Penumbra Width Research Articles

Comprehensive analysis of peripheral dose in electron beam therapy with a Varian TrueBeam Linac

ObjectiveTo investigate the characteristics of peripheral doses outside electron-beam applicators in Varian TrueBeam linacs. MethodPeripheral doses outside the electron applicator were measured for 6-, 9- and 12-MeV beams at the maximum dose depth (Dmax) for each energy source and at a source-to-surface distance of 100 cm. Measurements were performed using EBT3 films in solid water phantoms. The impact of field size on the penumbra width and peripheral doses was studied using various cutouts, including 3 cm × 3 cm, 6 cm × 6 cm, and 10 cm × 10 cm in a 10 cm × 10 cm applicator with the gantry and collimator at 0°. The influence of the applicator size was investigated using a circular cutout of 5 cm in diameter for various applicator sizes, including 6 cm × 6 cm, 10 cm × 10 cm, 15 cm × 15 cm, 20 × 20 cm, and 25 cm × 25 cm, at Dmax for each energy, while keeping the gantry and collimator angle at 0°. The measured dose profiles were compared with the Eclipse treatment planning system (TPS) predicted dose profiles. The effect of varying gantry angles (0°, 90°, and 270°) for a 3 cm × 3 cm cutout in a 10 cm × 10 cm applicator for each energy source and varying collimator angles (0°, 90°, and 270°) for a 10 cm × 10 cm field were investigated to determine their effects on the penumbra widths and peripheral doses. ResultsBoth the penumbra width and peripheral dose values increased with energy across different field sizes, gantry angles, collimator angles, and applicator sizes. Root Mean Square Deviation (RMSD) analysis indicated minimal differences between the measured profiles and TPS data. Peripheral doses remained below 5% of the maximum dose approximately 10–15 mm away from the field edges, suggesting the potential for implementing additional shielding where required. ConclusionsThis study highlights the importance of considering peripheral doses in electron radiotherapy. It is important to note the impact on healthy tissues beyond the treatment area to ensure patient safety and prevent the long-term side effects of treatment. These findings emphasize the necessity of implementing appropriate measures to minimize peripheral doses.

Beam profile characteristics of a Varian linear accelerator across different photon energy levels

Radiotherapy employs variety of energy radiation to destroy cancer cells. A linear accelerator(also called LINAC) is the machine used to delivered external beam radiation therapy.In order to confirm the treatment planning systems that deliver the best radiation to the tumors while sparing the surrounding normal tissues, extensive measurements of dosimetric parameters are part of the commissioning procedure of a LINAC for clinical usage. This work aimed to compare and assess photon beam profiles with respect to penumbra width, symmetry, and beam flatness. Beam profile measurements were performed for this study atTMSS Cancer Center, Bogura, Bangladesh, using a linear accelerator (Varian VitalBeam SN: 5199) with 6MV,10MV and 15MV photon energies for a set of field sizes (4 × 4, 10 × 10 and 20 × 20 cm2) and various reference depths keeping the same environmental conditions. A 3D water phantom, CC13 ionization chamber (SN:18635)as a reference chamber, CC04 ionization field chamber (SN:18616) as a and IBA myQA Accept software version 1.6 were used to measure the profiles of photon energies 6 MV, 10 MV 15 MV respectively. Utilizing the Eclipse (Version: 16.1) external treatment planning system, the profile calculation was carried out. According to the manufacturer's and IEC specifications, the photon beam profile data from the current investigation are compatible, and all of the tolerances fall within the ranges of clinically acceptable tolerance.

Investigation of a novel 2.5 MV sintered diamond target beam for intracranial linac-based stereotactic treatments.

This work investigates the small-field dosimetric characteristics of a 2.5 MV sintered diamond target beam and its feasibility for use in linac-based intracranial stereotactic treatments. Due to the increased proportion of low energy photons in the low-Z beam, it was hypothesized that this novel beam would provide sharper dose fall-off compared to the 6 MV beam owing to the reduced energy, and therefore range, of secondary electrons. Stereotactic treatments of ocular melanoma and trigeminal neuralgia were simulated for 2.5 MV low-Z and 6 MV beams using Monte Carlo to calculate dose in a voxelized anatomical phantom. Two collimation methods were investigated, including a 5x3 mm2 HDMLC field and a 4 mm cone to demonstrate isolated and combined effects of geometric and radiological contributions to the penumbral width. The measured 2.5 MV low-Z dosimetric profiles demonstrated reduced penumbra by 0.5 mm in both the inline and crossline directions across all depths for both collimation methods, compared to 6 MV. In both treatment cases, the 2.5 MV low-Z beam collimated with the 4 mm cone produced the sharpest dose fall off in profiles captured through isocenter. This improved fall-off resulted in a 59% decrease to the maximum brainstem dose in the trigeminal neuralgia case for the 2.5 MV low-Z MLC collimated beam compared to 6 MV. Reductions to the maximum and mean doses to ipsilateral and contralateral OARs in the ocular melanoma case were observed for the 2.5 MV low-Z beam compared to 6 MV with both collimation methods. While the low dose rate of this novel beam prohibits immediate clinical translation, the results of this study support the further development of this prototype beam to decrease toxicity in intracranial SRS treatments.&#xD.

