Collimator Size Research Articles

Salvage Breast-Conserving Surgery and Reirradiation With Intraoperative Electrons for Recurrent Breast Cancer: A Multicentric Study on Behalf of Italian Association of Radiotherapy and Clinical Oncology (AIRO)

AimIntraoperative radiotherapy with electrons (IOERT) may represent a viable choice for partial breast re-irradiation (rePBI) after repeat quadrantectomy for local recurrence (LR) for primary breast cancer (BC) in lieu of mastectomy. Materials and MethodsA database collecting data on rePBI with IOERT from 8 Italian centres was set up in 2016- 2018, providing data on cumulative incidence (CumI) of 2nd LR nd survival with a long follow-up (FU) ResultsFrom 2002 to 2015, 109 patients underwent the conservative retreatment.The median primary BC -1stLR interval was 11.1 years (range: 2.4-27.7). The median 1stLR size was 0.9 cm (range: 0.3-3.0) and 43.6% were Luminal A. Median IOERT dose was 18 Gy (range: 12-21) and median collimator was 4 cm (range: 3-6). Median FU was 11.7 years (interquartile range: 7.7-14.6). The 2ndLR CumI was 12.2% (95% CI: 6.8-19.2) at 5 years and 32.3% at 10 years (95% CI: 22.8-42.2), occurring in the same site as the 1stLR in about half of the cases. HER2 status and collimator size were independent LR predictors. The 5- and 10-year overall survival were 95.2% and 88.3%, respectively, while 5- and 10-year BC specific survival were 98% and 94.5%. The development of a 2ndLR significantly reduced BCSS (HR=9.40, P<0.001). Grade ≥3 fibrosis was 18.9%. Patient-reported cosmesis was good/excellent in 59.7% of the cases. Conclusion2ndLR CumI was within the range of the literature, but higher than expected, opening questions on radiation field extension and fractionation schedule. Since a 2ndLR worsened the outcome, salvage modality must be carefully planned.

Comparative analysis of plan quality and delivery efficiency: ZAP-X vs. CyberKnife for brain metastases treatment

Purpose/ObjectivesZAP-X, a novel and dedicated radiosurgery (SRS) system, has recently emerged, while CyberKnife has solidified its position as a versatile solution for SRS and stereotactic body radiation therapy over the past two decades. This study aims to compare the dosimetric performance and delivery efficiency of ZAP-X and CyberKnife in treating brain metastases of varying target sizes, employing circular collimation.Methods and materialsTwenty-three patients, encompassing a total of 47 brain metastases, were included in the creation of comparative plans of ZAP-X and CyberKnife for analysis. The comparative plans were generated to achieve identical prescription doses for the targets, while adhering to the same dose constraints for organs at risk (OAR). The prescription isodose percentage was optimized within the range of 97–100% for each plan to ensure effective target-volume coverage. To assess plan quality, indices such as conformity, homogeneity, and gradient (CI, HI, and GI) were computed, along with the reporting of total brain volumes receiving 12Gy and 10Gy. Estimated treatment time and monitor units (MUs) were compared between the two modalities in evaluating delivery efficiency.ResultsOverall, CyberKnife achieved better CI and HI, while ZAP-X exhibited better GI and a smaller irradiated volume for the normal brain. The superiority of CyberKnife’s plan conformity was more pronounced for target size less than 1 cc and greater than 10 cc. Conversely, the advantage of ZAP-X’s plan dose gradient was more notable for target sizes under 10 cc. The homogeneity of ZAP-X plans, employing multiple isocenters, displayed a strong correlation with the target’s shape and the planner’s experience in placing isocenters. Generally, the estimated treatment time was similar between the two modalities, and the delivery efficiency was significantly impacted by the chosen collimation sizes for both modalities.ConclusionThis study demonstrates that, within the range of target sizes within the patient cohort, plans generated by ZAP-X and CyberKnife exhibit comparable plan quality and delivery efficiency. At present, with the current platform of the two modalities, CyberKnife outperforms ZAP-X in terms of conformity and homogeneity, while ZAP-X tends to produce plans with a more rapid dose falloff.

