CC13 Ion Chamber Research Articles

The role of volume averaging effects, beam hardening, and phantom scatter in dosimetry of grid therapy

Abstract Objective: Current reference dosimetry methods for spatially fractionated radiation therapy (SFRT) assume a negligible beam quality change, perturbation, or volume-averaging correction factor. Therefore, the aim of this work was to investigate the impact of the grid collimators on the dosimetric characteristics of a 6 MV photon beam. A detector-specific correction factor, k_(Q_grid,Q_msr)^(f_grid,f_msr ), was proposed. Several dosimeters were evaluated for their ability to measure both reference dose and grid output factors (GOFs).
Approach: A Monte Carlo model of a grid collimator was created to study the change in the depth dose characteristics with the grid collimator. The impact of the collimator on the percent depth dose (PDD), electron contamination, and average photon energy was investigated. The k_(Q_grid,Q_msr)^(f_grid,f_msr) correction factors were calculated for two reference-class micro ion chambers. The reference dose and GOFs were measured with a grid collimator using six ion chambers, two silicon diodes, and a diamond detector.
Main results: The PDD in the presence of the grid was observed to be steeper compared to the open field. The average photon energy increased from 1.33 MeV to 1.74 MeV with the presence of the grid collimator. The dose contribution by scattered photons was significantly higher at deeper regions for the open field compared to the grid field. The k_(Q_grid,Q_msr)^(f_grid,f_msr) correction was calculated to be <0.5%. The reference dose for all detectors, except for the CC13 and CC04 chambers, was within 1% of each other. The CC13 under-responded up to 3.2% due to volume-averaging effects. The GOFs calculated for all detectors, except Razor and A16, were within 1% of each other.
Significance: The phantom scatter dictates the change in the PDD with the presence of the grid. The micro ion-chambers exhibit negligible k_(Q_grid,Q_msr)^(f_grid,f_msr) correction. All detectors, except the CC13 ion chamber, were found to be suitable for SFRT reference dosimetry.

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

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

Dosimetric verification of annual quality assurance for a linear accelerator using a transmission type detector

The purpose of our study is to establish an efficient quality assurance (QA) procedure using a transmission-type detector (IBA, Stealth chamber), a reference signal detector, as a field chamber. Relative dosimetry items, including monitor unit linearity, output constancy based on dose rate and field size, and output factor were measured and compared with results obtained from the Farmer-type chamber (IBA, Wellhofer, FC65-G). Moreover, output for each field size was measured to assess its applicability to small fields. Results using the Stealth chamber were in good agreement with the FC65-G within 1.0%, except for output constancy according to gantry angle, which had a 1.1% error rate for the Stealth chamber and 2.7% for the FC65-G. Differences of up to − 6.26% output factor were observed for the Stealth chamber and up to − 0.56% for the CC-13 ionization chamber (IBA) in the 3 × 3 cm2 field. Our study confirmed the possibility of using Stealth chambers for relative dosimetry measurement in QA.

Cylindrical ionization chamber response in static and dynamic 6 and 15 MV photon beams

Purpose. To investigate the response of the CC13 ionization chamber under non-reference photon beam conditions, focusing on penumbra and build-up regions of static fields and on dynamic intensity-modulated beams. Methods. Measurements were performed in 6 MV 100 × 100, 20 × 100, and 20 × 20 mm2 static fields. Monte Carlo calculations were performed for the static fields and for 6 and 15 MV dynamic beam sequences using a Varian multi-leaf collimator. The chamber was modelled using EGSnrc egs_chamber software. Conversion factors were calculated by relating the absorbed dose to air in the chamber air cavity to the absorbed dose to water. Correction and point-dose correction factors were calculated to quantify the conversion factor variations. Results. The correction factors for positions on the beam central axis and at the penumbra centre were 0.98–1.02 for all static fields and depths investigated. The largest corrections were obtained for chamber positions beyond penumbra centre in the off-axis direction. Point-dose correction factors were 0.54–0.71 at 100 mm depth and their magnitude increased with decreasing field size and measurement depth. Factors of 0.99–1.03 were obtained inside and near the integrated penumbra of the dynamic field at 100 mm depth, and of 0.92–0.94 beyond the integrated penumbra centre. The variations in the ionization chamber response across the integrated dynamic penumbra qualitatively followed the behaviour across penumbra of static fields. Conclusions. Without corrections, the CC13 chamber was of limited usefulness for profile measurements in 20-mm-wide fields. However, measurements in dynamic small irregular beam openings resembling the conditions of pre-treatment patient quality assurance were feasible. Uncorrected ionization chamber response could be applied for dose verification at 100 mm depth inside and close to large gradients of dynamically accumulating high- and low-dose regions assuming 3% tolerance between measured and calculated doses.

