Relative Dose Profiles Research Articles

Development of a Relative Dosimetric System for Calibration of 32P Eye Applicators Using Radiochromic Films.

Beta irradiation after bare scleral surgery of primary pterygium is an effective and safe treatment, which reduces the risk of local recurrence. Obtaining the reference dose rate for a radioactive applicator consisting of a plate as a 32P absorber, a steel window and a steel capsule. Relative dosimetry and dose profile were measured using two types of radiochromic films, HD-810 and EBT1, for the 32P applicator and were compared with Monte Carlo simulation data. Dose uniformity in the 32P applicator was obtained with radiochromic HD-810 film. The measurement depth dose distribution data at distances up to 3.8 mm were compared with calculation data, and the values were not found to differ statistically. Depth dose distribution with a large dose gradient was determined and the dose rate data obtained 0.0053 ± 9.9% in unit of Gy/s.mCi at a 0.1 mm depth distance. Practical results indicated that the dose nonuniformity and the maximum symmetrical for the 32P applicator were 11.5% and 9.2%, respectively. Our experiments show that the use of the radiochromic film to perform the relative dosimetric checks is feasible and the activity value with acceptable error can be determined through this indirect method.

Carbon-fiber alternative to the commercial gating surrogate for the Varian Truebeam™

Background In this study we present the Tracking Accessory 3 (TA3) as an alternative to the commercial gating block (GB) surrogate for the Varian Truebeam™ gating system (TGS). The TGS requires three visible reflectors to track the surrogate, presenting an opportunity for a surrogate to be made with less material and thus smaller dosimetric footprint than the commercial four reflector model. Materials and methods Relative dose and depth dose profiles below the TA3 and the GB were measured with radiosensitive film. Accuracy and reproducibility of the detected motion amplitude for three TA3s and one GB were determined using a respiratory phantom with surrogate to determine the camera’s tracking volume. Clinical performance was evaluated prospectively in 10 breast cancer patients treated with deep inspiration breath hold monitored with TA3 and compared to previously published results. Non-parametric statistics were applied to test for significance. Results and conclusions Surface doses were increased up to 94% and 187% for the TA3 and GB, respectively, compared to no surrogate. The surface area influenced by at least 25% increase in dose was 12 cm2 and 105 cm2 for the TA3 and GB, respectively. The water equivalent thickness of the surrogates was found to be 1 mm for the TA3 and 3 mm for GB. The difference in measured amplitude were <0.2 mm for TA3 compared to the GB. The TA3s and GB were detected at all extremes of the clinically relevant tracking volume of the TGS. Clinical performance showed no significant differences. The TA3 caused less surface dose increase compared to the commercial GB. In the tested range all surrogates measured motion amplitude within 0.2 mm of reference value, which is not a clinically relevant difference. The TA3 showed no significant differences in clinical performance to similarly positioned surrogates.

Commissioning Intracranial Stereotactic Radiosurgery for a Magnetic Resonance-Guided Radiation Therapy (MRgRT) System: MR-RT Localization and Dosimetric End-to-End Validation

This is the first reporting of the MRIdian A3iTM intracranial package (BrainTxTM) and benchmarks the end-to-end localization and dosimetric accuracy for commissioning an magnetic resonace (MR)-guided stereotactic radiosurgery program. We characterized the localization accuracy between MR and radiation (RT) isocenter through an end-to-end hidden target test, relative dose profile intercomparison, and absolute dose validation. BrainTx consists of a dedicated head coil, integrated mask immobilization system, and high-resolution MR sequences. Coil and baseplate attenuation was quantified. An in-house phantom (Cranial phantOm foR magNetic rEsonance Localization of a stereotactIc radiosUrgery doSimeter, CORNELIUS) was developed from a mannequin head filled with silicone gel, film, and MR BB with pinprick. A hidden target test evaluated MR-RT localization of the 1×1×1 mm3 TrueFISP MR and relative dose accuracy in film for a 1 cm diameter (International Electrotechnical Commission (IEC)-X/IEC-Y) and 1.5 cm diameter (IEC-Y/IEC-Z) spherical target. Two clinical cases (irregular-shaped target and target abutting brainstem) were mapped to the CORNELIUS phantom for feasibility assessment. A 2-dimensional (2D)-gamma compared calculated and measured dose for spherical and clinical targets with 1 mm/1% and 2 mm/2% criteria, respectively. A small-field chamber (A26MR) measured end-to-end absolute dose for a 1 cm diameter target. Coil and baseplate attenuation were 0.7% and 2.7%, respectively. The displacement of MR to RT localization as defined through the pinprick was 0.49 mm (IEC-X), 0.27 mm (IEC-Y), and 0.51 mm (IEC-Z) (root mean square 0.76 mm). The reproducibility across IEC-Y demonstrated high fidelity (<0.02 mm). Gamma pass rates were 97.1% and 95.4% for 1 cm and 1.5 cm targets, respectively. Dose profiles for an irregular-shaped target and abutting organ-at-risk-target demonstrated pass rates of 99.0% and 92.9%, respectively. The absolute end-to-end dose difference was <1%. All localization and dosimetric evaluation demonstrated submillimeter accuracy, per the TG-142, TG-101, MPPG 9.a. criteria for SRS/SRT systems, indicating acceptable delivery capabilities with a 1 mm setup margin.