Mixed-size spot scanning with a compact large momentum acceptance superconducting (LMA-SC) gantry beamline for proton therapy

Objective. Lowering treatment costs and improving treatment quality are two primary goals for next-generation proton therapy (PT) facilities. This work will design a compact large momentum acceptance superconducting (LMA-SC) gantry beamline to reduce the footprint and expense of the PT facilities, with a novel mixed-size spot scanning method to improve the sparing of organs at risk (OAR). Approach. For the LMA-SC gantry beamline, the movable energy slit is placed in the middle of the last achromatic bending section, and the beam momentum spread of delivered spots can be easily changed during the treatment. Simultaneously, changing the collimator size can provide spots with various lateral spot sizes. Based on the provided large-size and small-size spot models, the treatment planning with mixed spot scanning is optimized: the interior of the target is irradiated with large-size spots (to cover the uniform-dose interior efficiently), while the peripheral of the target is irradiated with small-size spots (to shape the sharp dose falloff at the peripheral accurately). Main results. The treatment plan with mixed-size spot scanning was evaluated and compared with small and large-size spot scanning for thirteen clinical prostate cases. The mixed-size spot plan had superior target dose homogeneities, better protection of OAR, and better plan robustness than the large-size spot plan. Compared to the small-size spot plan, the mixed-size spot plan had comparable plan quality, better plan robustness, and reduced plan delivery time from 65.9 to 40.0 s. Significance. The compact LMA-SC gantry beamline is proposed with mixed-size spot scanning, with demonstrated footprint reduction and improved plan quality compared to the conventional spot scanning method.

Physical dose validation of dynamic treatment for Gamma Knife radiosurgery.

Dynamic treatment in Gamma Knife (GK) radiosurgery systems delivers radiation continuously with couch movement, as opposed to stationary step-and-shoot treatment where radiation is paused when moving between isocenters. Previous studies have shown the potential for dynamic GK treatment to give faster treatment times and improved dose conformity and homogeneity. However, these studies focused only on computational simulations and lack physicalvalidation. This study aims conduct dynamic treatment dosimetric validation with physical experimental measurements. The experiments aim to (1) address assumptions made with computational studies, such as the validity of treating a continuous path as discretised points, (2) investigate uncertainties in translating computed plans to actual treatment, and (3) determine ideal treatment planning parameters, such as interval distance for the path discretization, collimator change limitations, and minimum isocenter treatmenttimes. This study uses a GK ICON treatment delivery machine, and a motion phantom custom-made to attach to the machine's mask adapter and move in 1D superior-inferior motion. Phantom positioning is first verified through comparisons against couch motion and computed doses. For dynamic treatment experiments, the phantom is moved through a program that first reads the desired treatment plan isocenters' position, time, and collimator sizes, then carries out the motion continuously while the treatment machine delivers radiation. Measurements are done with increasing levels of complexity: varying speed, varying collimator sizes, varying both speed and collimator sizes, then extends the same measurements to simulated 2D motion by combining phantom and couch motion. Dose comparisons between phantom motion radiation measurements and either couch motion measurements or dose calculations are analyzed with 2 mm/2% and 1 mm/2% gamma indices, using both local and global gamma indexcalculations. Phantom positional experiments show a high accuracy, with global gamma indices for all dose comparisons 99%. Discretization level to approximate continuous path as discrete points show the good dose matches with dose calculations when using 1 and 2-mm gaps. Complex 1D motion, including varying speed, collimator sizes, or both, as well as 2D motion with the same complexities, all show good dose matches with dose calculations: the scores are 92.0% for the strictest 1 mm/2% local gamma index calculation, 99.8% for 2 mm/2% local gamma index, and 97.0% for all global gamma indices. Five simulated 2D treatments with optimized plans scored highly as well, with all gamma index scores 95.3% when compared to stationary treatment, and scores 97.9% when compared to plan calculateddose. Dynamic treatment computational studies are validated, with dynamic treatment shown to be physically feasible and deliverable with high accuracy. A 2-mm discretization level in treatment planning is proposed as the best option for shorter dose calculation times while maintaining dose accuracy. Our experimental method enables dynamic treatment measurements using the existing clinical workflow, which may be replicated in other centers, and future studies may include 2D or 3D motion experiments, or planning studies to further quantify potential indication-specificbenefits.

