Collimator Angle Research Articles

Estimation of Out-of-Field Dose Variation using Markus Ionization Chamber Detector

Objective: The aim of This work to provide evaluation for the out-of-field dose with different plan parameters as field size and depth using Markus ionization chamber detector in the measurement that are frequently used in electron and superficial dosimetery, in radiotherapy. Methods: This is carried out through the application of these detector in estimation of the out-of-field dose with important dosimetric parameters such as field size (from 5×5 to 30×30 cm2) and depth (from 1.5 to 30 cm) at energy 6 MV and collimator angle 0° at SSD 100 cm. Results: Results show that, the Markus detector reported an increase in out-of-field dose with field size, depth in almost all measurements. For 6 MV and 0° collimator angle, the out-of-field dose at field size of 5×5 cm2 (depth of 1.5 cm) is 1.1% and at field size of 30×30 cm2 (depth of 1.5 cm) is 4.4% . The out-of-field dose for a depth of 1.5 cm (field size of 10×10 cm2) is 2.3% and for a depth of 30 cm (field size of 10×10 cm) is 5.5%. the measured out-of-field dose by Markus detector overestimated in the calculated at different field sizes (2.7% instead of 2.3% at field size of 10×10 cm2 and 5.2% instead of 4.4% at field size of 30×30 cm2) and different depths (2.7% instead of 1.1% at depth of 1.5 cm and 4.1% instead of 3.4% at depth of 30 cm). Analysis: The result reported an increase in mean out-of-field dose with field size, depth, energy and SSD. Markus ionization chamber detector show overestimation of the measured out-of-field dose in the calculated values at all field sizes and depths, this may be attributed to the poor detection of out-of-field dose by TPS.

Abstract P4-12-10: Clinical implementation of the machine learning-based automated treatment planning tool for whole breast radiotherapy

Abstract Objectives: The machine learning (ML)-based automated treatment planning tool has been developed and evaluated for whole breast radiotherapy. This study implemented the tool in clinic and compared plan quality and planning efficiency with the manual treatment process for whole breast radiotherapy using irregular surface compensator technique. Methods: This study involved two phases. The 1st phase was to evaluate and the 2nd phase was to implement in the clinic. In the 1st phase, thirty whole breast or chest wall cases were planned by using the irregular surface compensator technique with fluence maps manually and iteratively edited to achieve uniform dose distribution to the target by planners. Patients were treated with these manual plans after physician’s approval. The evaluation phase thus compared our in-house ML-based automated treatment planning tool implemented in Eclipse Scripting Application Programming Interface (ESAPI) to these manual plans. The ML-based planning tool generated the fluence maps with the same beam parameters such as beam energy, gantry angle, collimator angle, and aperture shape as manual plans. Breast or chest wall clinical target volume (CTV) coverage based on the percentage CTV volume receiving 95% of the prescribed dose (V95%) and high-dose volume based on V105% were compared to evaluate the plan quality as well as the planning efficiency. Two-tailed Wilcoxon Signed-Rank test was performed to test the null hypothesis that the two planning schemes yield equivalent plan quality. In the 2nd phase, the planners used the automated planning tool for fourteen plans (twelve patients) followed by manual fluence modification as needed. Physicians reviewed and approved the plans, and patients were treated. Results: For the 1st phase, the mean planning time was 110.2 min with standard deviation (SD) of 62.8 min for the manual planning with the range from 25 to 270 min, and 6.4 min with SD of 2.1 min for the automated planning with the range from 4 to 12 min (p<0.01). CTV mean V95% was 96.7% (SD: 5.0%) for the manual planning and 96.7% (SD: 4.8%) for the automated planning (p=0.89). CTV mean V105% was 21.6% (SD: 29.8%) for the manual planning and 20.4% (SD: 30.5%) for the automated planning (p=0.22). Differences in doses to heart and lungs were negligible between the paired plans as the two planning schemes used the same beam parameters. For the 2nd phase, the mean planning time was 16.4 min (SD: 9.1 min) and the mean time for additional manual editing was 12.7 min (SD: 12.5 min). The mean total treatment planning time was 29.1 min (SD: 14.8 min).). CTV mean V95% was 97.2% (SD: 4.2%) and mean V105% was 8.2 % (SD: 3.6%). The manual post modifications were added by the planners with intention to improve the target coverage or to reduce high doses, yet, the difference between the plans without and without the manual post modification was negligible. Conclusion: The ML-based automated treatment planning tool through Varian ESAPI has been successfully implemented for clinical use going through two phases of study. Abiding to the same plan quality as manual process, the automated tool significantly reduced the planning time as the ML-based tool automate the iterative fluence editing process. Citation Format: Sua Yoo, Yang Sheng, Rachel Blitzblau, Suzanne Catalano, Jay Morrison, Leigh O'Neill, Fangfang Yin, Q. Jackie Wu. Clinical implementation of the machine learning-based automated treatment planning tool for whole breast radiotherapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-12-10.

