Variable Collimator Research Articles

Low-loss tunable beam collimator and expander assembly with no moving parts using anengineered diffuser and varifocal lenses.

In this paper, we present a novel design for a tunable beam collimator. A variable collimator assists in achieving an adaptive size of an output collimated beam. Alternatively, it can also provide an adjustable output beam divergence angle for a noncollimated beam output. Tunable collimators are highly desirable for various applications in testing, engineering, and measurements. Such devices are also useful in providing tunable illumination of samples or targets in microscopes and emulating different target distances for characterizing the performance of camera systems in laboratory settings. The proposed collimator has two distinct advantages: it is light-efficient compared with pinhole-based collimator designs, and it delivers a large range of output beam sizes without involving the mechanical motion of bulk components. These attributes are achieved via the use of an engineered diffuser (in the place of a pinhole) and a pair of large aperture tunable focus lenses, which deliver a tunable magnification to the output collimated beam. In laboratory experiments, we achieve an optical transmission efficiency of 90% for the proposed tunable collimator.

A novel concept of adjustable three-layer collimator in a pixelated photon-counting detector for gamma-ray imaging: Monte Carlo simulation study

In gamma-ray imaging, a collimator plays a crucial role in determining the sensitivity of the system and the spatial resolution of the acquired image. Herein, we presented a novel concept for a variable collimator in a pixelated photon-counting detector. The gamma-ray imaging system was modeled using the Monte Carlo simulation tool Geant4 Application for Tomographic Emission program. A detector using cadmium telluride and a 99mTc point source was used. The collimators comprised three tungsten layers, and the septal height the beam finally penetrated from the incident gamma ray was adjusted differently. The first collimator layer constituted a hole with four times the pixel size, and the second and third layers comprised a hole with twice the pixel size, overlapping asymmetrically. Finally, the emitted gamma ray equaled the pixel. The spatial resolution showed different results depending on the source-to-collimator distance for each collimator configuration. Additionally, we determined that the sensitivity improved as the collimator height where gamma rays are incident and initially interact increased. We have proposed a novel collimator concept in a gamma-ray imaging system using a pixelated photon-counting detector, which may be applied in various fields in the future.

Dose Rate Control of FLASH Proton Beam for Animal Testing: Monte Carlo Simulation

<h3>Purpose/Objective(s)</h3> Compared to conventional proton therapy (PT), ultra-high dose rate (FLASH) PT can reduce the toxicity to normal tissues while maintaining tumor control. The biological effect of FLASH beam will depend on its dose rate. In this study, we proposed a method that can freely change the dose rate of FLASH proton beam and deliver its uniform dose to mice for animal testing. <h3>Materials/Methods</h3> A FLASH proton beam nozzle was built in our proton therapy system for animal experiments. Ultra-high dose rate higher than 200 Gy/s can be obtained at the plateau region in the pristine Bragg peak of 230 MeV, 300 nA proton beam. The nozzle consisted of two main parts: a graphite scatterer that provides a uniform dose distribution from a Gaussian proton beam with ultra-high dose rate and a lead scatterer that can control the dose rate of the FLASH proton beam. A variable collimator system and an animal irradiation stage were added for the study of FLASH effects. The graphite scatterer was designed to obtain a uniform dose distribution with a size of 2 cm without reducing the dose rate significantly. It had a Gaussian shape and was installed downstream of a window extracting proton beam into the air. The lead scatterer was designed to control the dose rate of proton beam by changing its thickness and position. Its position was adjustable from 10 to 50 cm from the isocenter. It had a flat plate shape and was installed at the end of the proton beam nozzle. The graphite scatterer is slidable and the lead scatterer is detachable not to interfere with the existing nozzles. The dose rates and lateral dose profiles were calculated using a simulation toolkit. <h3>Results</h3> A FLASH proton beam nozzle was designed to control the dose rate under the ultra-high dose rate condition and to partially irradiate a mouse with uniform dose. Dose rates exceeding 40 Gy/s for 230 MeV proton beam were obtained at the plateau region in the pristine Bragg peak. The dose rate could be changed from 100 to 44 Gy/s, and the field size with ±10% dose uniformity was about 2 cm. <h3>Conclusion</h3> Our FLASH proton beam nozzle can conduct animal testing on the flash effect without interfering with existing proton beam nozzle. In addition, various studies of the FLASH effect depending on dose rate became possible. This approach designing a FLASH proton beam nozzle may be applicable to other proton therapy facilities. In the near future, animal experiments to demonstrate the FLASH effect will follow in our institute.