Determination of output factor for CyberKnife using scintillation dosimetry and deep learning

Objective. Small-field dosimetry is an ongoing challenge in radiotherapy quality assurance (QA) especially for radiosurgery systems such as CyberKnifeTM. The objective of this work is to demonstrate the use of a plastic scintillator imaged with a commercial camera to measure the output factor of a CyberKnife system. The output factor describes the dose on the central axis as a function of collimator size, and is a fundamental part of CyberKnife QA and integral to the data used in the treatment planning system. Approach. A self-contained device consisting of a solid plastic scintillator and a camera was build in a portable Pelicase. Photographs were analysed using classical methods and with convolutional neural networks (CNN) to predict beam parameters which were then compared to measurements. Main results. Initial results using classical image processing to determine standard QA parameters such as percentage depth dose (PDD) were unsuccessful, with 34% of points failing to meet the Gamma criterion (which measures the distance between corresponding points and the relative difference in dose) of 2 mm/2%. However, when images were processed using a CNN trained on simulated data and a green scintillator sheet, 92% of PDD curves agreed with measurements with a microdiamond detector to within 2 mm/2% and 78% to 1%/1 mm. The mean difference between the output factors measured using this system and a microdiamond detector was 1.1%. Confidence in the results was enhanced by using the algorithm to predict the known collimator sizes from the photographs which it was able to do with an accuracy of less than 1 mm. Significance. With refinement, a full output factor curve could be measured in less than an hour, offering a new approach for rapid, convenient small-field dosimetry.

Experimental Investigation of X-Ray Radiation Shielding and Radiological Properties for Various Natural Composites.

Shielding from radiation and plan dose verification is vital during the potential applications in industrial and medical applications. A number of natural composites have been investigated for protecting against high-energy X-ray shielding. The aim is to learn about how natural composites behave under various X-ray energies at STP. The radiological parameters of wood samples were determined using computed tomography imaging, specifically relative electron density (RED), Hounsfield units (HUs), and mass density (MD). Percentage attenuation was measured using a semiflux ionization chamber incorporated with a brass build-up cap and an ionization chamber placed at the beam Isocenter for a different type of natural composite. Measurements are being carried out on a Linear accelerator at an SSD of 110 cm with different collimator sizes. Measured values of HUs, RED, and MD were -232 ± 40, 0.738 ± 0.039, 0.768 ± 0.024 g/cc,-368 ± 41, 0.662 ± 0.047, 0.632 ± 0.024 g/cc, -334 ± 44, 0.639 ± 0.042, 0.666 ± 0.026 g/cc, -370±61, 0.604±0.059, 0.63± 0.036 g/cc, -433±39, 0.543±0.038, 0.608 ± 0.035 g/cc, -382±54, 0.5±0.052, 0.618 ± 0.0316 g/cc, -292±68, 0.680±0.066, 0.708 ± 0.039 g/cc, -298±27, 0.680±0.0229, 0.702± 0.131 g/cc, for Acacia Nilotica, Mangifera Indica, Azadirachta Indica, Tectona Grandis L, Ficus Religiosa, Tecomella Undulata, Sesamum Indicum, Pinus respectively. Measurements show that attenuation is affected by the energy of incident photons, collimator opening, and the type of density of the wood. Various radiological parameters were determined for wood samples that can be utilized to create inhomogeneous phantoms in dosimetry. The largest attenuation is found in Acacia Nilotica and Sesamum Indicum, while the lowest attenuation is found in Ficus religiosa.

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.

Simulation of Leksell Gamma Knife-4C System with Different Phantoms Using PHITS and Geant4

This study used PHITS and Geant4 code packages to simulate a Leksell Gamma Knife system in order to determine radiation dose distribution in two types of phantoms. The results observed in the water phantom with configurations of single source and 201 sources are in good accord with the prior research, including both simulation and experiment. Several characteristics of Leksell Gamma Knife 4C, such as dose profiles, output factor, FWHM, and penumbra size, are calculated based on Monte Carlo simulations, which show the best consistency with other results. The output factors for collimators of 14 mm, 8 mm, and 4 mm are 0.984, 0.949, and 0.872, respectively. The simulation results with an adult mesh-type reference phantom reveal considerable similarities with the established radiosurgery plans. It indicates that the absorbed dose in brain tumors was highest when utilizing the 18 mm collimator and subsequently reduced with collimator size to 0.65, 0.25, and 0.5 with the 14 mm, 8 mm, and 4 mm collimators, respectively. The absorbed dose has a very low value for other essential organs and decreases with distance from the brain tumor. These findings may explain why the dose to organs decreases linearly as target distance, volume, and collimator size increase.