Dosimetric evaluation of off-axis fields and angular transmission for the 1.5 T MR-linac

Objective. GPU-oriented Monte Carlo dose (GPUMCD) is a fast dose calculation algorithm used for treatment planning on the Unity MR-linac. Treatments for the MR-linac must be calculated quickly and accurately, and must account for two important MR-linac aspects: off-axis positions and angular transmission through the cryostat, couch and MR-coils. Therefore, the aim of this research is to quantify the system-related errors for GPUMCD calculations over the range of clinically-relevant field configurations and gantry angles. Approach. Dose profiles (crossline, inline and PDD) were measured and calculated for varying field sizes, off-axis positions and depths. Eleven different (off-axis) positions were included. The angular transmission was investigated by measuring and calculating the transmission for multiple angles, taking the cryostat, couch and coils into account. Main results. Differences between absolute point doses were found to be within 1.7% for field sizes 2 × 2 cm2 and larger. The relative dose profiles in the crossline, inline and PDD direction illustrated maximum mean dose differences of 0.9pp, 0.8pp and 0.7pp of D max in the central region for field sizes 2 × 2 cm2 and larger. The 1 × 1 cm2 field size showed larger dosimetric errors for absolute point doses and relative dose profiles. The maximum mean DTA in the penumbra was 0.7 mm. The mean difference in angular transmission ranged from −0.33% ± 0.60% to 0.27% ± 0.91% using three treatment machines. Additionally, 77.1%–93.7% of the datapoints remained within 1% transmission difference. The largest transmission differences were present at the edges of the table. Significance. This research showed that the GPUMCD algorithm provides reliable dose calculations with a low uncertainty for field sizes 2 × 2 cm2 and larger, focusing on off-axis fields and angular transmission.

Experimental Validation of Monte Carlo Simulation for the Leksell Gamma Knife Perfexion Using Gafchromic EBT3 Dosimetry Film and Diamond Detector T60019 PTW

This work presents experimental measurements performed using GAFChromic EBT3 dosimetry film and diamond detector T60019 to validate penEasy Monte Carlo based model developed previously for the Leksell Gamma Knife Perfexion. The relative dose profiles along X and Z coordinate axes, for each collimator size 4, 8, and 16 mm, and the relative output factors have been investigated. The penEasy model has been validated comparing the calculated output factors and dose profiles with the corresponding experimental results. The differences between the calculated output factors and the corresponding data measured with diamond detector were found to be 1.8 and 2.1% for the 8 and 4 mm collimator, respectively. Excellent agreement was found between penEasy and EBT3 results. The differences in the output factors were found to be 0.4% and −0.7% for the 8 and 4 mm collimators, respectively. The relative dose profiles were found to be in good agreement for both detectors and penEasy data. The gamma index test returns values (3%, 1 mm) < 1 for all profiles comparison calculated between penEasy and EBT3 film.