Optimization of each layer's hole of a variable collimator for environmental radiation monitoring using GATE

The purpose of this work is to optimize the number of holes in each layer with the Monte Carlo simulation tool, GATE, a variable collimator of the non-replaceable types that makes up for pinhole collimator's shortcomings. The material is tungsten and the detector size is 25.8 × 25.8 × 3.0 mm3 GAGG with 0.7 × 0.7 × 3.0 mm3 pixels arranged in 29 × 29 × 1 and performance is evaluated based on the field-of-view, sensitivity, and spatial resolution by magnification for the point source 137Cs. Simulations were designed to optimize the five layers from 44, 55, 11, 55, 44 to 6, 9, 3, 9, and 6 to meet the feasibility of manufacturing collimators. We also confirmed performance errors of 5.7% sensitivity and 5.2% spatial resolution compared to using an ideal variable collimator for all parameters, such as specific purposes and magnification scale. Through these results, we propose an environmental monitoring system that can be used for diverse radiation disasters by enabling image magnification and transforming the shape of the collimator.

Treatment plan prediction for lung IMRT using deep learning based fluence map generation

PurposeRecently, it has been shown that automated treatment planning can be executed by direct fluence prediction from patient anatomy using convolutional neural networks. Proof of principle publications utilise a fixed dose prescription and fixed collimator (0°) and gantry angles. The goal of this work is to further develop these principles for the challenging lung cancer indication with variable dose prescriptions, collimator and gantry angles. First we investigate the impact of clinical applicable collimator angles and various input parameters. Then, the model is tested in a complete user independent planning workflow. MethodsThe dataset consists of 152 lung cancer patients, previously treated with IMRT. The patients are treated with either a left or a right beam setup and collimator angles and dose prescriptions adjusted to their tumour shape and stage. First we compare two CNNs with standard vs. personalised, clinical collimator angles. Next, four CNNs are trained with various combinations of CT and contour inputs. Finally, a complete user free treatment planning workflow is evaluated. ResultsThe difference between the predicted and ground truth fluence maps for the fluence prediction CNN with all anatomical inputs in terms of the mean mean absolute error (MAE) is 4.17 × 10−4 for a fixed collimator angle and 5.46 × 10−4 for variable collimator angles. These differences vanish in terms of DVH metrics. Furthermore, the impact of anatomical inputs is small. The mean MAE is 5.88 × 10−4 if no anatomical information is given to the network. The DVH differences increase when a total user free planning workflow is examined. ConclusionsFluence prediction with personalised collimator angles performs as good as fluence prediction with a standard collimator angle of zero degrees. The impact of anatomical inputs is small. The combination of a dose prediction and fluence prediction CNN deteriorates the fluence predictions. More investigation is required.

Optimization of Each Layer's Hole of a Variable Collimator for Environmental Radiation Monitoring Using Gate

Optimization of Each Layer's Hole of a Variable Collimator for Environmental Radiation Monitoring Using Gate

Efficacy of Frameless Radiosurgical Entorhino-Amygdalohippocampectomy for Pharmacoresistant Medial Temporal Lobe Epilepsy