A Multiobjective Approach for Sector Duration Optimization in Stereotactic Radiosurgery Treatment Planning

Sector duration optimization (SDO) is a problem arising in treatment planning for stereotactic radiosurgery on Gamma Knife. Given a set of isocenter locations, SDO aims to select collimator size configurations and irradiation times thereof such that target tissues receive prescribed doses in a reasonable amount of treatment time and healthy tissues nearby are spared. We present a multiobjective linear programming model for SDO to generate a diverse collection of solutions so that clinicians can select the most appropriate treatment. We develop a generic two-phase solution strategy based on the ε-constraint method for solving multiobjective optimization models, 2phasε, which aims to systematically increase the number of high-quality solutions obtained, instead of conducting a traditional uniform search. To improve solution quality further and to accelerate the procedure, we incorporate some general and problem-specific enhancements. Moreover, we propose an alternative version of 2phasε, which makes use of machine learning tools to reduce the computational effort. In our computational study on eight previously treated real test cases, a significant portion of 2phasε solutions outperformed clinical results and those from a single-objective model from the literature. In addition to significant benefits of the algorithmic enhancements, our experiments illustrate the usefulness of machine learning strategies to reduce the overall run times nearly by half while maintaining or besting the clinical practice. History: Accepted by Paul Brooks, Area Editor for Applications in Biology, Medicine, and Healthcare. Funding: This work was supported in part by the Natural Sciences and Engineering Research Council of Canada [Discovery Grant RGPIN-2019-05588]. Supplemental Material: The software that supports the findings of this study is available within the paper and its Supplementary Information [ https://pubsonline.informs.org/doi/suppl/10.1287/ijoc.2022.1252 ] or is available from the IJOC GitHub software repository ( https://github.com/INFORMSJoC ) at [ http://dx.doi.org/10.5281/zenodo.7048848 ].

Do radiographers collimate? A retrospective analysis of radiographic collimation of common musculoskeletal examinations at an adult trauma centre.

Collimation of the primary beam is an important factor in radiography to reduce dose and improve image quality. The introduction of larger detector plates in direct digital radiography (DR) allows the exposed area to be calculated by removing cropping applied to the image. The aim of this study was to assess whether the exposed area was larger than a reference standard across five different projections on different body types, with the reference size being the corresponding cassette size used in traditional film/screen or computed radiography (CR). A retrospective clinical audit of five common musculoskeletal radiographic projections (AP knee, AP shoulder, horizontal beam lateral hip, lateral cervical spine and lateral facial bones), of 359 patients was undertaken. The electronic cropping was removed from projections, and the superior-inferior, antero-posterior and medio-lateral collimation size was measured, depending on the projection. The two measurements were multiplied to give an exposed field of view area. The three measurements were compared with a reference standard, being the size of the corresponding cassette size used in the department on film/screen or computed radiography. From the five projections, 1071 measurements were analysed. 416 (38.8%) of these measurements were less than or equal to the agreed reference standard. 655 (61.2%) were greater than the agreed reference standard. The study demonstrates that the majority (61.2%) of the measurements taken were above the reference standard. This results in an increase in radiation dose to patients and detrimental impacts on image quality.

Ten-year results of hypofractionated whole breast radiotherapy and intraoperative electron boost in premenopausal women

AimTo evaluate outcome of intraoperative electron boost (IOERT) and hypofractionated whole breast irradiation (HWBI) for breast cancer (BC) in young women. Methods and MaterialsWomen aged ≤ 48 with pT1-2 N0-1 BC received 12 Gy IOERT boost during conservative surgery followed by 3-dimensional conformal HWBI in 13 fractions (2.85 Gy/die). Local relapses (LR) and survival (disease-free, DFS; specific, BCSS; overall, OS) were analyzed. Results481 consecutive BC patients, mostly node negative, with median age of 42 were treated between 2004 and 2014. Median tumor size was 1.48 cm and median IOERT collimator was 4 cm. After 25-day mean interval, HWBI was delivered. At a median follow-up of 9.6 years, there were 23 LRs (4.8 %, 9 of which were in the boost region). Ten-year LR cumulative incidence was 4.1 % (95 %CI, 2.5–6.3). Over time, local control rate decreased for Luminal A and HER2 positive with negative hormonal receptors, while remained steady for triple negative. At multivariate analysis, LR predictors included age < 40, extensive intraductal component and the use of 4-cm IOERT collimator size. Ten-year survival outcomes were as follows: DFS 80.0 % (95 % CI, 75.8–83.5), BCSS 97.5 % (95 % CI, 95.5–98.6 %), OS 96.5 % (95 % CI, 94.3–97.9). Luminal B HER2 negative had the worse survival outcomes. Perioperative complications were uncommon (16.4 %), acute toxicity was mild (<2% Grade 3), but moderate/severe fibrosis was described in 40.8 % of the cases. Cosmesis was scored as excellent/good in 86 % of the cases. ConclusionsELIOT boost and HWBI achieved an excellent local control at the cost of tumor bed fibrosis. IOERT boost dose lower than 12 Gy is advisable.