Experimental and Monte Carlo based dosimetric investigation of a novel 3mm radiosurgery 3MV beam using the microSilicon detector.

The ZAP-X system is a novel gyroscopic radiosurgical system based on a 3MV linear accelerator and collimator cones with a diameter between 4 and 25mm. Advances in imaging modalities to detect small and early-stage pathologies allow for an early and less invasive treatment, where a smaller collimator matching the anatomical target could provide better sparing of surrounding healthy tissue. A novel 3mm collimator cone for the ZAP-X was developed. This study aims to investigate the usability of a commercial diode detector (microSilicon) for the dosimetric characterization of this small collimator cone; and to investigate the underlying small field perturbation effects. Profile measurements in five depths as well as PDD and output ratio measurements were performed with a microSilicon detector and radiochromic EBT3 films. In addition, comprehensive Monte Carlo simulations were performed to validate the measurement observations and to quantify the perturbation effects of the microSilicon detector in these extremely small field conditions. It is shown that the microSilicon detector enables an accurate dosimetric characterization of the 3mm beam. The profile parameters, such as the FWHM and 20%-80% penumbra width, agree within 0.1 to 0.2mm between film and detector measurements. The output ratios agree within the measurement uncertainty between microSilicon detector and films, whereas the comparisons of the PDD results show good agreement with the Monte Carlo simulations. The analysis of the perturbation factors of the microSilicon detector reveals a small field correction factor of approximately 3% for the 3mm circular beam and a correction factor smaller than 1.5% for field diameters above 3mm. It could be shown that the microSilicon detector is well-suitable for the characterization of the new 3mm circular beam of the ZAP-X system.

Dosimetric comparison of 6 MV and 10 MV flattening filter-free beams using small diameter cone for trigeminal neuralgia radiosurgery

Abstract Purpose: To compare the dosimetric characteristics and treatment delivery efficiency of trigeminal neuralgia (TN) stereotactic radiosurgery (SRS) patients previously treated with a 6 MV-FFF (flattening filter-free; radiation beam obtained by removing the flattening filter) beam versus those re-planned with a 10 MV-FFF beam using a conical collimator on the TrueBeam Novalis STx linear accelerator. Methods: Eleven patients with TN previously treated with a 6 MV-FFF beam following the SRS protocol of 90 Gy in a single fraction were selected. Plans were recalculated using 10 MV-FFF beam, maintaining the same dose prescription and beam angle configuration used with 6 MV-FFF beam. The dose gradient, volumes receiving 20 and 10 Gy, maximum dose and dose to 10% of the brainstem were recorded for both the energies. Efficiency was assessed by the average monitor unit (MU) and time per arc. The 10 MV-FFF machine was configured in the treatment planning system (TPS) to measure the tissue phantom ratio (TPR), dose profiles and scatter factors using RAZOR, PTW-60012 diodes and EBT3 radiochromic films. Results: Compared to the 6 MV-FFF, the 10 MV-FFF plans exhibit average increments in dose gradient, volume of 20 Gy and volume of 10 Gy of 3.8, 17.1 and 17.8%, respectively. Average increases of 6.5 and 18.1% were obtained for maximum dose and dose to 10% of the brainstem, respectively. An average increase of 31 MU/arc was observed for the 10 MV-FFF plans, with a 40% reduction in treatment time per arc. The TPR for the 10 MV-FFF beams increased by 10%, and a penumbra width of 0.3 mm was observed. Scatter factor increments of 15, 13.5, 12.7 and 10.3% were observed for the 6 MV-FFF over the 10 MV-FFF for cones of 4, 5, 6 and 7 mm, respectively. Conclusions: In TN SRS, the utilisation of 10 MV-FFF beams reduces treatment duration but results in an increased brainstem radiation dose. To mitigate this increase in brainstem dose, it is necessary to adjust the isocentre position.