<h3>Purpose/Objective(s)</h3> Evaluate the efficacy and safety of Linac based surgery in the treatment of pharmaco-resistant epilepsies of mesial temporal lobe origin due to medial temporal sclerosis. <h3>Materials/Methods</h3> Retrospective single institution study comprised of 12 MTLE pts. (10 pts. with MT Sclerosis on Dominant side) who underwent SRS between August 2015 to Jan 2019. The epileptogenic substrate was localized using invasive video-EEG monitoring with foramen ovale electrodes, magnetic resonance imaging, metabolic (PET) imaging. All patients underwent detailed neuropsychological testing and the ESI-55 quality-of-life questionnaire. The radiosurgical target included Anterior part of parahippocampal region; entorhinal area adjacent to the collateral sulcus & rhinal sulcus; hippocampus - head & Anterior part and Amygdalofugal part of the amygdaloid complex. Non-coplanar Volumetrically modulated plans or non-isocentric CK plans using variable collimator were generated for prescribed mean dose of 22gy (range: 20-24 Gy), without any dose heterogeneity. Normal structures doses were within acceptable limits. Radiosurgical treatment was performed by using the C-Arm Linac with hexapod robotics (9 pts.) or frameless robotic radiosurgery system with skull tracking (3pts.). A minimum 2-year follow-up period included clinical, neuropsychological, ophthalmic (Visual field charting) & radiologic evaluations. <h3>Results</h3> Median age at diagnosis is 28 yrs (Range: 17-42yrs). At each 6-month follow-up evaluation, the frequency of seizures was significantly smaller than that at the previous visit. The median seizure frequency of 10.25 the month before treatment was reduced to 0.33 at 2 years after treatment. At 2 years, 67% of the patients (8 of 12) were seizure free - 4 Engel class 1A, 3 Engel class 1B, 1 Engel class 2A responses. The median latent interval from SRS to seizure cessation was 9.5 months (range 6–20 months). One patient was immediately seizure free. The median latency in aura cessation was 14 months (range 9–22 months). Morphological changes, edema, on magnetic resonance imaging were visible at 10 months (median) after SRS (range 7–22 months) in nearly all patients. During the onset period of these radiological changes, three patients experienced headache associated, in two cases, with nausea and vomiting. In these three patients the signs resolved immediately after short course of low doses of steroids. No cases of permanent neurological deficit (except three cases of monosymptomatic visual field deficit), or morbidity, or mortality were observed <h3>Conclusion</h3> Initial experience of Frameless SRS, as non-invasive technique, for MTLE provides benefits especially in young patients requiring high social skills with dominant lobe involvement in the traditional realms of safety with short- to middle-term efficacy & superior neuropsychological outcomes, similar to published series in GKS studies but with appreciated realms of patient acceptance, comfort and costs. Further long-term follow up is required.

P13.21 PET-based dose painting radiation therapy strategy in a glioblastoma rat model using the small animal radiation research platform

Abstract BACKGROUND Previously, a rat glioblastoma model to mimic chemo-radiation treatment of human glioblastoma in the clinic was established. Similarly to the clinic, CT and MRI were combined during the treatment planning process. PET imaging was subsequently added which allowed us to implement sub-volume boosting using a micro-irradiation system. However, combining three imaging modalities (CT, MRI and PET) using a micro-irradiation system, proved to be labour intensive because multimodal imaging, treatment planning and dose delivery have to be completed sequentially in the preclinical setting. MATERIAL AND METHODS Two different methodologies were compared in silico for performing preclinical [18F]FET PET based radiation therapy (20 Gy based on MRI, 8 Gy boost based on PET) based on three different cases. Method 1 is based on the previously published methods1,2. However, the process is automated using an in-house developed MATLAB code. Method 2 consists of a more sophisticated method where a series of isocenters and jaw dimensions for the motorised variable collimator were determined based on the [18F]FET PET uptake. Both methods were evaluated by means of the dose volume histograms (DVH) and Q-volume histograms. RESULTS The setup parameters for both methods were calculated. The DVHs for method 2 are systematically closer to the ideal dose distribution compared to method 1. These findings are confirmed by the D90 and D50 values which are considerably lower for method 1. When observing the Q-factor, method 2 always results in dose distributions that are closer to the dose objectives (method 1: 0.141±0.046; method 2: 0.064±0.011). CONCLUSION The described novel method to optimize the preclinical treatment planning process has many advantages in terms of dose delivery, time efficiency and variability, when compared to the previously used methods1,2. These improvements are important to narrow the gap between clinical and preclinical radiation research and for the development of new therapeutics and/or radiation therapy procedures for glioblastoma. 1. Bolcaen, J., Descamps, B., Boterberg, T., Vanhove, C. &amp; Goethals, I. PET and MRI Guided Irradiation of a Glioblastoma Rat Model Using a Micro-irradiator. J. Vis. Exp. 1–10 (2017) doi:10.3791/56601. 2. Verhoeven, J. et al. Technical feasibility of [18F]FET and [18F]FAZA PET guided radiotherapy in a F98 glioblastoma rat model. Radiat. Oncol. 14, (2019).