Calorimeter measurements of absolute dose in aluminum, a surrogate of bone, to validate dose-to-medium in Acuros XB

Objective. While the accuracy of dose calculations in water with Acuros XB is well established, experimental validation of dose in bone is limited. Acuros XB reports both dose-to-medium and dose-to-water, and these values differ in bone, but there are no reports of measurements of validation in bone. This work compares Acuros XB calculations to measurements of absolute dose in aluminum (medium similar to bone). The validity of using selected relative dosimeters in aluminum is also investigated. Approach. A calorimeter with an aluminum core embedded in an aluminum phantom was selected as bone surrogate for the measurement of absolute dose. Matching the medium of the core to the medium of the phantom allowed eliminating the calculation of the conversion between media. The dose was measured at the fixed depth of 3.3 cm in aluminum (∼9 g·cm−2) with 6X, 10X, 6FFF and 10FFF photon beams from a TrueBeam Varian linac. In addition, experimental cross-calibration between water and aluminum was performed for an IBA CC13 ionization chamber, a PTW microDiamond and EBT3 Gafchromic film. Main results. Calculations with Acuros XB dose-to-medium in aluminum differed from the calorimetry data by −2.8% to −3.5%, depending on the beam. Use of dose-to-water would have resulted in about 39% discrepancy. The cross calibration coefficient between water and aluminum yielded values of about 0.87 for the CC13 chamber, 0.91 for the microDiamond, and 0.88 for the film, and independent of the beam within about ±1%. Significance. It was demonstrated the value of the dose-to-medium in aluminum (surrogate of bone) computed with Acuros XB is close to the value of the absolute dose measured with a calorimeter, and there is a significant discrepancy when dose-to-water is used instead. The use of an ionization chamber, a microDiamond and Gafchromic film in aluminum required a considerable correction from calibration in water.

Validation of Dosimetric Comissioning Accuracy of IMRT for Versa HD Linear Accelerator with AAPM TG-119 protocol

Purpose: Verification of the IMRT prior to the introduction the new Versa HD (Elekta) Linac into operation using test examples given in the AAPM TG-119 report.
 Materials and methods: The verification of plans created both on the basis of the dose prescriptions given in the AAPMTG-119 report and on the basis of the constraints used in the NMRRC was made. The
 plans were verified using a matrix of ionization chambers (MatriXX Evolution) placed in a mini Phantom
 (IBA Dosimetry), using gamma criteria of 3 %/3 mm and 3 %/2 mm at a dose threshold of 10 %. In addition to checking the coincidence of the calculated and measured dose distribution in the plane, a
 comparison of the dose at the point calculated in the Monaco planning system (Elekta) and measured
 by the CC13 ionization chamber (0.13 cm3
 , Iba Dosimetry) inside the PTV and in the organ at risk was
 carried out. 
 Results: The difference in the dose measured by the ionization chamber and calculated in the planning
 system was less than 2 %, and correlates well with the results of accredited clinics. The measured dose
 distribution in the plane well (97-100 % of the points) coincided with the calculated one for all localizations. An exception was the test case with several targets, in which the assessment was carried out
 according to the 3 %/2 mm gamma criterion. 
 Conclusions: The results obtained when testing IMRT according to the protocol described in the AAPM
 TG-119 report, can be used to compare their results with the results of other accredited clinics.

Monte Carlo Modeling and Verification of 6 MV Linear Accelerator

Purpose: To model the ELEKTA COMPACT accelerator head by using EGSnrc/BEAMnrc/DOSXYZnrc and to validatethe simulation according to the depth-dose and lateral profiles of different radiation fields measured by the water phantom. Methods: IBA Blue Water Phantom2 and CC13 Ionization Chamber were used to measure the depth-dose curves at 10 cm × 10 cm field and profile curves at 10 cm depth underwater. In BEAMnrc, the main components of accelerator head and the initial electron beam are established based on the specifications file, and the phase space file containing the photon beam information is generated. In DOXYZnrc, phase space files were used to irradiate a homogeneous water phantom of the same size as the IBA water phantom, and the simulated percentage depth dose curves and lateral profiles were outputted. The accuracy of the model was evaluated by mean square error (MSE) compared with the measured data. PDD curves are used to determine the energy of the initial electron beam. Dose profile curves are used to adjust the flattening filter. The penumbra on lateral profiles is used to adjust the full-width half-maximum (FWHM) of the electron source. Result: The electron energy of 5.8 MeV was considered the best match after comparing the PDD curves of 5.6 - 6.2 MeV electron beams. The flattening filter can only be adjusted by trial. In the final result, the maximum fluctuation of profile curve within 80% of the maximum field size is less than 3%, which meets the requirements of field flatness. The optimum FWHM for different fields is not consistent due to the Transmission penumbra. But a match can be approached by adjusting the FWHM every 10 cm field size.