Representation and illustration of the initial parameters in GATE 8.1 monte carlo simulation of an Elekta Versa-HD linear accelerator

The purpose of this study is to validate GATE 8.1 Radiotherapy Monte Carlo (MC) simulation of a 6 MV Elekta Versa-HD linear accelerator at King Abdulaziz University Hospital (KAUH). The simulation data using GATE 8.1 was benchmarked against measured data at KAUH. The simulation comprises treatment head, Multi-Leaf Collimators (MLCs), and homogeneous water phantom, whereas the measured data was performed using MP3 phantom and PTW 31,010, 0.125 cm3 Semiflex Chamber. Dose depth distribution and dose profiles were carried out at three different field sizes 10x10, 20x20, and 30 × 30 cm2. The calculated TPR20,10 for the simulated data was found to be 0.658, 1.1% less than the measured TPR20,10 which was found to be 0.666. The obtained results indicate good agreement between the simulated and measured data, where Gamma Index 3%/3 mm criteria reached values of 97% and 90% for relative dose and dose profiles, respectively.

The potential impact of ultrathin filter design on dosimetry and relative biological effectiveness in modern image-guided small animal irradiators.

Modern image-guided small animal irradiators like the Xstrahl Small Animal Radiation Research Platform (SARRP) are designed with ultrathin 0.15 mm Cu filters, which compared with more heavily filtrated traditional cabinet-style biological irradiators, produce X-ray spectra weighted toward lower energies, impacting the dosimetric properties and the relative biological effectiveness (RBE). This study quantifies the effect of ultrathin filter design on relative depth dose profiles, absolute dose output, and RBE using Monte Carlo techniques. The percent depth-dose and absolute dose output are calculated using kVDoseCalc and EGSnrc, respectively, while a tally based on the induction of double-strand breaks as a function of electron spectra invoked in PENELOPE is used to estimate the RBE. The RBE increases by >2.4% in the ultrathin filter design compared to a traditional irradiator. Furthermore, minute variations in filter thickness have notable effects on the dosimetric properties of the X-ray beam, increasing the percent depth dose (at 2 cm in water) by + 0.4%/0.01 mm Cu and decreasing absolute dose (at 2 cm depth in water) by -1.8%/0.01 mm Cu for the SARRP. These results show that modern image-guided irradiators are quite sensitive to small manufacturing variations in filter thickness, and show a small change in RBE compared to traditional X-ray irradiators. We quantify the consequences of ultrathin filter design in modern image-guided biological irradiators on relative and absolute dose, and RBE. Our results show these to be small, but not insignificant, suggesting laboratories transitioning between irradiators should carefully design their radiobiological experiments.

Toward a novel dosimetry system using acrylic disk radiation sensor for proton pencil beam scanning.

Fabricate an acrylic disk radiation sensor (ADRS) and characterize the photoluminescence signal generated from the optical device as basis for the development and evaluation of a new dosimetry system for pencil beam proton therapy. Based on the characteristics of the proposed optical dosimetry sensor, we established the relation between the photoluminescence output and the applied dose using an ionization chamber. Then, we obtained the relative integral depth dose profiles using the photoluminescence signal generated by pencil beam irradiation at energies of 99.9 and 162.1 MeV, and compared the results with the curve measured using a Bragg peak ionization chamber. The relation between the photoluminescence output and applied dose was linear. In addition, the ADRS was dose independent for beam currents up to 6 Gy/min, and the calibration factor for energy was close to 1. Hence, the energy dependence on the optical device can be disregarded. The integral depth dose profiles obtained for the ADRS suitable agreed with the curve measured in the Bragg peak ionization chamber without requiring correction. These results suggest that the ADRS is suitable for dosimetry measurements in pencil beam scanning, and it will be employed as a low-cost and versatile dosimetry sensor in upcoming developments.

Characterisation of two new radiochromic gel dosimeters TruView™ and ClearView™ in combination with the vista™ optical CT scanner: A feasibility study

