The edge enhancement effect in the flattened beam was more evident than in the FFF beam. The 3D NIPAM gel dosimeter can be used for dose verification of small field for radiation therapy with high dose rate.

Part I: Out-of-field dose mapping for 6X and 6X-flattening-filter-free beams on the TrueBeam for extended distances

PURPOSE

With increasing cancer treatment success rates, many patients go on to live long, productive lives following recovery. Therefore, minimizing potential side effects due to dose outside the treated field is becoming a significant consideration in radiation therapy. With many potential treatment configurations available, it is important to quantify how out-of-field dose varies with common variables such as distance from isocenter, couch angle, jaw size, and flattening-filter setting. The accurate quantification of out-of-field dose at extended distances could also benefit researchers and detector developers. While data exist for out-of-field dose from older linear accelerator (Linac) models, the phenomenon has not been described for the latest generation of machines, such as the Varian TrueBeam. The purpose of this study was to comprehensively quantify out-of-field dose for the Varian TrueBeam Linac low energy photons in a wide range of positions and treatment geometries.

METHOD AND MATERIALS

Out-of-field doses were measured using two phantom setups: (a) A large volume ion chamber with a buildup sleeve to quantify head leakage and collimator scatter background dose; and (b) A farmer ion chamber in solid water to incorporate phantom scatter in addition to collimator scatter, and head leakage background dose. In both cases, the ion chamber was positioned with its length along the slowly varying transverse direction (perpendicular to the radial from isocenter). Doses were measured for four symmetric jaw settings (2 × 2 cm² , 4 × 4 cm² , 10 × 10 cm² , and 20 × 20 cm² ) for a range of distances from the isocenter (0-100 cm). The angular dependence of the out-of-field dose was measured using four different angles: 0°, 45°, 90°, and 135° with respect to the in-plane direction. All measurements were performed for both 6X and 6X-flattening-filter-free (FFF) beams.

RESULTS

The lowest out-of-field doses were observed at 60 cm away from isocenter in both in-plane and cross-plane directions for fields smaller than 10 × 10 cm² . Out-of-field dose decreased with decreasing jaw size (a factor of 4.7 for 6X-FFF and a factor of 3.1 for 6X going from 20 × 20 cm² to 2 × 2 cm² at 60 cm from isocenter in the in-plane direction). The 6X-FFF beam produced out-of-field doses as low as 64% of the 6X beam.

CONCLUSION

This study presents a comprehensive description of 6X and 6X-FFF out-of-field doses on a Varian TrueBeam Linac including measurements at a range of positions, angles, and jaw settings and with and without phantom scatter.

Feasibility study of using flattening-filter-free photon beams to deliver conventional flat beams

Various dosimetric benefits such as increased dose rate, and reduced leakage and out of field dose have led to the growth of flattening-filter-free (FFF) technology in the clinic. In this study, we concentrate on investigating the feasibility of using FFF beams to deliver conventional flat beams, since completely getting rid of the flattening-filter module from the gantry head can not only simplify the gantry design but also decrease the workload on machine maintenance and quality assurance. Two intensity modulated radiotherapy techniques, step-and-shoot (S&S) and sliding window (SW), were used to generate flat beam profiles for 6 regular-shaped beams and 3 clinical beams while operating in FFF mode. The inverse plans were generated based on uniform dose optimization. Degree of flatness, MU efficiency, and beam delivery time for both methods were assessed. S&S technique is able to achieve a degree of flatness less than 2.5% for most field configurations. While SW technique was able to generate relatively flat beams for field sizes less than 18 × 18 cm². For all field configurations, S&S beams resulted in a longer delivery time compared to reference flat beams and SW beams. For field sizes less than 18 × 18 cm², SW modulated FFF beams resulted in a faster delivery time compared to reference flat beams. The ability to deliver conventional flat beams is not absent when operating in FFF mode. Utilizing beam modulation, FFF mode can achieve reasonable flat profiles and comparable efficiency to conventional flat beams. The ability to deliver most clinical treatments from the same treatment unit will allow for less quality assurance as well as maintenance, and completely eliminate the need for the flattening filter on modern linacs.