Optimization using Monte Carlo simulations of the thickness of a variable collimator for radiation monitoring

In this study, we used GATE v8.0 to optimize the design parameters for the layer thickness and the shape of holes in variable collimators with new methods for moving detectors and acquiring magnified images in real time. The optimized thickness was a combination of five layers with thicknesses of 6.0, 2.0, 4.0, 2.0, and 6.0 mm. Image magnification on variable and conventional pinhole collimators, designed with the same parameters, confirmed that the greater the magnification is, the more distinct the two sources are. The increase in the penetration and the scatter ratio caused by empty areas created during the process of implementing the pinhole form into layers resulted in an average sensitivity error of 3.95%, which reduced spatial resolution slightly.

Gamma Tocotrienol Protects Mice From Targeted Thoracic Radiation Injury.

Radiation injury will result in multiorgan dysfuntion leading to multiorgan failure. In addition to many factors such as radiation dose, dose rate, the severity of the injury will also depend on organ systems which are exposed. Here, we report the protective property of gamma tocotrienol (GT3) in total as well as partial body irradiation (PBI) model in C3H/HeN male mice. We have carried out PBI by targeting thoracic region (lung-PBI) using Small Animal Radiation Research Platform, an X-ray irradiator with capabilities of an image guided irradiation with a variable collimator with minimized exposure to non-targeted tissues and organs. Precise and accurate irradiation of lungs was carried out at either 14 or 16 Gy at an approximate dose rate of 2.6 Gy/min. Though a low throughput model, it is amenable to change the field size on the spot. No damage to other non-targeted organs was observed in histopathological evaluation. There was no significant change in peripheral blood counts of irradiated mice in comparison to naïve mice. Femoral bone marrow cells had no damage in irradiated mice. As expected, damage to the targeted tissue was observed in the histopathological evaluation and non-targeted tissue was found normal. Regeneration and increase of cellularity and megakaryocytes on GT3 treatment was compared to significant loss of cellularity in saline group. Peak alveolitis was observed on day 14 post-PBI and protection from alveolitis by GT3 was noted. In irradiated lung tissue, thirty proteins were found to be differentially expressed but modulated by GT3 to reverse the effects of irradiation. We propose that possible mode of action of GT3 could be Angiopoietin 2-Tie2 pathway leading to AKT/ERK pathways resulting in disruption in cell survival/angiogenesis.

82. Variable tilt-angle, parallel-hole collimation system for molecular imaging gamma tomosynthesis

Purpose Nuclear medicine imaging based on commonly used gamma cameras, which are large, bulky and designed for general-purpose imaging, is limited in detecting small lesions often indicative of early stage diseases. To overcome this limitation, the authors propose a gamma tomosynthesis method based on variable tilt-angle collimation system that, complemented by a conventional gamma camera, is able to perform high-resolution three-dimensional imaging. Methods The variable tilt-angle collimator allows to acquire a set of planar images at different angles, which are then 3D reconstructed through the Shift And Add (SAA) method [1] . Spatial resolutions were measured in reconstructed images using a point source at different source-to-collimator distances; sensitivity was evaluated over the range of slant angles (from −45°to 45°) using a disk source. Image contrast (IC) and contrast to noise ratio (CNR) of sub-centimeters tumours were evaluated using a breast phantom. Furthermore, the capability of the tomosynthesis technique in preserving axial resolution was tested by using the Mini Defrise phantom. Gamma tomosynthesis images were compared with those obtained with a standard circular-orbit SPECT scan. Results The proposed system allows reaching spatial resolutions in the x-y plane ranging from 9 to 14 mm over a depth range of 6-10 cm; spatial resolution in the depth dimension becomes two times greater than those in the other two dimensions. The measured sensitivity decreased from 9 cps/ μ Ci to 6 cps/ μ Ci varying the slant angle from 5° to 45degr . The IC and CNR resulted from gamma tomosynthesis images demonstrated a significant improvement compared to SPECT. Conclusions The proposed gamma tomosynthesis demonstrated the potential for superior lesion detection in nuclear medicine imaging. Differently from the currently used SPECT, a conventional gamma camera equipped with the variable tilt-angle collimator could be located at the minimum distance from the patient, thus improving detection, localisation and characterisation of sub-centimetre lesions.