PurposeThis study aims at characterising the properties of TruView™ and ClearView™ two new gel dosimeters (Modus Medical Devices Inc.) and at studying the feasibility of relative dosimetry using these dosimeters and the Vista™ Optical CT scanner to accurately evaluate dose. MethodsIn this work, we investigated key dosimetric aspects (dose response, energy and dose rate dependence) and stability of these radiochromic gels initiated in preliminary works (Huet et al., 2017; Colnot et al., 2017) using spectrophotometric measurements. Moreover, by mean of optical CT scanning (Vista™), their performances to measure relative depth dose (PDD) and cross profiles were analysed. ResultsTruView™ and ClearView™ present a linear dose response up to 20 Gy and up to 80 Gy respectively, independent of both photon beam energy (4–18 MV) and dose rate (up to 9.9 Gy/min) (Huet et al., 2017; Colnot et al., 2017). ClearView™ response proves to be stable for a week post-irradiation and uniform within the batch whereas TruView™ presents an unstable but uniform response. Optical CT scanning generates errors due to stray light that need to be corrected in order to use these gels; ClearView™ scanning particularly requires important precautions. After corrections, those gels used in combination with the Vista™ scanner show promising spatial and dosimetric precision (dose difference <5%). Finally, TruView™ is reusable and presents excellent reproducible response (maximum 3% difference) and the ClearView™ dosimeter presents good spatial stability (0.5% difference after 6 days). ConclusionThis study provides important knowledge about two gel dosimeters presenting interesting dosimetric properties. A study is ongoing to benchmark those promising candidates for clinical dose verification.

P264] Time resolved dose rate distributions in radiosurgery and their impact on biological effectiveness of treatment

Purpose To obtain time-resolved dose rate distributions (TR-DRD) in radiosurgery applications, and evaluate the impact of TR-DRD on the biological effectiveness of treatment. Methods Treatment plans of 74 acoustic schwannoma patients treated with Leksell Gamma Knife® C or PERFEXION™ were meta-analyzed. The 3D dose distribution within the Planning Target Volume (PTV) for each patient was obtained from the superposition of relevant data for each delivered GK-shot. Single shot dosimetry calculations were based on the nominal dose rate at the center of each shot and corresponding relative dose profile data. TR-DRD were estimated neglecting 60Co source decay during the irradiation of each shot. A formalism including frequently used radiobiological tools that accounts for the effect of dose rate and incorporates fast and slow components of repair, was developed on the basis of the Biologically Effective Dose (BED). Patient and plan characteristics as well as dose and dose rate indexes incorporating the TR-DRD were introduced and their correlation with the corresponding radiobiological parameters was examined using the Spearman rank test. Results The nominal mean dose rate at focus (NMDR-F) presented a high correlation with the percentage of the total dose delivered with dose rates lower than 1 Gy/min, TD 1 Gy / min , ( r = - 0.837 and p = 5.32 E - 20 ) and the percentage of the PTV volume receiving dose rates lower than 1 Gy/min from all shots, V 1 Gy / min ( TR-DRD ) ( r = - 0.830 and p = 6.43 E - 20 ). A statistically significant correlation was also found between the number of iso-centers used per PTV volume (Unit Isocenter, UI) and the V 1 Gy / min ( TR-DRD ) ( r = 0.268 and p = 0.021 ) as well as the percentage of the PTV volume receiving dose values lower than 15 Gy, V 15 Gy ( r = 0.543 and p = 5.90 E - 07 ). The Equivalent Uniform BED (EUBED) was significantly correlated with the UI ( r = - 0.4318 and p = 0.0001 ), the UI/NMDR-F ( r = - 0.3955 and p = 0.0005 ), the V 1 Gy / min ( TR-DRD ) ( r = - 0.3155 and p = 0.0062 ) and the V 15 Gy ( r = - 0.569 and p = 1.25 E - 07 ). Conclusion The observed correlation between the TR-DRD based parameters and the EUBED suggests that the former should be taken into account at the planning stage of radiosurgery, either directly or indirectly via plan characteristics readily evaluated in every day clinical routine (NMDR-F, UI) that are correlated with TR-DRD based dose and dose rate parameters.

Abstract ID: 230 Experimental and Monte Carlo investigation of the depth-dependent fluence perturbation of parallel-plate chambers in clinical electron beams