Radiobiological assessment of nasopharyngeal cancer IMRT using various collimator angles and non-coplanar fields

AbstractAim:The aim of this study was to evaluate clinical efficacy and radiobiological outcome of intensity-modulated radiation therapy (IMRT) modalities using various collimator angles and non-coplanar fields for nasopharyngeal cancer (NPC).Materials and methods:A 70-Gy planning target volume dose was administered for 30 NPC patients referred for IMRT. Standard IMRT plans were constructed based on the target and organs at risk (OARs) volume; and dose constraints recommended by Radiation Therapy Oncology Group (RTOG). Using various collimator angles and non-coplanar fields, 11 different additional IMRT protocols were investigated. Homogeneity indexes (HIs) and conformation numbers (CNs) were calculated. Poisson and relative seriality models were utilised for estimating tumour control probability (TCP) and normal tissue complication probabilities (NTCPs), respectively.Results:Various collimator angles and non-coplanar fields had no significant effect on HI, CN and TCP, while significant effects were noted for some OARs, with a maximum mean dose (Dmax). No significant differences were observed among the calculated NTCPs of all the IMRT protocols. However, the protocol with 10° collimator angle (for five fields out of seven) and 8° couch angle had the lowest NTCP. Furthermore, the standard and some of non-coplanar IMRT protocols led to the reduction in OARs Dmax.Conclusions:Using appropriate standard/non-coplanar IMRT protocols for NPC treatment could potentially reduce the dose to the OARs and the probability of inducing secondary cancer in patients.

Localized extra focal dose collimator angle dependence during VMAT: An out-of-field Monte Carlo study using PRIMO software

PurposePatients undergoing 3DCRT on TrueBeam linac equipped with HD_120_MLC are subjected to localized extra-focal dose (LEFD) in a specific region out of the treatment field. The dose in this region is up to 160% of the mean transmitted dose through MLC. Although VMAT provides a more conformal dose to PTV and better sparing to OAR, it implies more monitor units and irradiating more healthy tissues outside the treatment field. One aims to investigate the dose distribution due to LEFD during VMAT and the collimator rotation angle optimization to minimize this extra dose. MethodsEight VMAT plans for a hypothetical abdominal tumor were contoured on an anthropomorphic pediatric phantom and calculated on Eclipse TPS. The plans were optimized to the same dosimetric objectives at PTV and OAR with different collimator rotation angles. They were also simulated using PRIMO Monte Carlo software. The dose out of the treatment field was evaluated in eight spherical structures distributed at different distances from the isocenter. To validate the results, QA verification plans were created, were measured by EPID and simulated in a virtual phantom of the EPID. Measured and simulated QA plans were compared using gamma-index methodology. Point dose measurements were also used to verify Monte Carlo simulations accuracy out of the treatment field. ResultsBoth measured and simulated QA dose distributions showed that each treatment arc has two LEFD regions out of the treatment field which were not calculated by TPS. The simulated treatment plans showed LEFD as cylindrical dose distributions, 10 cm height, with the diameter of X-jaws aperture. At 0° rotation the two LEFD regions corresponding to the two-treatment arcs are superimposed. As rotation angle increases the two cylindrical distributions start to drift apart but their intersection remains localized along the central axis until they are completely separated. ConclusionThe LEFD can be avoided during VMAT without compromising the treatment plan objectives by increasing the collimator rotation angle to relocate LEFD out of the body.