Commissioning and Initial Validation of Commercial Treatment Planning System for the Electron FLASH Research Extension

The aim of this study was to investigate the feasibility of commissioning the 16 MeV electron FLASH beam in a commercial treatment planning system (TPS) for pre-clinical research purposes. The delivery system consisted of a new commercial solution for which a linear accelerator was modified into a FLASH Research Extension platform. Additionally, preliminary radiation biology results were highlighted to showcase the future use of this system. To commission a commercial electron Monte Carlo (MC) for dose calculation of a 16 MeV FLASH beam in the TPS, radiochromic film was used to measure the vendor-required beam data, e.g., profiles and percent depth dose (PDD) curves for cone sizes of 6 × 6 cm2, 10 × 10 cm2, and 15 × 15 cm2 as well as an in-air profile for a 40 × 40 cm2 open field (no cone). Once the electron MC beam model was generated, additional measurements were collected for validation and compared against the calculated dose from the TPS. A treatment planning comparison between the newly commissioned FLASH beam and the conventional electron beam was conducted. Specifically, the dose-volume histograms (DVHs) for target volumes and organs at risk were investigated for skin cancer cases previously treated with conventional electron beams. Lastly, the FLASH dose distribution predicted by the electron MC for an in vitro cell study setup was validated with radiochromic film measurements, and initial radiobiology tests were conducted using FLASH and conventional dose-rate electron beams. The electron MC calculated dose for the 16 MeV electron FLASH beam agreed with measured PDDs within 1% for all field sizes. The beam profile characteristics, such as penumbra, shape, and full width at half maximum, demonstrated good agreement with less than 0.5 mm difference between the TPS and measurements. There were noticeable differences in the profiles of large fields between the FLASH and conventional dose-rate beam models due to the more forward-peaked FLASH beam. For treatment planning, Regarding DVH, the FLASH dose-rate plan provided comparable plan quality to the conventional dose-rate plan, achieving adequate coverage for the target volumes and sparing the healthy organs and tissues. The electron MC dose prediction for the FLASH beam was also found to be in good agreement with the film measurements of the in vitro cell study setup. Furthermore, the FLASH beam was observed to be more effective with a 20 % increase in killing pancreatic cancer cells compared to the conventional dose rate. The study successfully incorporated the 16 MeV electron FLASH Research Extension into the commercial TPS using electron Monte Carlo for dose calculation. This will be valuable for pre-clinical cell and animal studies. This research also enables FLASH treatment planning studies, a key component for the future implementation of FLASH into clinical care. Further research is necessary to incorporate the radiation biology effect of FLASH into the treatment planning system.

A Multicenter Study of Clinical to Planning Target Volume Margins for Adjuvant Partial Breast Irradiation Delivered on the 1.5T MR-Linear Accelerator

Adjuvant partial breast irradiation (APBI) for early-stage breast cancer patients delivered on a conventional Linac commonly uses a clinical to planning target volume (CTV-PTV) margin of 10 mm. Published data suggest this margin could be reduced using an MR-guided workflow. This study quantifies the CTV to PTV margin for APBI delivered on the 1.5T MR-Linac (MRL) using an Adapt to Position (ATP) workflow. All target contouring was done as per the IMPORT LOW trial and MRL Consortium guidelines. The CTV is the tumor bed defined by surgical clips including postsurgical changes. A single center cohort of ten patients was used to assess delineation error on ProKnow DS v1.28.0 by measuring CTV contour displacements on the CT planning scans (pCT) delineated by five breast radiation oncologists. All other error components were measured on treatment planning software on another single center cohort of ten patients. Target deformation error was measured as surgical clip displacements between the pCT and daily pre-treatment (pre-Tx) MRI scans. Intrafraction motion was determined by the CTV displacement between pre- and post-treatment MRIs (post-Tx) in available paired images from five patients. Matching error was estimated as the interobserver variation of three MRL radiographers registering the pCT with daily pre-Tx MRI. Technical delivery accuracy was estimated using the results from routine quality assurance measurements. Beam penumbral width (p) was estimated from the clinical treatment plans. The systematic (Σ) and random errors (σ) for each component were calculated in the left/right (X), superior/inferior (Y) and anterior/posterior (Z) directions. The contribution of these errors to the PTV margin, M was calculated using van Herk's formula with α and β being 2.50 and 1.64 respectively. For APBI using an MRL ATP workflow, a CTV-PTV margin of 5.7 to 7.6 mm is required to achieve a 90% confidence of CTV coverage by the 95% isodose. Individual error components are in. Table 1 delineation error remains the largest component of error. A minimum CTV-PTV margin of 6-8 mm is required for APBI using an MRL ATP workflow. Although smaller than margins used in conventional Linacs, the clinical benefits (in terms of fibrosis risk) of treating APBI patients on an MRL are likely to be modest. Further margin reductions may be possible using an "Adapt to Shape" workflow with daily online recontouring.