The electron fluence inside a parallel-plate ionization chamber positioned in a water phantom and exposed to a clinical electron beam deviates from the unperturbed fluence in water in absence of the chamber. One reason for the fluence perturbation is the "in-scattering effect" as described in ICRU 35. Aim of this work is a detailed experimental and with Monte Carlo simulations investigation of this effect. The experiments and Monte Carlo Simulations, with EGSnrc [1], were performed in a solid-water phantom (RW3) sized 30 × 30 × 30 cm3. One of the RW3 plates of height h = 0.5 cm had a hole with radius r = 1 cm which acts as the air cavity in the phantom. The phantom was placed below an electron applicator of field size 10 × 10 cm2 and irradiated with 6 MeV electrons from an Elekta Synergy accelerator. The air cavity was positioned at depths of 1.0, 1.5, 2.0 and 2.5 cm in the phantom. For the exerpiments the spatial resolved dose directly below the cavity was scored with GAFCHROMIC ETB3 films. The films were scanned with an Epson 10000xl-scanner (resolution 600 dpi, color depth 48 bit); a dose response calibration curve for the red channel was applied. At 1.0 cm depth the relative dose profile shows the well known dose increase at the air/RW3 boundary of about 4% relative to the dose at the center of the cavity and a de-crease of about 8% outside the air cavity within the RW3-phantom. The amplitude of this "dose oscillation" decreases with increasing depth of the cavity within the phantom and disappears at about 2 cm, which is less than the half value thickness of the electron beam. The results are in agreement with already published Monte Carlo results [2]. The results are partly in contradiction to the ideas summarized in ICRU 35 but confirm recent Monte Carlo simulations from Zink et al.

53. Characterisation of TruView™: A new 3-D reusable radiochromic MethylThymol Blue based gel dosimeter for ionising radiations

Introduction Cutting-edge radiotherapy techniques deliver highly conformal treatments with steep dose gradients around the target volume. Thus, the verification of the complex resultant dose distributions presents a real dosimetric challenge requiring accurate and spatially precise dosimeters. Gel dosimetry is a promising technique as it can inform about the whole 3-D dose distribution in one irradiation and without conventional dosimeters issues [1] . This study focuses on the characterisation of the new TruView™ gel dosimeter (Modus Medical Devices Inc.). This dosimeter is based on Fricke-MTB composition [2] but presents a new MethylThymol Blue (MTB) reactive dye and a reusable structure. The dosimetric properties of this gel dosimeter and the influence of different parameters on its response are presented. Methods The key dosimetric aspects (dose response, dependence with energy and dose rate) and the TruView™ dosimeter variability with different parameters (gel temperature, light exposure, time between irradiation and reading, etc.) were studied thanks to spectrophotometric measurements. Finally, by mean of optical CT scanning (Vista™, Modus Medical Devices Inc.), its performances to measure relative depth dose (PDD) and cross profiles have been analysed. Results TruView™ presents linear response with dose up to 20 Gy independent of both photon beam energy and dose rate on the studied ranges. The study of the numerous parameters influencing its dose response enabled to determine the optimal conditions for using this gel. In those conditions, the comparison of TruView™ relative depth dose (PDD) and of cross profiles with ion chamber measurements shows a good dosimetric and spatial precision with 90% of the points passing the 3 mm 3% gamma test for the PDD and 87% for the cross profiles. Seven days after irradiation, ion diffusion inside the gel leads to return to homogeneity allowing a new irradiation to be performed. The TruView™ response presents an excellent reproducibility as 100% of the points pass the gamma test for the comparison between the PDD of the two uses and 94% for the cross profiles. Conclusions This study provides important knowledge regarding the 3-D gel dosimeter TruView™. This dosimeter presents interesting dosimetric properties but remains delicate to use due to the great response sensitivity to numerous parameters. A further study is currently ongoing to benchmark this promising candidate for clinical dose verification.

Source modeling for Monte Carlo dose calculation of CT examinations with a radiotherapy treatment planning system.