25. CyberKnife SBRT using a new MLC: Advantages, limitations and new treatment opportunities

Purpose The CyberKnife (CK) M6 series is equipped with a MLC intended to achieve higher delivery efficiency than the circular IRIS variable collimator and to expand the feasibility of CK treatments to larger tumors. This study aimed to evaluate the CK-MLC performance for SBRT plans focusing on (i) comparison with IRIS (ii) large volume tumor treatments (iii) dosimetric delivery accuracy. Methods (i) IRIS and MLC plans in 3–5 fractions were created for 30 liver, 15 pancreas and 10 prostate cases and compared in terms of coverage, conformity (nCI), dose gradient (R50), homogeneity (HI), OAR doses, PTV gEUD, MU normalized to prescription dose (MU/PD), treatment time both estimated by TPS (tTPS) and measured (tmeas). Prostate plans MU/PD and tTPS were compared with similar published data [1] , [2] . (ii) SBRT MLC plans (30 Gy/5 fractions) were optimized for large PTVs (240 cm3 ÷ 1140 cm3, diameters 8 cm ÷ 14 cm) in one pancreas and three liver cases. Plan quality, treatment time and feasibility to meet OAR constraints were evaluated. (iii) Delivery QA was performed for 50 MLC plans using different techniques. Results (i) Compared to IRIS ( Table 1 ), MLC achieved equivalent coverage, conformity, homogeneity and EUD without compromising OAR sparing and improving (p Conclusions MLC improves dose fall-off and delivery efficiency compared to IRIS. Time reduction is partially lost when measured and is highly variable among different studies. MLC permits to extend CK SBRT treatments to larger PTVs maintaining high dosimetric accuracy.

Preclinical Modeling of Image-Guided Craniospinal Irradiation for Very-High-Risk Medulloblastoma

Craniospinal irradiation (CSI) is a crucial component of treatment for medulloblastoma (MB), a brain tumor clinically stratified into prognostically distinct molecular subgroups. Preclinical models of clinically relevant CSI offer the potential to study radiation dose and volume effects in these subgroups and to identify subgroup-specific combination adjuvant therapies, particularly for very-high-risk MB in which treatments are often unsuccessful. The commercially available Small Animal Radiation Research Platform equipped with a motorized variable collimator was used for image-guided CSI. Mice were implanted in brain cortices with patient-derived orthotopic xenografts (PDOXs) of very-high-risk Group 3 (G3) or Sonic Hedgehog (SHH) MB and were treated with fully fractionated CSI at 2Gy/fraction for a cumulative 36Gy. Radiation therapy dose response effects on tumor burden and overall survival were assessed. The pattern of treatment failure was determined using bioluminescence and then confirmed histologically. Acute toxicity was appraised by body weight measurements and blood work. We established an accurate and efficient preclinical protocol to administer CSI reproducibly to mice harboring MB. CSI improved the survival of mice bearing very-high-risk G3 or SHH MB PDOXs. However, radiation therapy dose responses across models suggested significant radio-responsiveness to conventionally fractionated CSI ≥20Gy. CSI was well tolerated; mice had no significant changes in body weight, and acute leukopenia developed but resolved soon after therapy completion. Our protocol for preclinical CSI delivery was effective and well tolerated, and it can be readily integrated into preclinical pipelines for MB and other central nervous system-seeding tumors.

Monte Carlo simulation of accurate dose distribution with high resolution for the single source channel of Leksell 4C gamma knife

Objective To obtain the high-resolution dose distribution for the single source channel of Leksell 4C gamma knife. Methods A parallel computing platform based on the Message Passing Interface (MPI) and Monte Carlo Code MCNPX was established. The ring-shaped detector and two pre-validated variance reduction techniques (emission direction-biased sampling of source and termination of electron tracking in partial structures) were adopted to derive the high-resolution dose distribution for the single source channel of Leksell 4C gamma knife. The effect of cut-off energy for both photon and electron on the accuracy of simulation outcomes was evaluated and statistically compared. Results Compared with previous findings, the spatial resolution of the dose distribution for the single source channel obtained in this study was higher (radial resolution=0.1 mm) with less statistical error (<1%). The calculation time was acceptable (approximately 24 h). For the 4-, 8-, 14-and 18-mm variable collimators, the penumbra and full width at the half maximum (FWHM) for single side were 1.0, 1.1, 1.2, 1.3 mm and 2.2, 4.3, 7.3, 9.3 mm, respectively, which were consistent with previous studies. The difference of the simulation results was extremely small between different cut-off energy for photon (1 keV vs.10 keV). However, the simulation results significantly differ between 1 and 521 keV electronic cut-off energy. Conclusions The MCNPX parallel computing platform based on the MPI environment can be utilized to derive highly accurate dose distribution with high resolution in acceptable calculation time. The cut-off energy of the photon and electron should be cautiously set up during simulation. Key words: Gamma knife; Monte Carlo simulation; Leksell 4C; Single source channel

Clinical usefulness of MLCs in robotic radiosurgery systems for prostate SBRT.