Optimizing the Small Animal Radiation Research Platform (SARRP) for High-Dose Rate Focal Irradiation Studies

<h3>Purpose/Objective(s)</h3> The objective of the study is to enable high-dose rate focal irradiation in small animal models through modification of a commercial small animal radiation research platform (SARRP), which is widely used by investigators for preclinical research in radiation oncology. Small animal pre-clinical research is a fundamental part of clinical translation of novel treatment techniques. Currently, the SARRP system only enables dose rate of 3.5 Gy/min at the nominal isocenter. However, preclinical studies to investigate SBRT ablative models, such as those for the lung, liver, and more recently the heart, require much higher dose rates (ideally at least 10-20 Gy/min) and the current SARRP system setup requires long anesthesia time to deliver the required radiation dose to the animals. In addition, emerging applications such as lattice SBRT for larger tumors (which requires the delivery of a large grid of focal irradiation spots in a short time), is currently impossible with the current SARRP system. Here, we present our work to optimize the SARRP system using a modified beam collimation system to allow high-dose rate focal irradiation studies <i>in vivo</i>, while enabling a highly flexible set of collimator sizes and shapes to be used. <h3>Materials/Methods</h3> A commercial SARRP system with nominal source-to-axis distance (SAD) of 35 cm was studied in this work. In order to be able to perform experiments at a higher dose rate, a dedicated beam collimation apparatus was designed to provide a short source-to-surface distance (SSD) of 14 cm. Measurements were performed using an ionization chamber and radiochromic film dosimeter to measure the dose rate using the original beam collimation assembly at the isocenter (35 cm SSD), and using our dedicated beam collimation assembly at 14 cm SSD. Measurements were at the surface and at 2 cm depth in phantom with and without the x-ray filter in place. <h3>Results</h3> The SARRP system with standard collimation assembly provides a dose rate of 3.5 Gy/min at 2 cm depth in phantom at 35 cm SAD. The dose rate measured using our apparatus increased by a factor of 4.9, reaching 17 Gy/min, at 14 cm SSD and 2 cm depth in phantom. Removing the x-ray filter would additionally increase the dose rate by another factor of 6 at the surface and 1.7 at 2 cm depth in phantom. <h3>Conclusion</h3> The SARRP system with our modified collimation system would allow <i>in vivo</i> experiments with dose rates of 17-29 Gy/min at 14 cm SSD.

Comprehensive evaluation of dosimetric impact against position errors in accelerator-based BNCT under different treatment parameter settings.