Comparison of Flattening Filter and Flattening Filter-Free Volumetric Modulated Arc Radiotherapy in Patients with Locally Advanced Nasopharyngeal Carcinoma

BACKGROUND This study aimed to investigate the therapeutic role of flattening filter-free (FFF) mode in volumetric modulated arc therapy (VMAT) compared with flattening filter (FF) mode in patients with locally advanced nasopharyngeal carcinoma (NPC). MATERIAL AND METHODS Ten previously treated patients with NPC underwent treatment re-planning with FFF and FF VMAT. Radiotherapy dose distribution on planning target volume (PTV), organs at risk (OAR), target conformity index (CI), total monitor units (MUs), and therapeutic time were compared. RESULTS Maximum and mean radiotherapy dose in PTV and PGTV (primary lesions of NPC and cervical lymph node metastases) in FFF VMAT planning were significantly increased compared with FF VMAT planning, but PTV and OAR showed no significant differences. The CI value of PTV in FFF VMAT planning was significantly reduced compared with FF planning (P<0.05). No differences were found for the maximum radiotherapy dose in the spinal cord and left and right optic nerve, and the mean radiotherapy dose in the brainstem, left and right parotid gland (P>0.05). The maximum dose in the brainstem in the FFF planning was significantly higher compared with FF planning (P>0.05). The maximum radiotherapy dose in left and right crystalline lens (P<0.05) in FFF planning was significantly reduced compared with FF planning. The total hop count in FFF planning was significantly increased compared with FF planning (P<0.05). CONCLUSIONS Both 6 MV X-ray FFF mode and FF mode in the treatment of patients with NPC showed that FFF VMAT planning provided improved protection for OAR.

Study of dosimetric properties of flattened and unflattened megavoltage x ray beam on high Z implant materials

PURPOSE

Addition of high Z implants in the treatment vicinity or beam path is unavoidable in certain clinical situation. In this work, we study the properties of radiation interaction parameters such as mass attenuation coefficient (MAC), x ray beam transmission factor (indirect beam attenuation), interface effects like backscatter dose perturbation factor (BSDF) and forward dose perturbation factor (FDPF) for flattened (FF) and unflattened (UF) x ray beams.

METHODS

MAC for stainless steel and titanium alloy was measured using CC13 chamber with appropriate buildup in narrow beam geometry. The x ray beam transmission factors were measured for stainless steel and titanium alloy for different field size, off-axis, and depths. Profile analysis was performed using a radiation field analyzer (RFA) as a function of field size and depth to study the influence of phantom scattering and spectral variation in the beam. In addition, interface effects such as BSDF and FDPF were measured with gafchromic films at maximum BSDF peak position calculated using Acuros XB algorithm and with PPC40 chamber measured at exit side of high Z material, respectively.

RESULTS

The MAC in both cases decreases with increase in energy for stainless steel (SS) and titanium (Ti) alloy. The MAC increases with the change in x ray beam type from flattened to UF beam because of relatively lower mean energy. The x ray beam transmission factor increases with the increase in energy, field size, and depth owing to increase in penetration power phantom scatter, respectively. The measured BSDF and FDPF were found to be in good agreement with AXB algorithm.

CONCLUSION

The dosimetric properties of x ray photon beam were studied comprehensively in the presence of high Z material like stainless steel and titanium alloy using both flattened and UF beams to understand and incorporate the findings of various parameters in clinical condition due to the variation in energy spectrum from FF to UF x ray beam.

The effect of beam shape on physical parameters of head and neck simultaneous-integrated boost intensity-modulated radiation therapy

Aim

To evaluate the influence of the beam shape created by X-rays with "flat beams" and without "flattening-filter-free [FFF] beams" a flattening filter, and the isocenter locations for FFF beams on the treatment of a large irradiated volume for tumours.

Background

The increase of dose rate and the decrease of out-of-field dose can be expected for FFF beams and lead to effective and safety radiotherapy. On the other hand, the bell-shaped dose profile is thought to be a factor of negating these advantages.