Complex Target Volume Delineation and Treatment Planning in Radiotherapy for Malignant Pleural Mesothelioma (MPM)

Introduction: Radiotherapy alone or combined with surgery and/or chemotherapy is being investigated in the treatment of malignant pleural mesothelioma (MPM). This study aimed to simulate a Volumetric Modulated Arc Therapy (VMAT) treatment of a patient with MPM. Materials and Methods: CT images from a patient with intact lungs were imported via DICOM into the Pinnacle3 treatment planning (TP) system (TPS) and used as a model for MPM to delineate organs at risk (OAR) and both clinical and planning target volumes (CTV and PTV) with a margin of 5 mm. Elekta Synergy with 6 MV photons and 80 leafs MLCi2 was employed. VMAT plans were generated using two coplanar arcs with gantry rotation angles of 178&deg - 182&deg, the collimator angles of each arc were set to 90&deg, Octavius® 4D 729 was employed for quality assurance while the calculated and measured doses were compared using VeriSoft. Results: A TP was achieved. The Gamma volume analysis with criteria of 3 mm distance to agreement and 3% dose difference yielded the gamma passing rate = 99.9%. The reference isodose was 42.75 Gy with the coverage constraints for the PTV D95 and V95 = 95.0% of 45 Gy. The remaining dosimetric parameters met the recommendations from the clinically acceptable guidelines for the radiotherapy of MPM. Conclusion: Using well-defined TV and VMAT, a consistent TP compared to similar ones from publications was achieved. We obtained a high agreement between the 3D dose reconstructed and the dose calculated.

The Effect of Different Colimator Angles in Prostate Radiotherapy Plans with Volumetric Modulated Arc Therapy (VMAT) Technique

The aim of this study was to compare the differences in tumor and critical organ doses by using different collimator angles in the treatment plans of 10 prostate cancer patients who received radiotherapy treatment in the Radiation Oncology Clinic of Şişli Hamidiye Etfal Training and Research Hospital. The plans of 10 prostate cancer patients treated with the Varian Trilogy model linear accelerator were evaluated using the double field Volumetric Modulated Arc Therapy (VMAT) planning technique in the Eclipse (ver.13.6) treatment planning system. In VMAT technique, treatment plans were made when the collimator angles were clockwise (CW) and counter clockwise (CCW) 30°-330°. Critical organs and target organs are re-calculated by changing CW and CCW (15°-345°, 45°-315°, 60°-300°, 75°-285°, 90°-270°) in VMAT calculations without changing the optimization values. Dose distributions were obtained and the effect of collimator angle on dose distribution was examined. The collimator angles, which we obtained the best dose distribution, were determined for VMAT technique and evaluated for statistical significance. Quality control of the statistically significant treatment plans was performed with Electronic Portal Imaging Device (EPID) and the results were compared with 3% dose difference and 3 mm distance difference criteria for each patient in linear accelerator. It was found that the collimator angle which provides optimal dose distribution for the treatment of prostate cancer patients was consistent both statistically and as a result of measurements.

Toward the combined optimization of dynamic axes (CODA) for stereotactic radiotherapy and radiosurgery using fixed couch trajectories.