BackgroundPatients who undergo accelerator‐based (AB) boron neutron capture therapy (BNCT) for head and neck cancer in the sitting position are generally uncomfortably immobilized, and patient motion during this treatment may be greater than that in other radiotherapy techniques. Furthermore, the treatment time of BNCT is relatively long (up to approximately 1 h), which increases the possibility of patient movement during treatment. As most BNCT irradiations are performed in a single fraction, the dosimetric error due to patient motion is of greater consequence and needs to be evaluated and accounted for. Several treatment parameters are required for BNCT dose calculation.PurposeTo investigate the dosimetric impacts (DIs) against position errors using a simple cylindrical phantom for an AB‐BNCT system under different treatment parameter settings.MethodsThe treatment plans were created in RayStation and the dose calculation was performed using the NeuCure® dose engine. A cylindrical phantom (16 cm diameter × 20 cm height) made of soft tissue was modeled. Dummy tumors in the form of a 3‐cm‐diameter sphere were arranged at depths of 2.5 and 6.5 cm (denoted by T 2.5 and T 6.5, respectively). Reference plans were created by setting the following parameters: collimator size = 10, 12, or 15 cm in diameter, collimator‐to‐surface distance (CSD) = 4.0 or 8.0 cm, tumor‐to‐blood ratio (T/B ratio) using 18F‐fluoro‐borono‐phenylalanine = 2.5 or 5.0, and 10B concentration in blood = 20, 25, or 30 ppm. The prescribed dose was D 95% ≥ 20 Gy‐eq for both T 2.5 and T 6.5. Based on the reference plans, phantom‐shifted plans were created in 26 directions [all combinations of left–right (LR), anterior–posterior (AP), and superior–inferior (SI) directions) and three distances (1.0, 2.0, and 3.0 cm). The DIs were evaluated at D 80% of the tumors. The shift direction dependency of the DI in the LR, AP, and SI directions was evaluated by conducting a multiple regression analysis (MRA) and other analyses where required.ResultsThe coefficients of the MRA of the DIs for LR, AP, and SI shifts were −0.08, 2.16, and −0.04 (p‐values = 0.084, <0.01, and 0.334) for T 2.5 and −0.05, 2.08, and 0.15 (p‐values = 0.526, <0.01, and 0.065) for T 6.5, respectively. The analysis of variance showed that DIs due to the AP shift were significantly greater for smaller collimator sizes on T 2.5 and smaller CSD on T 6.5. Dose reduction due to SI or LR (lateral) shifts was significantly greater for smaller collimator sizes on both T 2.5 and T 6.5 and smaller CSD on T2.5, according to the Student's t‐test. There were no significant differences in the DIs against both the AP shift and the lateral shift between the different T/B ratios and 10B concentrations.ConclusionThe DIs were largely affected by the shift in the AP direction and were influenced by the different treatment parameters.

Initial Beam Characteristics Calculations of 6 MV FF X-Ray Beam from Elekta Versa-HD Linac Using Topas MC Simulation Code

The focus of this research was to calculate initial beam characteristics of a 6 MV X-rays with flattening filter (FF) from Elekta Versa-HD linear accelerator (Linac) using the Tool for Particle Simulation (TOPAS) Monte Carlo (MC) simulation code. TOPAS 3.6 simulated data was compared with the measured data. During simulation, all the main component were simulated including Linac head, Multi-Leaf Collimators (MLCs), and uniform water phantom, while the SNC 3D scanner phantom and SNC125c Chamber were used for measurements. At collimator sizes of 5x5, 10x10, and 20x20 cm2, the depth dose scans (PDD) distribution and beam profiles (BP) were measured. The estimated tissue phantom ratio (TPR20,10) was 0.669, which was -1.1 percent lower than the measured TPR20,10 of 0.681. The obtained results show that the simulated and measured data were in good agreement, with gamma rate analysis of 3%/3 mm criteria reaching 95% and 99 % for PDD and BP respectively.

Analysis of Radiation Dose Distribution Inhomogenity Effects in Gamma Knife Radiosurgery using Geant4

The Monte Carlo method is widely used in the Gamma Knife dose distribution calculations. In this study, Monte-Carlo simulation with Geant4 was applied to determine Leksell Gamma Knife dose distribution for homogeneous solid water and heterogeneous brain phantoms, and primarily focused on the differences arising from the effects of inhomogeneity. The relative dose graphs were displayed for different collimator diameters. The dose values from the maximal interval regions were determined and compared. It seen that when the collimator size was 4mm, the mean maximum relative dose point was at 47mm for water phantom and at 44mm for the brain. The difference found was about 6.38%. The differences among the results determined for water and brain were ranged from 2 to 17%. As a result, the inhomogeneity effect should be considered in dose calculation in Gamma Knife planning system.

The dosimetric enhancement of GRID profiles using an external collimator in pencil beam scanning proton therapy.