Radiation dose to patients undergoing examinations with Multislice Computed Tomography (MSCT) as well as Cone Beam Computed Tomography (CBCT) is a matter of concern. Risk management could benefit from efficient replace rational dose calculation tools. The paper aims to verify MSCT dose calculations using a Treatment Planning System (TPS) for radiotherapy and to evaluate four different variations of bow-tie filter characterizations for the beam model used in the dose calculations. A TPS (RayStation™, RaySearch Laboratories, Stockholm, Sweden) was configured to calculate dose from a MSCT (GE Healthcare, Wauwatosa, WI, USA). The x-ray beam was characterized in a stationary position the by measurements of the Half-Value Layer (HVL) in aluminum and kerma along the principal axes of the isocenter plane perpendicular to the beam. A Monte Carlo source model for the dose calculation was applied with four different variations on the beam-shaping bow-tie filter, taking into account the different degrees of HVL information but reconstructing the measured kerma distribution after the bow-tie filter by adjusting the photon sampling function. The resulting dose calculations were verified by comparison with measurements in solid water as well as in an anthropomorphic phantom. The calculated depth dose in solid water as well as the relative dose profiles was in agreement with the corresponding measured values. Doses calculated in the anthropomorphic phantom in the range 26-55 mGy agreed with the corresponding thermo luminescence dosimeter (TLD) measurements. Deviations between measurements and calculations were of the order of the measurement uncertainties. There was no significant difference between the different variations on the bow-tie filter modeling. Under the assumption that the calculated kerma after the bow-tie filter replicates the measured kerma, the central specification of the HVL of the x-ray beam together with the kerma distribution can be used to characterize the beam. Thus, within the limits of the study, a flat bow-tie filter with an HVL specified by the vendor suffices to calculate the dose distribution. The TPS could be successfully configured to replicate the beam movement and intensity modulation of a spiral scan with dose modulation, on the basis of the specifications available in the metadata of the digital images and the log file of the CT.

A quality assurance test for the validation of the spatial and dosimetric accuracy of a new technique for the treatment of multiple brain mestastases

Introduction Radiation treatment planning, delivery and patient monitoring have been enriched with the increased use of MRI in radiotherapy clinical practice. MRI Polymer gel dosimetry has been proposed as a sensitive 3D dosimetry and QA tool. Purpose To develop and implement a QA test for the validation of spatial and dosimetric accuracy of a recently introduced dynamic conformal arcs technique for the treatment of multiple brain metastases. Materials and methods A 3D head avatar phantom with radiologically bone-equivalent material, filled with a sensitive polymer gel dosimeter was fabricated using anonymized CT scans of a specific patient. Irradiation was performed utilizing a single setup isocenter dynamic conformal arcs technique. A 1.5 T MRI clinical scanner was used as the reading device for dose quantification on gel material. Calculated and measured 3D dose distributions were compared in terms of spatial agreement as well as dose profiles, 3D gamma indices (5%/2 mm, 15% dose threshold), DVHs and dose-volume indices. Results A spatial agreement within 2 mm was observed between high dose regions and gel polymerized areas on MRI. Detected offset is a combination of MR-CT registration inaccuracies, set-up errors and MRI-related geometric distortions. Comparison of relative dose profiles showed good dosimetric agreement between the two datasets, while corresponding 3D gamma index passing rate reached 90%. Dosimetric agreement was further verified in terms of DVH comparison, especially for structures lying in high dose areas. Conclusion The 3D head avatar phantom and methods used, demonstrated efficacy in validating spatial and dosimetric accuracy of the specific radiotherapy technique.

Characterization of ytterbium-doped silica optical fibre as scintillator dosimeter

Introduction Scintillator dosimeters (SD) have been a research topic by many groups over the last decade. The recent availability of a commercial system (Exradin W1 by Standard Imaging) represents a significant accomplishment. The attractiveness of SD would be furtherly enhanced by a scintillator free from any spectral superposition with the Cherenkov light, so to avoid any sensitive calibration procedure for the stem effect correction. Yb-doped silica optical fibres, thanks to their near-infrared (NIR) emission, proved to be a promising option. Purpose This study aims to characterize the dosimetric properties of Yb-doped fibers in radiotherapy and to compare their results with those obtained by various reference dosimeters like micro ion-chambers, the commercial SD, and diodes designed for small field dosimetry. Materials and methods Yb-doped fibres were prepared by sol–gel. The scintillation was detected with a laboratory-made photon counting system based on an avalanche photodiode (APD), using a long-pass filter at 950 nm. Irradiations were carried out with photons and electron beams generated by a Varian Trilogy accelerator. Results The NIR scintillation proved to be unaffected by the stem effect, even in unfavorable large field irradiations. The system showed a satisfactory reproducibility, good sensitivity, linear dose-rate response, independence of the signal (total counts) of dose rate and impinging beam orientation. The results were in good agreement with reference dosimeters in terms of relative dose profiles and output factors. Conclusion Findings pave the way to the engineering design of the system, which could be an interesting option also for real-time in-vivo dosimetry applications. Disclosure Authors declare no relationship that may bias the presentation.