Move up to MRI guided Gamma Knife radiosurgery: A Monte Carlo study on dose distribution changes

This study explores the feasibility of integrating Magnetic Resonance Imaging (MRI) with the Gamma Knife Icon (GKI™) for real-time monitoring during radiosurgery. The use of MRI could improve treatment accuracy and safety, but combining it with GKI™ poses dosimetric challenges due to the Lorentz force exerted by the MR scanner Magnetic Field (MF). A comprehensive GKI™ Monte Carlo (MC) simulation was conducted to investigate the changes induced by MF on the dose distribution of GKI™. The simulation was validated using profile curves and output factors (OFs). Five MF intensities were applied for different collimator sizes, and changes in physical penumbra width and Full Width at Half Maximum (FWHM) were calculated. The study found that the effect of MF on the dose distribution of the GKI™ is more prominent in high-intensity fields and smaller collimator sizes. The observed lateral displacement of up to 1 mm is significant, considering the device's nominal 0.25 mm precision. The results indicate that a 7 T magnetic field is the most suitable choice for the hypothetical MR-GK device, considering the penumbra width reduction and potential improvements in image quality. However, significant engineering and cost-related challenges are associated with integrating these two devices, and further research is necessary to investigate the feasibility and safety of combining these two technologies in clinical settings.

Commissioning and validation of a Monte Carlo algorithm for spine stereotactic radiosurgery.

A 6FFF Monte Carlo (MC) dose calculation algorithm was commissioned for spine stereotactic radiosurgery (SRS). Model generation, validation, and ensuing model tuning are presented. The model was generated using in-air and in-water commissioning measurements of field sizes between 10 and 400 mm2 . Commissioning measurements were compared to simulated water tank MC calculations to validate output factors, percent depth doses (PDDs), profile sizes and penumbras. Previously treated Spine SRS patients were re-optimized with the MC model to achieve clinically acceptable plans. Resulting plans were calculated on the StereoPHAN phantom and subsequently delivered to the microDiamond and SRSMapcheck to verify calculated dose accuracy. Model tuning was performed by adjusting the model's light field offset (LO) distance between physical and radiological positions of the MLCs, to improve field size and StereoPHAN calculation accuracy. Following tuning, plans were generated and delivered to an anthropomorphic 3D-printed spine phantom featuring realistic bone anatomy, to validate heterogeneity corrections. Finally, plans were validated using polymer gel (VIPAR based formulation) measurements. Compared to open field measurements, MC calculated output factors and PDDs were within 2%, profile penumbra widths were within 1mm, and field sizes were within 0.5mm. Calculated point dose measurements in the StereoPHAN were within 0.26%±0.93% and -0.10%±1.37% for targets and spinal canals, respectively. Average SRSMapcheck per-plan pass rates using a 2%/2mm/10% threshold relative gamma analysis was 99.1%±0.89%. Adjusting LOs improved open field and patient-specific dosimetric agreement. Anthropomorphic phantom measurements were within -1.29%±1.00% and 0.27%±1.36% of MC calculated for the vertebral body (target) and spinal canal, respectively. VIPAR gel measurements confirmed good dosimetric agreement near the target-spine junction. Validation of a MC algorithm for simple fields and complex SRS spine deliveries in homogeneous and heterogeneous phantoms has been performed. The MC algorithm has been released for clinical use.