Stereotactic body radiation therapy (SBRT) using recently introduced multileaf collimators (MLC) is preferred over circular collimators in the treatment of localized prostate cancer. The objective of this study was to assess the clinical usefulness of MLCs in prostate SBRT by comparing the effectiveness of treatment plans using fixed collimators, variable collimators, and MLCs and by ensuring delivery quality assurance (DQA) for each. For each patient who underwent conventional radiation therapy for localized prostate cancer, mock SBRT plans were created using a fixed collimator, a variable collimator, and an MLC. The total MUs, treatment times, and dose–volume histograms of the planning target volumes and organs at risk for each treatment plan were compared. For DQA, a phantom with a radiochromic film or an ionization chamber was irradiated in each plan. We performed gamma‐index analysis to evaluate the consistency between the measured and calculated doses. The MLC‐based plans had an ~27% lower average total MU than the plans involving other collimators. Moreover, the average estimated treatment time for the MLC plan was 31% and 20% shorter than that for the fixed and variable collimator plans respectively. The gamma‐index passing rate in the DQA using film measurements was slightly lower for the MLC than for the other collimators. The DQA results acquired using the ionization chamber showed that the discrepancies between the measured and calculated doses were within 3% in all cases. The results reinforce the usefulness of MLCs in robotic radiosurgery for prostrate SBRT treatment planning; most notably, the total MU and treatment time were both reduced compared to the cases using other types of collimators. Moreover, although the DQA results based on film dosimetry yielded a slightly lower gamma‐index passing rate for the MLC than for the other collimators, the MLC accuracy was determined to be sufficient for clinical use.

ANTARES: Cold neutron radiography and tomography facility

The neutron imaging facility ANTARES, operated by the Technische Universität München, is located at the cold neutron beam port SR-4a. Based on a pinhole camera principle with a variable collimator located close to the beam port, the facility provides the possibility for flexible use in high resolution and high flux imaging.

SU‐E‐T‐163: Characterization of a Novel High Resolution 1D Silicon Monolithic Array for Small Field Commissioning and Quality Assurance

Purpose:To study the suitability of a novel 1D silicon monolithic array for dosimetry of small radiation fields and for QA of high dose gradient treatment modalities (IMRT and SBRT).Methods:A 1D array composed of 4 monolithic silicon modules of 64 mm length and 1 mm pixel pitch was developed by IBA Dosimetry. Measurements were carried out for 6MV and 15MV photons on two commercial different linacs (TrueBeam and Clinac iX, Varian Medical Systems, Palo Alto, CA) and for a CyberKnife G4 (Accuray Inc., Sunnyvale, CA). The 1D array was used to measure output factors (OF), profiles and off axis correction factors (OACF) for the Iris CyberKnife variable collimator (5–60 mm). In addition, dose profiles (at the isocenter plane) were measured for multiple IMRT and SBRT treatment plans and compared with those obtained using EDR2radiographic film (Carestream Health, Rochester NY), a commercial 2D diode array and with the dose distribution calculated using a commercial TPS (Eclipse, Varian Medical Systems, Palo Alto, CA).Results:Due to the small pixel pitch of the detector, IMRT and SBRT plan profiles deviate from film measurements by less than 2%. Similarly, the 1D array exhibits better performance than the 2D diode array due to the larger (7 mm) pitch of that device. Iris collimator OFs measured using the 1D silicon array are in good agreement with the commissioning values obtained using a commercial stereotactic diode as well as with published data. Maximum deviations are &lt; 3% for the smallest field (5 and 7.5mm) and below 1% for all other dimensions.Conclusion:We have demonstrated good performances of the array for commissioning of small photon fields and in patient QA, compared with diodes and film typically used in these clinical applications. The technology compares favorably with existing commercial solutionsThe presenting author is founded by a Marie Curie Early Initial Training Network Fellowship of the European Communitys Seventh Framework Programme under contract number (PITN‐GA‐2011‐289198‐ARDENT).The research activity is hosted by IBA Dosimetry, Gmbh.