To develop a novel system for patient-specific combined optimization of couch, collimator, and gantry angles for use in volumetric modulated arc therapy (VMAT) treatment planning. The system was designed to produce highly compact dose distributions by extensively sampling the 4π space. Automated fixed couch trajectory planning was used to reduce normal tissue doses by avoiding beams-eye-view (BEV) overlap with organs-at-risk (OARs) and improve monitor unit (MU) efficiency through collimator angle optimization. By merging distinct BEV objective functions used to optimize the couch rotation angle and collimator angle, a three-dimensional (3D) cost space (the CODA cube) was constructed with axes of gantry, couch, and collimator rotation angles. At each voxel in this CODA cube, the cost of implementing this combination of axes positions in fixed couch trajectories was quantified. The CODA cube was sampled and explored using a modified constrained Bellman-Ford algorithm to suggest low-cost fixed candidate arcs on each plane of the space, from which 10-arcs are chosen throughout the 3D space using a k-means clustering algorithm. These fixed couch trajectories were then imported into the Eclipse treatment planning system (v.11) and inverse-optimized according to clinical standards. Eight artificial cranial targets were contoured in a test-patient anatomy, and seven treatment plans were generated from combinations of three and four targets. The CODA cube optimized plans were compared to standard 4-arc VMAT plans for cranial stereotactic radiotherapy/surgery that were optimized for the same sets of targets; maximum dose to each OAR, V12Gy to normal brain, conformity, and total MUs were compared. Both planning methods were inverse-optimized with identical dosimetric objectives. CODA plans resulted in a reduction in maximum dose to OARs of 20.6% (P<0.01), with maximum brainstem dose decreased by 2.63Gy (P=0.031) on average when compared to the standard arc arrangement. The mean reduction in total MU was 8.6% (P=0.156), the mean increase in the inverse of the van't Riet conformation number was 0.1%, (P=0.67) and the mean decrease in normal brain tissue receiving 12Gy or higher was 3.9% (P=0.16), when compared to the standard VMAT arc configuration (n=7). The optimization of couch, collimator, and gantry angles simultaneously using a 3D optimization space achieved improvement on multiple clinical metrics when compared to the standard VMAT arc configuration. A statistically significant sparing to OAR maximum doses was seen. Combining these optimizations may yield superior results to independent optimization.

Correction: Effect of Collimator Angles on the Dosimetric Results of Volumetric Modulated Arc Therapy Planning for Patients with a Locally-Advanced Nasopharyngeal Carcinoma

The affiliation of Yongkan Ki should be corrected as follow:

Dosimetric Effect Resulting From the Collimator Angle, the Isocenter Move, and the Gantry Angle Errors

Introduction: Dose distribution can be affected by diverse parameters, such as beam orientations, and collimator angles. These parameters should respect and maintain the international recommended levels during the realization of the quality assurance protocols of linear accelerators. This study aimed at evaluating the dosimetric effects on treatment quality considering the mechanical error fluctuations in the recommended range. Material and Methods: This study included ten patients with head and neck cancer. All of them were treated using three-dimensional conformal radiotherapy with the simple 3-field classic technique. Initially, an optimized treatment plan was computed for each patient. Afterward, similar calculations were executed by varying isocenter position, gantry and collimator angles. Eventually, dosimetric evaluations based on dose-volume histograms were studied and analyzed by Wilcoxon signed rank test for each plan. Results: The analysis of the dose-volume histograms of tumor volumes and organs at risk, as well as the dosimetry calculation, revealed that the small errors of 0.5° in gantry and collimator angles have minimal effects on dose distribution. However, the variation in isocenter coordinating up to 1 mm may influence the patients’ treatment quality, particularly in the spinal cord and the brainstem, in which Wilcoxon's test showed significant effects in all plans. Conclusion: According to the results, the quality of the treatment plans is almost insensitive to the errors of the gantry and the collimator angles of the order 0.5° though it is relatively sensitive to isocenter errors (1 mm). These should be reduced in order to avoid overdose when applying the conventional 3-field technique.

Implementation of Machine Learning-Based Treatment Planning Tool for Whole Breast Radiotherapy Using Irregular Surface Compensator Technique