The radiobiological benefits afforded by spatially fractionated (GRID) radiation therapy pairs well with the dosimetric advantages of proton therapy. Inspired by the emergence of energy-layer specific collimators in pencil beam scanning (PBS), this work investigates how the spot spacing and collimation can be optimized to maximize the therapeutic gains of a GRID treatment while demonstrating the integration of a dynamic collimation system (DCS) within a commercial beamline to deliver GRID treatments and experimentally benchmark Monte Carlo calculation methods. GRID profiles were experimentally benchmarked using a clinical DCS prototype that was mounted to the nozzle of the IBA-dedicated nozzle system. Integral depth dose (IDD) curves and lateral profiles were measured for uncollimated and GRID-collimated beamlets. A library of collimated GRID dose distributions were simulated by placing beamlets within a specified uniform grid and weighting the beamlets to achieve a volume-averaged tumor cell survival equivalent to an open field delivery. The healthy tissue sparing afforded by the GRID distribution was then estimated across a range of spot spacings and collimation widths, which were later optimized based on the radiosensitivity of the tumor cell line and the nominal spot size of the PBS system. This was accomplished by using validated models of the IBA universal and dedicated nozzles. Excellent agreement was observed between the measured and simulated profiles. The IDDs matched above 98.7% when analyzed using a 1%/1-mm gamma criterion with some minor deviation observed near the Bragg peak for higher beamlet energies. Lateral profile distributions predicted using Monte Carlo methods agreed well with the measured profiles; a gamma passing rate of 95% or higher was observed for all in-depth profiles examined using a 3%/2-mm criteria. Additional collimation was shown to improve PBS GRID treatments by sharpening the lateral penumbra of the beamlets but creates a trade-off between enhancing the valley-to-peak ratio of the GRID delivery and the dose-volume effect. The optimal collimation width and spot spacing changed as a function of the tumor cell radiosensitivity, dose, and spot size. In general, a spot spacing below 2.0 cm with a collimation less than 1.0 cm provided a superior dose distribution among the specific cases studied. The ability to customize a GRID dose distribution using different collimation sizes and spot spacings is a useful advantage, especially to maximize the overall therapeutic benefit. In this regard, the capabilities of the DCS, and perhaps alternative dynamic collimators, can be used to enhance GRID treatments. Physical dose models calculated using Monte Carlo methods were experimentally benchmarked in water and were found to accurately predict the respective dose distributions of uncollimated and DCS-collimated GRIDprofiles.

Impact of Irradiation Field Aperture Controller Settings on Patient-specific Quality Assurance

The aperture shape controller (ASC) is a tool on a radiation therapy planning system to reduce complexity by increasing the aperture size of multileaf collimator (MLC). The purposes of this study were to clarify the effect of the dose index on the treatment plan when the intensity of ASC is changed and then to clarify the effect on the verification result in the individual patient-specific quality assurance (QA) using the verification phantom. For four types of mock structures presented at AAPM TG-119, volumetric modulated arc therapy (VMAT) treatment plans with three dose levels were set without using the ASC for each of these four types. ASC settings were changed to very low, low, moderate, high and very high for the treatment plan, and the treatment plan was recreated without changing the planning target volume (PTV) and/or OAR dose constraints. The dose index of the treatment plan was then evaluated. The plan was also transferred by the true composite method to the assessment phantom, and patient-specific QA was evaluated. The frequency of dose constraint deviation by changing the ASC was 3.6% (2/55). In patient-specific QA, when the ASC setting was set to moderate, the γ analysis passing rate increased by 1.5% compared to the case without the ASC. In the treatment plan with a γ analysis pass rate of 95% or less without ASC, a 2.7% increase in the γ analysis pass rate was observed by setting the ASC to moderate. We found that setting the ASC tended to improve patient-specific QA. Compared to when the ASC was not set, the evaluation result of patient-specific QA was improved without violating the dose constraints of the PTV and/or organ at risk (OAR). In particular, the improvement was notable for the case where the evaluation result of patient-specific QA was 95% or less when the ASC was not used.

The current status and shortcomings of stereotactic radiosurgery.