SU‐F‐T‐513: Dosimetric Validation of Spatially Fractionated Radiotherapy Using Gel Dosimetry

Purpose:Spatially fractionated radiation therapy, also known as GRID therapy, is used to treat large solid tumors by irradiating the target to a single dose of 10–20Gy through spatially distributed beamlets. We have investigated the use of a 3D gel for dosimetric characterization of GRID therapy.Methods:GRID therapy is an external beam analog of volumetric brachytherapy, whereby we produce a distribution of hot and cold dose columns inside the tumor volume. Such distribution can be produced with a block or by using a checker‐like pattern with MLC. We have studied both types of GRID delivery. A cube shaped acrylic phantom was filled with polymer gel and served as a 3D dosimeter. The phantom was scanned and the CT images were used to produce two plans in Pinnacle, one with the grid block and one with the MLC defined grid. A 6MV beam was used for the plan with a prescription of 1500cGy at dmax. The irradiated phantom was scanned in a 3T MRI scanner.Results:3D dose maps were derived from the MR scans of the gel dosimeter and were found to be in good agreement with the predicted dose distribution from the RTP system. Gamma analysis showed a passing rate of 93% for 5% dose and 2mm DTA scoring criteria. Both relative and absolute dose profiles are in good agreement, except in the peripheral beamlets where the gel measured slightly higher dose, possibly because of the changing head scatter conditions that the RTP is not fully accounting for. Our results have also been benchmarked against ionization chamber measurements.Conclusion:We have investigated the use of a polymer gel for the 3D dosimetric characterization and evaluation of GRID therapy. Our results demonstrated that the planning system can predict fairly accurately the dose distribution for GRID type therapy.

SU‐C‐201‐04: Noise and Temporal Resolution in a Near Real‐Time 3D Dosimeter

Purpose:To characterize the performance of a real‐time three‐dimensional scintillation dosimeter in terms of signal‐to‐noise ratio (SNR) and temporal resolution of 3D dose measurements. This study quantifies its efficiency in measuring low dose levels characteristic of EBRT dynamic treatments, and in reproducing field profiles for varying multileaf collimator (MLC) speeds.Methods:The dosimeter prototype uses a plenoptic camera to acquire continuous images of the light field emitted by a 10×10×10 cm3 plastic scintillator. Using EPID acquisitions, ray tracing‐based iterative tomographic algorithms allow millimeter‐sized reconstruction of relative 3D dose distributions. Measurements were taken at 6MV, 400 MU/min with the scintillator centered at the isocenter, first receiving doses from 1.4 to 30.6 cGy. Dynamic measurements were then performed by closing half of the MLCs at speeds of 0.67 to 2.5 cm/s, at 0° and 90° collimator angles. A reference static half‐field was obtained for measured profile comparison.Results:The SNR steadily increases as a function of dose and reaches a clinically adequate plateau of 80 at 10 cGy. Below this, the decrease in light collected and increase in pixel noise diminishes the SNR; nonetheless, the EPID acquisitions and the voxel correlation employed in the reconstruction algorithms result in suitable SNR values (&gt;75) even at low doses. For dynamic measurements at varying MLC speeds, central relative dose profiles are characterized by gradients at %D50 of 8.48 to 22.7 %/mm. These values converge towards the 32.8 %/mm‐gradient measured for the static reference field profile, but are limited by the dosimeter's current acquisition rate of 1Hz.Conclusion:This study emphasizes the efficiency of the 3D dose distribution reconstructions, while identifying limits of the current prototype's temporal resolution in terms of dynamic EBRT parameters. This work paves the way for providing an optimized, second‐generational real‐time 3D scintillation dosimeter capable of highly efficient and precise dose measurements.The presenting author is financially supported by an Alexander‐Graham Bell doctoral scholarship from the Natural Sciences and Engineering Research Council of Canada (NSERC).