Dosimetric delivery validation of dynamically collimated pencil beam scanning proton therapy

Objective. Pencil beam scanning (PBS) proton therapy target dose conformity can be improved with energy layer-specific collimation. One such collimator is the dynamic collimation system (DCS), which consists of four nickel trimmer blades that intercept the scanning beam as it approaches the lateral extent of the target. While the dosimetric benefits of the DCS have been demonstrated through computational treatment planning studies, there has yet to be experimental verification of these benefits for composite multi-energy layer fields. The objective of this work is to dosimetrically characterize and experimentally validate the delivery of dynamically collimated proton therapy with the DCS equipped to a clinical PBS system. Approach. Optimized single field, uniform dose treatment plans for 3 × 3 × 3 cm3 target volumes were generated using Monte Carlo dose calculations with depths ranging from 5 to 15 cm, trimmer-to-surface distances ranging from 5 to 18.15 cm, with and without a 4 cm thick polyethylene range shifter. Treatment plans were then delivered to a water phantom using a prototype DCS and an IBA dedicated nozzle system and measured with a Zebra multilayer ionization chamber, a MatriXX PT ionization chamber array, and Gafchromic™ EBT3 film. Main results. For measurements made within the SOBPs, average 2D gamma pass rates exceeded 98.5% for the MatriXX PT and 96.5% for film at the 2%/2 mm criterion across all measured uncollimated and collimated plans, respectively. For verification of the penumbra width reduction with collimation, film agreed with Monte Carlo with differences within 0.3 mm on average compared to 0.9 mm for the MatriXX PT. Significance. We have experimentally verified the delivery of DCS-collimated fields using a clinical PBS system and commonly available dosimeters and have also identified potential weaknesses for dosimeters subject to steep dose gradients.

Calculation of Experimental Penumbral Width of X-ray Dose Distributions Using Multiple Detectors

X-ray penumbral width depends on the size of the detector in the off-axis ratio of medical linear accelerators. The use of detectors with appropriate sizes according to the irradiation field is recommended. However, when measuring dose distributions, the shape of the off-axis ratio differs depending on detectors even if the dose distribution remains unchanged. This study aims to calculate the penumbral width using multiple detectors and obtain the independent penumbral width according to irradiation size. Using the X-ray output from a medical linear accelerator, off-axis ratios in water were measured using nine types of detectors. Penumbral width was calculated from each measured off-axis ratio by linear approximation and determining the intercept for analysis. The experimental irradiation fields were square areas (1×1 cm2 to 10×10 cm2). Penumbral widths were obtained from three types of detectors with different sensitive region widths of at least 1.2 mm. The values estimated from the nine detectors were 2.51-4.07 and 2.93-4.70 mm for 6 and 10 MV X-rays, respectively. The penumbral width and variation due to detector size increased with the irradiation field. The results estimated from the three selected detectors varied within ±0.5 mm compared with those from the nine detectors and were generally consistent. The reliability of the results was evaluated by comparing with the results from past studies and Monte Carlo simulations. Calculation of the penumbral width can be done regardless of size for various detectors. Thus, dose distributions can be compared for the linear accelerator at different facilities.

The feasibility of the pretreatment verification of 2D dose distributions in radiation therapy with small fields using the electronic portal imaging device.

The present study aims to evaluate the performance of an Electronic portal imaging device (EPID) for measuring dosimetric parameters and for verification of dose in small photon fields. In this study, the beam profiles were obtained using the amorphous silicon (a-Si) EPID for field sizes ranging from 1 × 1 to 10 × 10 cm 2 at energies 6 and 18 mega-voltage (MV). For comparison, the dosimetric parameters, including penumbra widths and field sizes, were measured with the pinpoint, diode, and Semiflex dosimeters. Finally, Rando Phantom was used to compare the two-dimensional (2D) Dose distribution between EPID and Treatment Planning System (TPS). In both 5 cm and 10 cm depths, there were large differences between the measured doses obtained from TPS, Pinpoint detector, and Farmer detector in 1 × 1 field size. The differences become negligible as the field sizes increase and from 3 × 3 field size to 10 × 10 field size, the maximum observed differences are 2 cGy and 2.4 cGy for 5 cm and 10 cm depths, respectively. The results indicate that the penumbra widths are smaller in the Gantry-Target (GT) direction compared to the Right-Left (RL) direction. The maximum difference (47.6%) was observed for EPID in the 10 × 10 field size, and the minimum difference (16.6%) was observed for TPS in the 1 × 1 field size. Finally, 2D dose distributions obtained by EPID and TPS exhibit excellent agreement. EPID is an excellent tool for the measurement of dosimetry parameters such as dose profiles, penumbra widths, field sizes, and pretreatment verification of 2D dose distributions, especially in small fields.

Evaluation of Perkin Elmer Amorphous Silicon Electronic Portal Imaging Device for Small Photon Field Dosimetry.