Materials and methods

Treatment plans for 15 patients with head and neck cancer were created using XiO (Elekta, Stockholm AB, Sweden) in fixed-gantry step-and-shoot delivery under the same dose constraints. Seven fields of FFF beams with 7 MV and flat beams with 6 MV were used with the technique of intensity-modulated radiation therapy (IMRT). We compared the dose homogeneity and conformity of targets and dose constraints for organs as the plan quality and evaluated physical parameters: monitor unit (MU) values, number of segments and their locations from the isocenter in beam's-eye-view.

Results

No significant differences were found in the plan quality. The isocenter locations do not affect the physical parameters for FFF beams. It has been confirmed that the number of segments and MU values were 40% higher with FFF beams than with flat beams (p < 0.05).

Conclusion

This study demonstrates flat dose distribution is more suitable for IMRT with large and complex targets.

Comparative Evaluation of a 6MV Flattened Beam and a Flattening Filter Free Beam for Carcinoma of Cervix – IMRT Planning Study

Purpose:

Intensity modulated radiotherapy (IMRT) plan quality, beam on time and integral dose were compared using 6MV FB (Flattened Beam) and FFFB (Flattening filter free beam) for carcinoma of cervix.

Materials and Methods:

Ten patients with stage II–IIIB cervix cancer (Ca.Cx) were retrospectively identified from the department database. Target volume (TV) and organ at risk (OAR) were delineated as per Radiation Therapy Oncology Group (RTOG) cancer guidelines. Dose prescribed to planning target volume (PTV) was 50.4Gy in 28 fractions. Two plans (6MV FB IMRT and 6MV FFFB IMRT) were generated to achieve 95% of prescription dose to PTV and sparing OAR as per normal tissue guidelines. Numbers of beams and their orientations were the same for all plans. The homogeneity index (HI), conformity index (CI), treatment monitor unit (MU), beam on time (BOT) and non-tumor integral dose (NTID) were chosen for comparison.

Results:

FFFB generated plans were clinically acceptable. There was a statistically significant difference among the FB IMRT and FFFB IMRT plans with respect to CI, HI, D50%, D2% in PTV coverage, bladder V50Gy, MU, mean NTID and non-tumor low dose volume.

Conclusions:

6MV flattened and flattening filter free photon beams produce comparable plans by IMRT. FFF beams allow time efficient treatment delivery and may help reduce the risk of secondary malignances in carcinoma cervix cases.

Flattening filter-free technique in volumetric modulated arc therapy for lung stereotactic body radiotherapy: A clinical comparison with the flattening filter technique

The present study sought to evaluate the impact of the flattening filter-free (FFF) technique in volumetric modulated arc therapy for lung stereotactic body radiotherapy. Its clinical safety and availability were compared with the flattening filter (FF) method. The cases of 65 patients who underwent lung volumetric modulated arc therapy-stereotactic body radiotherapy (VMAT-SBRT) using FF or FFF techniques were reviewed. A total of 55 Gy/4 fractions (fr) was prescribed for peripheral lesions or 56 Gy/7 fr for central lesions. The total monitor units (MU), treatment time, dose to tumors, dose to organs at risk, tumor control (local control rate, overall survival, progression-free survival) and adverse events between cases treated with FF and cases treated with the FFF technique were compared. A total of 35 patients were treated with conventional FF techniques prior to November 2014 and 30 patients were treated with FFF techniques after this date. It was revealed that the beam-on time was significantly shortened by the FFF technique (P<0.01). Other factors were similar for FFF and FF plans in respect to conformity (P=0.95), homogeneity (P=0.20) and other dosimetric values, including total MU and planning target volume/internal target volume coverage. The median follow-up period was 18 months (range, 2-35). One-year local control rates were 97.1 and 90.0% in the FF group and FFF groups, respectively (P=0.33). Grade 3 pneumonitis was observed in 5.8% of FF patients and 3.4% of FFF patients (P=1.00). No other adverse events ≥grade 3 were observed. The results of the study suggest that VMAT-SBRT using the FFF technique shortens the treatment time for lung SBRT while maintaining a high local control rate with low toxicity.