To implement the machine learning (ML)-based treatment planning tool and compare plan quality and planning efficiency with the manual treatment process for whole breast radiotherapy using irregular surface compensator technique. Thirty whole breast or chest wall cases were planned by using the irregular surface compensator technique with fluence maps manually and iteratively edited to achieve uniform dose distribution to the target. Patients were treated with these manual plans after physician’s approval. For each case, an in-house ML-based automated treatment planning tool was implemented in a programming interface and generated the fluence maps with the same beam parameters such as beam energy, gantry angle, collimator angle, and aperture shape as manual plans.An analytical algorithm software with 0.25cm grid size was used in dose calculation for all plans. Breast or chest wall clinical target volume (CTV) coverage based on the percentage CTV volume receiving 95% of the prescribed dose (V95%) and high-dose volume based on V105% were compared to evaluate the plan quality as well as the planning efficiency. Two-tailed Wilcoxon Signed-Rank test was performed to test the null hypothesis that the two planning schemes yield equivalent plan quality. The mean planning time was 110.2 min with standard deviation (SD) of 62.8 min for the manual planning with the range from 25 to 270 min, and 6.4 min with SD of 2.1 min for the ML-based planning with the range from 4 to 12 min (p<0.01). CTV mean V95% was 96.7% (SD: 5.0%) for the manual planning and 96.7% (SD: 4.8%) for the ML-based planning (p=0.89). CTV mean V105% was 21.6% (SD: 29.8%) for the manual planning and 20.4% (SD: 30.5%) for the ML-based planning (p=0.22). Differences in doses to heart and lungs were negligible between the paired plans as the two planning schemes used the same beam parameters. This study successfully implemented the ML-based automated treatment planning tool through scripting software. Abiding to the same plan quality as manual process, the automated tool significantly reduced the planning time as the ML-based tool automate the iterative fluence editing process. This technique demonstrated promising potential to significantly improve efficiency of the treatment planning for whole breast or chest wall radiotherapy.

Optimized Volumetric Modulated Arc Technique for Noninvasive Cardiac Ablation to Treat Ventricular Tachycardia

Recent publications (Cuculich et al., NEJM-2017, Robinson et al., Circulation-2019) have shown promising outcomes of radioablation using a single-fraction SBRT to treat patients with refractory, life-threatening, VT. This prompted growing interest across the world to use SBRT to treat VT. The primary goal of this study is to explore an alternative VMAT planning technique to achieve greater Non-Target Heart-Tissue (NTHT) sparing while simultaneously allowing for breath hold (BH) treatments. 13 patients were selected who previously received a single fraction dose of 35 Gy to ITV and 25 Gy to PTV utilizing 3 non-coplanar VMAT arcs of 6 MV photon beams using HD-MLC on a radiosurgery-optimized linac. To further improve NTHT sparing we retrospectively re-planned each patient using 10 ipsi-lateral arcs with optimal gantry, couch and collimator angles. Two plans were generated using 6X FFF and 10X FFF beams with dynamic jaw-tracking. Gantry arc lengths were limited to 90° (< 15s/arc) for feasibility of a potential BH treatments. Couch angles were limited to ± 15° to avoid collisions. Plans were optimized to maximize the NTHT sparing while meeting the protocol constraints for the stomach, esophagus and colon (OAR). Plan quality metrics for both clinical and reoptimized plans are summarized in Table 1. Revised plans showed adequate target coverage (V95%Rx=>95%), better dose conformity and gradient, while meeting the dose constraints of the OARs. Excellent NTHT sparing is achieved by reducing the volumes of V20Gy by 43 cc and V5Gy by 250 cc. Average mean dose reduction of 2.5 (range: 0.6 – 6.0) Gy and 2.2 (0.7 – 5.7 Gy) was observed in 6X FFF and 10X FFF plans, respectively. Revised plans showed the possibility of treating each arc in a single BH. The standardized technique used in this study showed improved NTHT sparing and better dose conformity. This result may be important for the group of patients where cardiac toxicity may play a vital role in long-term survival. BH treatments may reduce the required PTV margins resulting in further NTHT sparing.Abstract 3778; Table 1Vol. (cc)Plan TypeTarget CoverageConformityMean D (Gy)%VRx%V95%RxConformity IndexGradient MeasureITV48.1 ± 29Clinical38.4 ± 1.197.6 ± 2.499.7 ± 0.66X FFF36.9 ± 0.494.2 ± 3.199.3 ± 0.810X FFF36.8 ± 0.890.5 ± 11.698.7 ± 2.0PTV138.9 ± 67.8Clinical33.6 ± 1.199.0 ± 0.899.7 ± 0.41.14 ± 0.122.01 ± 0.476X FFF32.3 ± 0.493.8 ± 2.596.8 ± 2.21.02 ± 0.041.76 ± 0.4710X FFF32.3 ± 0.494.2 ± 2.397.1 ± 1.81.03 ± 0.031.55 ± 0.26Non-Target Heart TissueMean D (Gy)%V25%V20%V15%V10%V5%V2Clinical6.2 ± 2.10.9 ± 0.84.9 ± 2.511.2 ± 5.421.4 ± 9.140.5 ± 13.966.7 ± 17.86X FFF3.7 ± 1.10.3 ± 0.31.9 ± 2.74.9 ± 1.610.1 ± 3.622.5 ± 8.945.9 ± 15.7Vol. spared (cc)9 ± 1143.± 3590 ± 76160 ± 113250 ± 146284 ± 20910X FFF4.0 ± 1.10.4 ± 0.32.2 ± 0.75.5 ± 1.711.4 ± 3.825.5 ± 9.649.6 ± 16.5Vol. spared (cc)8 ± 1039 ± 3381 ± 75142 ± 108208 ± 111233 ± 150 Open table in a new tab