Electronic portal imaging devices (EPIDs) are applied to measure the dose and verify patients' position. The present study aims to evaluate the performance of EPID for measuring dosimetric parameters in small photon fields. In this experimental study, the output factors and beam profiles were obtained using the amorphous silicon (a-Si) EPID for square field sizes ranging from 1×1 to 10×10 cm2 at energies 6 and 18 mega-voltage (MV). For comparison, the dosimetric parameters were measured with the pinpoint, diode, and Semiflex dosimeters. Additionally, the Monaco treatment planning system was selected to calculate the output factors and beam profiles. There was a significant difference between the output factors measured using the EPID and that measured with the other dosimeters for field sizes lower than 8×8 cm2. In the energy of 6 MV, the gamma passing rates (3%/3 mm) between EPID and diode profile were 98%, 98%, 95%, 94%, 93%, and 94% for 1×1, 2×2, 3×3, 4×4, 5×5, and 10×10 cm2, respectively. The measured penumbra width with EPID was higher compared to that measured by the diode dosimeter for both energies. The EPID can measure the dosimetric parameters in small photon fields, especially for beam profiles and penumbra measurements.

Gamma Knife Icon™ Single Sector Characterization Based on Monte Carlo Simulation

Purpose: Gamma Knife Radiosurgery refers to surgery using radiation to destroy intracranial tissues or lesions elusive or unsuitable for open surgery. This study aimed to simulate the Gamma Knife Icon™ (GKI™) single sector to assess various attributes of the output beam and evaluate the EGSnrc C++ Monte Carlo code capabilities to perform a complete simulation of GKI™ for more investigations.
 Materials and Methods: The single source is simulated, and the geometries of the 4 and 16 mm collimators are defined based on the manufacturer data. The phase space files (PSFs) are recorded at the end of each collimator, and dose distributions are saved for the final analysis process in the last step.
 Results: The beam spectrum has two energy peaks g1 =1.17 MeV and g2 = 1.33 MeV, and low energy photons from scattering are also evident. The Gamma Index (GI) values are less than 1 in comparing the dose profiles generated in simulation with reference data. The Full Width at Half Maximum (FWHM) is 4.55, 10.9, 5.13 (mm) and 16.7, 35.1, 17.65 (mm) for 4mm and 16 mm collimators along x, y, and z axes, respectively. The penumbra width (80%-20%) is also 1.48, 5.5, 1.54 (mm) and 3.76, 10.1, 2.78 (mm) for 4mm and 16 mm collimators along x, y, and z axes, respectively.
 Conclusion: Results are in good agreement with what is expected, and it is possible to perform a complete simulation of the GKI™ system using egs++ for more investigations in phantoms and patients.

Validation of pencil beam scanning proton therapy with multi-leaf collimator calculated by a commercial Monte Carlo dose engine.

This study aimed to evaluate the clinical beam commissioning results and lateral penumbra characteristics of our new pencil beam scanning (PBS) proton therapy using a multi-leaf collimator (MLC) calculated by use of a commercial Monte Carlo dose engine. Eighteen collimated uniform dose plans for cubic targets were optimized by the RayStation 9A treatment planning system (TPS), varying scan area, modulation widths, measurement depths, and collimator angles. To test the patient-specific measurements, we also created and verified five clinically realistic PBS plans with the MLC, such as the liver, prostate, base-of-skull, C-shape, and head-and-neck. The verification measurements consist of the depth dose (DD), lateral profile (LP), and absolute dose (AD). We compared the LPs and ADs between the calculation and measurements. For the cubic plans, the gamma index pass rates (γ-passing) were on average 96.5%±4.0% at 3%/3mm for the DD and 95.2%±7.6% at 2%/2mm for the LP. In several LP measurements less than 75mm depths, the γ-passing deteriorated (increased the measured doses) by less than 90% with the scattering such as the MLC edge and range shifter. The deteriorated γ-passing was satisfied by more than 90% at 2%/2mm using uncollimated beams instead of collimated beams except for three planes. The AD differences and the lateral penumbra width (80%-20% distance) were within±1.9% and±1.1mm, respectively. For the clinical plan measurements, the γ-passing of LP at 2%/2mm and the AD differences were 97.7%±4.2% on average and within±1.8%, respectively. The measurements were in good agreement with the calculations of both the cubic and clinical plans inserted in the MLC except for LPs less than 75mm regions of some cubic and clinical plans. The calculation errors in collimated beams can be mitigated by substituting uncollimated beams.