Dose Painting with a Variable Collimator for the Small Animal Radiation Research Platform (SARRP)

The goal of radiation treatment is to irradiate cancer cells (i.e., a target region) without destroying adjacent healthy tissue. Thus, it is advantageous to form the beam so that it best approximates the target, thereby reducing the amount of dose absorbed in critical regions outside the target area. While multi-leaf collimators are common in human clinical systems, small animal radiotherapy systems are typically limited to a set of fixed-size collimators. For these systems, dose painting can be used for conformal dose delivery, but is significantly slower than a multi-leaf collimator. As a compromise solution, a variable rectangular collimator has been developed for the Small Animal Radiation Research Platform (SARRP). This enables more efficient dose painting via the decomposition of a 2D target region into a minimum number of rectangles of variable size, which is the topic of this paper. The proposed method consists of several distinct steps and was implemented on the SARRP Treatment Planning System (TPS).

SU-E-T-59: Calculations of Collimator Scatter Factors (Sc) with and Without Custom-Made Build-Up Caps for CyberKnife

Purpose: To investigate the impact of custom-made build-up caps for a diode detector in robotic radiosurgery radiation fields with variable collimator (IRIS) for collimator scatter factor (Sc) calculation. Methods: An acrylic cap was custom-made to fit our SFD (IBA Dosimetry, Germany) diode detector. The cap has thickness of 5 cm, corresponding to a depth beyond electron contamination. IAEA phase space data was used for beam modeling and DOSRZnrc code was used to model the detector. The detector was positioned at 80 cm source-to-detector distance. Calculations were performed with the SFD, with and without the build-up cap, for clinical IRIS settings ranging from 7.5 to 60 mm. Results: The collimator scatter factors were calculated with and without 5 cm build-up cap. They were agreed within 3% difference except 15 mm cone. The Sc factor for 15 mm cone without buildup was 13.2% lower than that with buildup. Conclusion: Sc data is a critical component in advanced algorithms for treatment planning in order to calculate the dose accurately. After incorporating build-up cap, we discovered differences of up to 13.2 % in Sc factors in the SFD detector, when compared against in-air measurements without build-up caps.

SU‐E‐T‐282: Remove Field‐Size Limitation Using the Combination of Collimator Rotation and Jaw Tracking with MLC Aperture

Purpose:this study is to investigate the use of variable‐collimators and/or collimator‐rotation to overcome the field‐size limitation on TrueBeamSTX.Methods:The maximum static field‐size is 22‐ × 40‐cm for TrueBeamSTX and 40‐ × 40‐cm for Trilogy. Maximum displacement between adjacent leaf ends at a single carriage position is 15‐cm for TBSTX and 14‐cm for Trilogy. Our current practice for IMRT treatment at Trilogy is as follows, if MLC leaves travel more than 13.5 cm in the MLC traveling direction, the treatment fields will be split at isocenter to two groups of IMRT fields to cover the large tumors. However, TrueBeamSTX is not limited in this way because the jaws can track each segments of the MLC aperture. Radiation treatment plans for ten patients (different tumor sites) treated at Trilogy with one dimension of the field is greater than 22 cm and the other dimension is less than or equal to 22 cm. All the plans will be converted with jaws tracking each segment's MLC aperture. If necessary, the collimator rotation will also be applied for the planning. Comparison of the converted plans with the original clinical plans will be evaluated and based on the following criterion: (1) tumor coverage; (2) dose sparing to the critical structures; and (3) treatment time.Results:The tumor coverage is compatible between two plans. The dose of critical structures was evaluated in the cumulative DVH for spinal cord, parotids, and brainstem at V20 and V5. In all patients, the maximum reduction in V20 was never more than 5% and was typically less than 1%. In V5, the maximum reduction was 10% and 3% respectively. The significant difference can be found in using the variable collimator to shorten the treatment time.Conclusion:We conclude that we will use the variable‐collimator for all IMRT cases on TrueBeamSTX and as a potential backup machine for Trilogy.