Complete ray tracing simulation model of the solar simulator as a launch pad for prosperous Virtual lab on thermal vacuum testing simulation

The state-of-the-art ray tracing software is a powerful tool, which may be applied not only for the design and simulation of various kind of optical and illumination system. It may be used also for the simulations of some aspects of thermal vacuum testing. This assumption is based upon the importing of 3D-models of vacuum vessel, cryogenic panels and other features of thermal vacuum facility as well as importing of 3D-model of a specimen under test into the ray tracing model. The main stages of the making of ray tracing model are presented. This approach makes it possible to precisely specify the demands to the solar simulator, in particular to collimation angle and spectrum accuracy. The complete ray tracing model of the solar simulator is considered as a launch pad for building of more complicated model of thermal vacuum testing facility. Such model may then become a Virtual lab on thermal vacuum testing simulation.

Histogram error based algorithm for efficient collimation testing

This communication reports an investigation undertaken towards setting collimation of an optical beam using a self-imaging technique and histogram error (HE) based approach. The beam under test illuminates an amplitude type Ronchi grating. After the grating, a beam splitter is placed such that the grating’s self-images are formed in two perpendicular directions, at the different Talbot planes. The images are then recorded using two identical CCD cameras. Towards implementing a HE based algorithm, first, element-by-element subtraction of the normalized histogram of both self-images is computed. Next, the sum of the elements of the resultant image matrix is determined. Finally, the square of the sum yields the HE. HE provides an estimate of the collimation errors in the beam. For an incident collimated beam, the self-images recorded at different Talbot planes have identical unit magnification with respect to the grating; however, when the beam diverges or converges, the size and fringe width of self-images are differentially magnified or demagnified. Hence, when the beam is collimated, the HE is minimum. For the decollimated beam, the value of HE is higher, and increases as the decollimation errors increase. Using the proposed method, we could set the collimation position to a resolution of 1 μm, which relates to ±0.22 μ radians in terms of collimation angle (for a lens of focal length 300 mm and diameter 40 mm). Experimental results conclusively establish the viability of the technique. Good accuracy and precision in the measurement have been achieved.

Effect of collimator angle on HyperArc stereotactic radiosurgery planning for single and multiple brain metastases

We assessed the effect of collimator angle on the dosimetric parameters for targets and organs at risk (OARs) for collimator-optimized HA (CO-HA) and non-CO-HA (nCO-HA) plans. The nCO-HA and CO-HA plans were retrospectively generated for 26 patients (1 to 8 brain metastases). The dosimetric parameters for planning target volume (homogeneity index [HI]; conformity index [CI]; gradient index [GI]) and for OARs were compared. The modulation complexity score for volumetric modulated arc therapy (MCSV) and monitor units (MUs) were calculated. Doses were measured using the electronic portal imaging device and compared with the expected doses. Dosimetric parameters of the HI, CI, and GI for single (n = 12) and multiple (n = 14) metastases cases were comparable (p > 0.05). For multiple metastases cases, the CO-HA plan provided lower V4Gy, V12Gy, V14Gy, V16Gy for brain tissue compared to the nCO-HA plan (p < 0.05). Doses for OARs (D0.1cc) (brainstem, chiasm, Hippocampus, lens, optic nerves, and retinas) were comparable (p > 0.05). For multiple metastases cases, the CO-HA plan resulted in less complex multileaf collimator (MLC) patterns (MCSV = 0.19 ± 0.04, p < 0.01), lower MUs (8596 ± 1390 MUs, p < 0.01), and shorter beam-on time (6.2 ± 1.0 min, p < 0.01) compared to the nCO-HA plan (0.16 ± 0.04, 9365 ± 1630, and 6.7 ± 1.2 for MCSV, MUs, and beam-on time, respectively). For both treatment approach, the equivalent gamma passing rate was obtained with the 3%/3 mm and 2%/2 mm criteria (p > 0.05). The collimator optimization in the HA planning reduced doses to brain tissues and improved the treatment efficacy.