TU‐H‐CAMPUS‐TeP1‐05: Fast Processed 3D Printing‐Aided Urethane Resin (PUR) Bolus in Radiation Therapy

Purpose:3D printed custom bolus is regularly used in radiation therapy clinic as a compensator. However, usual method of bolus printing with 100% filling is very time‐consuming. The purpose of this study is to evaluate the feasibility and benefit of 3D printed bolus filled with UR.Methods:Two boluses were designed on nose (9e electrons) and ear (6× photons) for a head phantom in treatment planning system (TPS) to achieve dose coverage to the skin. The bolus structures (56–167cc) were converted to STereoLithographic (STL) model using an in‐house developed algorithm and sent to a commercial fused deposition modeling (FDM) printer. Only shells were printed with polylactic acid (PLA) material. Liquid UR was then placed in a vacuum pump and slowly poured into the hollow bolus from its top opening. Liquid UR hardened in around half an hour. The phantom was rescanned with custom boluses attached and the dosimetry was compared with original design in TPS. Basic CT and dose properties were investigated. GaF films were irradiated to measure dose profile and output of several open photon and electron beams under solid water and UR slabs of same thicknesses.Results:CT number was 11.2±7.3 and 65.4±7.8, respectively for solid water(∼1.04g/cc) and UR(∼1.08g/cc). The output measurement at dmax for 6× was within 2% for the two materials. The relative dose profiles of the two materials above dmax show 94–99% Gamma analysis passing rates for both photons and electrons. Dose distributions with 3D PUR boluses maintained great coverage on the intended skin regions and resembled that with computer generated boluses. Manufacturing 3D PUR boluses was 3–4 times faster than 100% printed boluses. The efficiency significantly improves for larger boluses.Conclusion:The study suggests UR has similar dose responses as solid water. Making custom bolus with UR greatly increases clinical workflow efficiency.

Benchmarking Monte Carlo simulations against experimental data in clinically relevant passive scattering proton therapy beamline configurations

Monte Carlo simulations are widely used to calculate secondary neutron doses in proton therapy. When using the passive scattering technique, a small variation in the tumor size or location requires a full adjustment of the beamline configuration. This work thus focuses on modeling all possible beamline elements of the local facility with the MCNPX code while covering the entire clinical range of proton beam energies. This paper documents the experimental validation of realistic beamline configurations dedicated to intracranial tumor treatments, with different sources and beamline elements. Simulations were compared against measurements and treatment planning system (TPS) data considering percentage depth dose distributions (PDD) and relative lateral dose profiles. The results show that simulated and measured PDD distributions differ by no more than 1.6 mm on the proton beam range (tolerance set at 2 mm/2%) for the 162 MeV configuration. Meanwhile, for the modulated lateral dose profiles, the 219 MeV configuration presented the largest difference for the in-plane field width at 90% with 2 mm difference in measurements. The modeled proton sources and beamline elements were validated, and the new realistic geometries can be used to calculate secondary neutron doses to healthy tissues.

On the development of a comprehensive MC simulation model for the Gamma Knife Perfexion radiosurgery unit

This work presents a comprehensive Monte Carlo (MC) simulation model for the Gamma Knife Perfexion (PFX) radiosurgery unit. Model-based dosimetry calculations were benchmarked in terms of relative dose profiles (RDPs) and output factors (OFs), against corresponding EBT2 measurements. To reduce the rather prolonged computational time associated with the comprehensive PFX model MC simulations, two approximations were explored and evaluated on the grounds of dosimetric accuracy. The first consists in directional biasing of the 60Co photon emission while the second refers to the implementation of simplified source geometric models. The effect of the dose scoring volume dimensions in OF calculations accuracy was also explored. RDP calculations for the comprehensive PFX model were found to be in agreement with corresponding EBT2 measurements. Output factors of 0.819 ± 0.004 and 0.8941 ± 0.0013 were calculated for the 4 mm and 8 mm collimator, respectively, which agree, within uncertainties, with corresponding EBT2 measurements and published experimental data. Volume averaging was found to affect OF results by more than 0.3% for scoring volume radii greater than 0.5 mm and 1.4 mm for the 4 mm and 8 mm collimators, respectively. Directional biasing of photon emission resulted in a time efficiency gain factor of up to 210 with respect to the isotropic photon emission. Although no considerable effect on relative dose profiles was detected, directional biasing led to OF overestimations which were more pronounced for the 4 mm collimator and increased with decreasing emission cone half-angle, reaching up to 6% for a 5° angle. Implementation of simplified source models revealed that omitting the sources’ stainless steel capsule significantly affects both OF results and relative dose profiles, while the aluminum-based bushing did not exhibit considerable dosimetric effect. In conclusion, the results of this work suggest that any PFX simulation model should be benchmarked in terms of both RDP and OF results.