BackgroundStereotactic radiosurgery (SRS) is a common treatment for intracranial lesions. This work explores the state of SRS treatment delivery to characterize current treatment accuracy based on treatment parameters.MethodsNCI clinical trials involving SRS rely on an end-to-end treatment delivery on a patient surrogate (credentialing phantom) from the Imaging and Radiation Oncology Core (IROC) to test their treatment accuracy. The results of 1072 SRS phantom irradiations between 2012 and 2020 were retrospectively analyzed. Univariate analysis and random forest models were used to associate irradiation conditions with phantom performance. The following categories were evaluated in terms of how they predicted outcomes: year of irradiation, TPS algorithm, machine model, energy, and delivered field size.ResultsOverall, only 84.6% of irradiations have met the IROC/NCI acceptability criteria. Pass rate has remained constant over time, while dose calculation accuracy has slightly improved. Dose calculation algorithm (P < .001), collimator (P = .024), and field size (P < .001) were statistically significant predictors of pass/fail. Specifically, pencil beam algorithms and cone collimators were more likely to be associated with failing phantom results. Random forest modeling identified the size of the field as the most important factor for passing or failing followed by algorithm.ConclusionConstant throughout this retrospective study, approximately 15% of institutions fail to meet IROC/NCI standards for SRS treatment. In current clinical practice, this is particularly associated with smaller fields that yielded less accurate results. There is ongoing need to improve small field dosimetry, beam modeling, and QA to ensure high treatment quality, patient safety, and optimal clinical trials.

Development of dosimetric procedures for experimental ultra-high dose rate irradiation at a clinical linear accelerator

As radiotherapy using ultra-high dose rates has gained new interest, the dosimetric challenges arising at these conditions needs to be addressed. Ionization chambers suffer from a large decrease in ion collection efficiency due to ion recombination, making on-line dosimetry difficult. In this work we present experimental setups and dosimetric procedures for FLASH irradiation of cells, zebrafish embryos and small animals using a 10 MeV electron beam at a modified clinical linear accelerator, and describe the dosimetric steps required to initiate clinical trials. The dosimetric equipment used for our pre-clinical experiments consisted of radiochromic film, thermoluminescent dosimeters, a Farmer-type ionization chamber and phantom material mimicking the experimental setup for irradiation. In preparation for small animal irradiation, dose profiles and depth dose curves were measured for all collimator sizes. The average dose rates were ≥620 Gy/s, ≥640 Gy/s and ≥400 Gy/s for cells, zebrafish embryos and small animals, respectively.

Dose kernel decomposition for spot-based radiotherapy treatment planning.

Pre-calculation of accurate dose deposition kernels for treatment planning of spot-based radiotherapies, such as Gamma Knife (GK) and Gamma Pod (GP), can be very time-consuming and may require large data storage with an enormous number of possible spots. We proposed a novel kernel decomposition (KD) model to address accurate and fast (real-time) dose calculation with reduced data storage requirements for spot-based treatment planning. The application of the KD model was demonstrated for clinical GK and GP radiotherapy platforms. The dose deposition kernel at each spot (shot position) is modeled as the product of a shift-invariant kernel based on a reference kernel and spatially variant scale factor. The reference kernel, one for each collimator, is defined at the center of the commissioning phantom for GK and at the center of the treatment target for GP and calculated using the Monte Carlo (MC) method. The spatially variant scale factor is defined as the ratio of the mean tissue maximum ratio (TMR) at the candidate shot position to that at the reference kernel position, and the mean TMR map is calculated within the entire volume through parallel beam ray tracing on the density image followed by averaging over all source directions. The proposed KD dose calculations were compared with the MC method and with the GK and GP treatment planning system (TPS) computations for various shot positions and collimator sizes utilizing a phantom and 14 and 12 clinical plans for GK and GP, respectively. For the phantom study, the KD Gamma index (3%/1mm) passing rates were greater than 99% (median 100%) relative to the MC doses, except for the shots close to the boundary. The passing rates dropped below 90% for 8mm (16mm) shots positioned within ∼1cm (∼2cm) of the boundary. For the clinical GK plans, the KD Gamma passing rates were greater than 99% (median 100%) compared to the MC and greater than 92% (median 99%) compared to the TPS. For the clinical GP plans, the KD Gamma passing rates were greater than 95% (median 98%) compared to the MC and greater than 91% (median 97%) compared to the TPS. The scale factors were calculated in sub-seconds with GPU implementation and only need to be calculated once before treatment plan optimization. The calculation of the dose kernel was also within sub-seconds without requiring beam-by-beam calculation commonly done in the TPS. The proposed model can provide an accurate dose and enables real-time dose and derivative calculations by kernel shifting and scaling without pre-calculating or requiring large data storage for GK and GP dose deposition kernels during treatment planning. This model could be useful for spot-based radiotherapy treatment planning by allowing an efficient global fine search for optimal spots.