Some considerations about a Soller collimator for neutron imaging

An analytical analysis of a Soller collimator for thermal neutrons concerning the intensity and collimation angle of the neutron beam at different distances from the exit of the collimator is presented. This analysis establishes the surfaces with uniform neutron intensity and the area of those surfaces. For those special surfaces, formulas in regards to the neutron intensity and collimation angle of the neutron beam were established. The considerations were made for the Soller collimator in contact with the neutron source, and placed at a distance from the neutron source. To demonstrate the validity of the analytical calculations, some arbitrary cases were also simulated using MCNP code. The obtained results from simulations showed a good agreement with the analytical estimations and confirmed their validity.

Technical Note: Improving VMAT delivery efficiency by optimizing the dynamic collimator trajectory.

To provide a method for optimizing the multileaf collimator angle trajectory in volumetric modulated arc therapy (VMAT) in order to make VMAT delivery and planning more efficient. Static fluence maps are optimized at a 10-degree spacing around the patient. Sliding window delivery time of each of these fields is computed for a large set of possible collimator orientations. An optimal trajectory, which selects a collimator angle for each field and assures that the collimator angles do not differ excessively between adjacent fields, is computed by solving a network flow model of a shortest path problem. For four clinical cases (two brains, an anal, and a spine), we demonstrate time reductions from 6% to 32% (average: 24%) for the optimal static angle vs the worst static angle. Further reductions from 3% to 17% (average 9%) are achievable when dynamic collimator trajectories are allowed. Dynamic collimator trajectories, which can be computed with an efficient linear programming formulation, can improve the efficiency of VMAT delivery.

Planning feasibility of extremely hypofractionated prostate radiotherapy on a 1.5 T magnetic resonance imaging guided linear accelerator.

Recently, intermediate and high-risk prostate cancer patients have been treated in a multicenter phase II trial with extremely hypofractionated prostate radiotherapy (hypo-FLAME trial). The purpose of the current study was to investigate whether a 1.5 T magnetic resonance imaging guided linear accelerator (MRI-linac) could achieve complex dose distributions of a quality similar to conventional linac state-of-the-art prostate treatments. The clinically delivered treatment plans of 20 hypo-FLAME patients (volumetric modulated arc therapy, 10 MV, 5 mm leaf width) were included. Prescribed dose to the prostate was 5 × 7 Gy, with a focal tumor boost up to 5 × 10 Gy. MRI-linac treatment plans (intensity modulated radiotherapy, 7 MV, 7 mm leaf width, fixed collimator angle and 1.5 T magnetic field) were calculated. Dose distributions were compared. In both conventional and MRI-linac treatment plans, the V35Gy to the whole prostate was >99% in all patients. Mean dose to the gross tumor volume was 45 Gy for conventional and 44 Gy for MRI-linac plans, respectively. Organ at risk doses were met in the majority of plans, except for a rectal V35Gy constraint, which was exceeded in one patient, by 1 cc, for both modalities. The bladder V32Gy and V28Gy constraints were exceeded in two and one patient respectively, for both modalities. Planning of stereotactic radiotherapy with focal ablative boosting in prostate cancer on a high field MRI-linac is feasible with the current MRI-linac properties, without deterioration of plan quality compared to conventional treatments.