Quality assurance of scanned proton beams at different gantry angles using an ionization chamber array in a rotational phantom

PurposeTo implement a single set-up monthly QA procedure for 9 different beam parameters at different gantry angles and evaluate its clinical implementation over a 12 month period. MethodsWe developed a QA procedure using an array detector (PTW Octavius 1500XDR) embedded in a rotational unit (PTW Octavius 4D) at our proton facility. With a single set-up we can monitor field central axis position, field symmetry, field size, flatness, penumbrae, output, spot size, spot position and range at different gantry angles (AAPM TG 224). The set-up is irradiated with homogenous 2D fields with dynamic aperture and spot patterns at five gantry angles. A modular top is used to check the range consistency. Absolute γ analysis were performed to compare measured dose distributions to calculated dose. All other parameters are directly extracted from the measurements. Additionally, the sensitivity of the set-up to small changes in beam parameters were compared to the Lynx detector (IBA). ResultsOver a 12 month period, output, symmetry, and flatness were within ± 2 %; FWHM, spot positions, penumbra widths, and central axis fields were within ± 1 mm. Range differences were all within 1/2 of the energy spacing (±0.6 MeV) relative to baseline. Most (2 %, 2 mm) γ-analysis showed agreement scores higher than 90 %. The sensitivity is comparable to the Lynx detector and measurement time is reduced by 40 %. ConclusionThe time-efficient monthly QA procedure that we developed can accurately be used to measure a large range of beam parameters at different gantry angles, within the TG 224 AAPM recommendations.

Assessing Clinical Feasibility of a LINAC-Based Electron FLASH Radiotherapy System Using an Anthropomorphic Phantom under Realistic Clinical Conditions

<h3>Purpose/Objective(s)</h3> Ultra-high dose rate (UHDR) radiotherapy (RT) at ≥40 Gy/s has been shown preclinically to reduce toxicity with similar tumor control compared to conventional RT, a phenomenon known as the FLASH effect. We previously described a configuration of a clinical-use LINAC that can output ∼17 MeV electron UHDR RT at 45-80 Gy/s with clinically feasible beam profiles, field sizes (3-20 cm), and SSDs (70-90 cm). Safe and effective clinical trials are needed to advance translation of FLASH treatment in humans, such as one proposed at our institution for cutaneous lymphomas. This study characterizes the beam output of our electron UHDR RT set-up under realistic clinical conditions for common anatomic sites using an anthropomorphic phantom, to assess its feasibility for clinical translation. <h3>Materials/Methods</h3> Our electron UHDR RT set-up utilizes a treatment delivery system with a configured program board of a decommissioned electron energy to adjust the beam output and control, without hardware modification. The electron cone is replaced with a flat applicator with custom cut-outs that lies in the accessory mount. We tested this set-up under realistic clinical conditions on a RANDO phantom for treatment of the chest/breast, leg, and scalp. We used 3 cm and 10 cm circular field sizes at 80 cm SSD with the gantry angled en-face to the targets. Tissue-equivalent bolus and flexible tungsten-silicone skin collimators (made from silicone sheets with tungsten powder cut into donut shapes of varying diameters) were used to mimic clinical treatment standards. Measurements were done using radiographic film and we evaluated the beam width, percent depth dose, and penumbra width. <h3>Results</h3> All results reported include 1 cm bolus and skin collimators unless otherwise specified. The values for R80 (depth at 80% dose), beam width of 80% dose (at R95), and penumbra width (at R85) for the 10 cm chest/breast field are 5.2, 8.7, and 0.9 cm, respectively. Without skin collimators, the values are 5.4, 9.0, and 1.2 cm, respectively. The values for the 3 cm chest/breast field are 2.9, 1.9, and 0.6 cm, respectively. Without skin collimators, the values are 3.2, 1.9, and 0.7 cm, respectively. The values for the 10 cm leg field are 4.0, 6.6, and 0.9 cm, respectively. The values for the 3 cm scalp field (with 4 cm bolus to minimize dose penetration to the brain) are 1.1, 1.6, and 0.9 cm, respectively. The steep curvature of the leg and scalp sites led to expected decrease in beam flatness but retained a treatment area that allows for a reasonable range of tumor sizes and depths. <h3>Conclusion</h3> Our configuration is able to deliver electron UHDR RT with clinically applicable beam profiles to various anatomic sites under realistic clinical conditions. Tungsten-silicone skin collimators are an easy-to-use tool to narrow the penumbra. Overall, these results show potential for treatment of superficial indications, including cutaneous lymphoma, extremity sarcoma, and partial breast irradiation, and place us one step closer toward the clinical translation of FLASH.

Clinical LINAC-Based Electron FLASH: Pathway for Practical Translation to Trials of FLASH Radiotherapy

<h3>Purpose/Objective(s)</h3> Studies show ultra-high dose rate (UHDR) radiotherapy (RT), at rates of ≥40 Gy per second, can produce the FLASH effect: reduced toxicity with equal tumor control when compared to conventional RT. Leveraging these benefits for clinical translation can profoundly impact RT. Early clinical trials exhibited safe delivery of UHDR-RT using specialized RT machines. Our prior study showed that a reversible configuration of a standard linear accelerator (LINAC) can be optimized for UHDR output. Here, we explored the components necessary for clinically practical delivery of LINAC-based electron FLASH RT. <h3>Materials/Methods</h3> Using a standard, clinically operational system for image-guided radiation, we configured the program board for a decommissioned electron beam energy for UHDR delivery. No LINAC hardware manipulation or exchange was performed. Key changes included adjustment of the dose per pulse and pulse delivery control using the gating interface. Of note, the scattering foil remained in place within the beam so that field sizes and depth doses could approximate those of standard clinical operations. A commercial electron arc applicator was installed at the face of the gantry using the standard applicator mount to allow for short SSD electron collimation. Copper electron blocks were fabricated with apertures ranging from 3.0 to 20.0 cm. Dose rates were measured at dmax for 100 to 59 cm source-to-surface distances (SSD) (level of machine head) and beam profile measurements were captured at 80 cm SSD using an IC profiler and radiographic film. <h3>Results</h3> UHDR rates ranging from 36.8 to 112.8 Gy/s were measured at 100 to 59 cm SSD, respectively. An 80 cm SSD was selected for treatment arrangement measurements to maintain UHDR and clinical utility, allowing for 13.5 cm of clearance. UHDR ≥60 Gy/s was observed at 80 cm SSD with all field sizes, using a standard, scattered electron beam, with an R80 depth of 6.1 cm, approximating to a 16-18 MeV energy. Average dose per pulse was measured at 0.33 Gy/pulse. Circular block apertures of 3, 10, and 20 cm exhibited beam flatness <5.0 % variability and symmetry of 2.2 to 6.2%. Penumbra width between the 80 and 20% isodose lines (IDL) measured at 0.7, 1.1, and 1.4 cm, respectively. <h3>Conclusion</h3> Optimizing a reversible setup for UHDR irradiation on a standard LINAC, electron delivery at dose rates expected to produce the FLASH effect was achieved. Flat, homogenous field sizes, ranging from 3 to 20 cm of coverage, were obtained at a clinically practical SSD of 80 cm with a treatment depth of 6.1 cm to the 80% IDL. This setup would enable UHDR irradiation at useful depths for a number of different clinical indications, bringing us a step closer to practical UHDR RT using a standard LINAC in future clinical trials.

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.

Performance of 3D diamond detectors in small field dosimetry: The impact of pixel size

Purpose:Small photon beams used in radiotherapy techniques have inherent characteristics of charge particle disequilibrium and high-dose gradient making accurate dosimetry for such fields very challenging. By means of a 3D manufacturing technique, it is possible to create arrays of pixels with a very small sensitive volume for radiotherapy dosimetry. We investigate the impact of 3D pixels size on absorbed dose sensitivity, linearity of response with dose rate, reproducibility and beam profile measurements. Methods:Diamond detectors with different pixel sizes have been produced in the 3DOSE experiment framework. To investigate the pixels size impact, they were tested using an Elekta Synergy LINAC. Dose rate dependence, absorbed dose sensitivity, reproducibility and beam profile measurement accuracy have been investigated and compared with PTW 60019 and IBA SFD reference dosimeters. Results:All of the 3D pixels had a linear and reproducible response to the dose rate. The sensitivity of a pixel decreases with its size, although even the smallest pixel has a high absorbed dose sensitivity (15 nC/Gy). The penumbra width measured with the smallest pixel size was consistent with the PTW microDiamond and differed by 0.2 mm from the IBA SFD diode. Conclusions:The study demonstrates that variation in pixel size do not affect the linearity of response with dose rate and the reproducibility of response. Due to the 3D geometry, the absorbed dose sensitivity of the detector remains high even for the smallest pixel, furthermore the pixel size was demonstrated to be of fundamental importance in the measurement of beam profiles.