Influence of collimator angle optimization on intensity-modulated radiotherapy planning for gastric cancer

Objective To investigate the impact of four different collimator angle optimization techniques on the planning target volume (PTV) and organ at risk (OAR) during intensity-modulated radiotherapy (IMRT) for gastric cancer. Methods Ten patients with gastric cancer undergoing IMRT in Zhongnan Hospital of Wuhan University from 2015 to 2016 years were recruited in this study. All IMRT plans were designed by conventional five fields (330°, 10°, 45°, 90°and 180°). In the Eclipse treatment planning system, four different collimator angle optimization techniques with consistent planning optimization parameters were employed to design the IMRT plan. Collimator angle optimization techniques included the following aspects. The collimator angle was set at 0 degree (CL0), collimator angle was set at 90 degree (CL90), Eclipse automatic collimator angle optimization (CLA) was adopted and collimator angle was set as the angle when the distance between X-Jaws and PTV (CLX) was the shortest. The dosimetric parameters mainly included the conformal index (CI) of PTV, the homogeneity index (HI), the mean dose (Mean), and the dosage of OAR.The treatment time (Time), monitor unit (MU), control point (CP), split field (SF) and conformal distance (Fx) were also considered. Results Regarding CL0 as the control, the CI, HI and Mean did not significantly differ among four collimator angle optimization techniques (all P>0.05), whereas CLX could significantly increase the average dose of PTV in the target area (P<0.05); CLX optimization reduced the liver(V30 reduction by 1.54%), left kidney(V12 decrease by 1.46%), right kidney and other OARs, whereas it slightly increased the maximum dose of the small intestine and spinal cord (<1%). CL90 and CLA optimization elevated the dose of OAR in gastric cancer. Among four different collimator angle optimization techniques, CLX optimization reduced the MU (25.02%), CP (26.03%), Fx (20.27%) and SF (by 1.3 separate fields on average) and treatment time (10.03%). CL90 and CLA optimization could decrease the MU, CP, Fx and SF.CL90 optimization had certain advantages in shortening the treatment time, whereas CLA optimization could prolong the treatment time by 5.04%. Conclusions During IMRT for gastric cancer, CL90, CLA and CLX collimator angle optimization techniques can obtain comparable dosimetry distribution to CL0 optimization technique, which can reduce the MU, decrease the radiation leakage, shorten the treatment time and improve treatment efficiency. Key words: Collimator angle optimization; Gastric neoplasms/intensity-modulated radiotherapy

PV-0477 A Monte Carlo study of collimator angle dependence of extra focal dose during VMAT

PV-0477 A Monte Carlo study of collimator angle dependence of extra focal dose during VMAT

Characterization of the HalcyonTM multileaf collimator system.

PurposeTo characterize the stacked and staggered dual‐layer multileaf collimator (MLC) on the HalcyonTM system.MethodsThe novel MLC assembly was reviewed and compared to the widely used MillenniumTM 120‐leaf MLC system. We investigated the MLC positioning stability over 70 days using Machine Performance Check (MPC) data. We evaluated the leaf transmission, penumbra, leaf end effect, and leaf edge effect. Leaf transmission through distal, proximal, and both MLC layers was measured with a Farmer chamber, by comparing an open and a closed field. Leaf penumbra was measured using film for three different MLC‐defined field sizes. The leaf end effect was measured with sweeping gap fields of varying gap sizes defined by the distal MLC. The leaf edge effect was evaluated using the Electronic Portal Imaging Device (EPID) for the different banks, gantry positions, and collimator angles. Point dose measurements for 10 test plans were compared to dose predictions of two dose calculation model versions.ResultsFrom MPC data, the largest measured MLC positioning accuracy deviation was within 0.1 mm. The proximal MLC exhibited greater deviations compared to the distal MLC. The distal‐and‐proximal‐combination had reduced inter‐leaf and intra‐leaf transmission compared to delivery with distal‐only. The measured leaf transmission was 0.41% for distal‐only, 0.40% for proximal‐only, and negligible for distal‐and‐proximal‐combination. The leaf end penumbra was wider compared to the leaf edge penumbra. The leaf end effect was measured to be −0.2 mm. The leaf edge effect showed minimal bank, gantry position, and collimator angle dependence. However, a systematic deviation between measurements and treatment planning system handling of the leaf edge effect was observed. The discrepancy between the measured and predicted dose in the 10 test plans improved with the latest version of the dose calculation algorithm.ConclusionThe characteristics of the stacked and staggered dual‐layer MLC on the HalcyonTM system were presented.