Dosimetry with a clinical linac adapted to FLASH electron beams.

PurposeTo assess dosimetric properties and identify required updates to commonly used protocols (including use of film and ionization chamber) pertaining to a clinical linac configured into FLASH (ultra‐high dose rate) electron mode.MethodsAn 18MV photon beam of a Varian iX linac was converted to FLASH electron beam by replacing the target and the flattening filter with an electron scattering foil. The dose was prescribed by entering the MUs through the console. Fundamental beam properties, including energy, dose rate, dose reproducibility, field size, and dose rate dependence on the SAD, were examined in preparation for radiobiological experiments. Gafchromic EBT‐XD film was evaluated for usability in measurements at ultra‐high dose rates by comparing the measured dose to the inverse square model. Selected previously reported models of chamber efficiencies were fitted to measurements in a broad range of dose rates.ResultsThe performance of the modified linac was found adequate for FLASH radiobiological experiments. With exception of the increase in the dose per MU on increase in the repetition rate, all fundamental beam properties proved to be in line with expectations developed with conventional linacs. The field size followed the theorem of similar triangles. The highest average dose rate (2 × 104 Gy/s) was found next to the internal monitor chamber, with the field size of FWHM = 1.5 cm. Independence of the dose readings on the dose rate (up to 2 × 104 Gy/s) was demonstrated for the EBT‐XD film. A model of recombination in an ionization chamber was identified that provided good agreement with the measured chamber efficiencies for the average dose rates up to at least 2 × 103 Gy/s.ConclusionDosimetric measurements were performed to characterize a linac converted to FLASH dose rates. Gafchromic EBT‐XD film and dose rate‐corrected cc13 ionization chamber were demonstrated usable at FLASH dose rates.

Monte Carlo calculated detector-specific correction factors for Elekta radiosurgery cones

A radiation field is considered small if its dimension is lower than the range of secondary electrons and the collimating devices partially occlude the source. Different detector types, such as unshielded diodes, diamond detectors, and small-volume ion chambers, are used for small-field measurements. Although the active volumes of these detectors are small, their non-water equivalent materials cause response variations. Herein, we aim to calculate the correction factors for our clinical detectors, EDGE detector (Sun Nuclear), 60017 diode (PTW), and CC01 ion chamber (IBA), for stereotactic radiosurgery cones of diameters of 5–15 mm in an Elekta Synergy linear accelerator using a Monte Carlo simulation. An Elekta Synergy linear accelerator treatment head was simulated using BEAMnrc Monte Carlo code as per the manufacturer specification. All three detectors were simulated as per the manufacturer specification. Three EGSnrc user codes were used for the detector simulation based on the detector geometry. The Monte Carlo model of the treatment head was validated against the measured data for a standard field size of 10 × 10 cm2. The off-axis profile, percentage depth dose, and tissue phantom ratio were verified in the validation procedure. The measured and Monte Carlo calculated relative output factors (ROFs) were not consistent. In a 5 mm field size, EDGE diode overestimated the ROF by 7.06%, and 60017 diode to 4.611%. In a 7.5 mm field size, the variations were 4.295% and 3.691% for EDGE and 60017 diodes, respectively. CC01 ion chamber under-responded up to 10% because of its low-density active volume. The maximum corrections were obtained in the smallest field size, which were 0.939(0.007), 0.962(0.006), and 1.117(0.008) for EDGE, PTW T60017, and CC01 detectors, respectively. After applying the Monte Carlo calculated correction factor to the measured ROF, it became consistent with the Monte Carlo calculated ROF.

Khảo sát một số đặc tính bức xạ của buồng ion hóa CC13 và diode bán dẫn razor đối với các chùm photon kích thước nhỏ trên máy gia tốc tuyến tính xạ trị Clinac IX

Purpose: Compare percent depth dose (PDD) and off-center ratio (OCR) measured by the CC13 ionization chamber and the RAZOR silicon diode in small photon beams. Method and Materials: Some dosimetric characteristics, such as PDD, OCR, penumbra and radiation field size, were considered in this study for 2x2, 3x3, and 4x4 cm2 field sizes. We used the CC13 ionization chamber and the RAZOR silicon diode to measure dose distribution with depth along the axis and off-center of the beam. From the results obtained, the team investigated the differences in radiation parameters measured by the two types of probes above. Results: There are significant differences in the radiation parameters investigated for the CC13 ionization chamber and the RAZOR silicon diode, especially the width of penumbra. For PDD curves, the difference is less than 5% from dmax to 30 cm, however the difference becomes greater in the build-up region, which reaches to 33% at the water phatom surface. The width of penumbra measured by CC13 is always larger than that of RAZOR, the ratio of the penumbra width between two detectors is 1.8 and 1.3 for energies of 6 MV and 15 MV, respectively. Conclusion: The RAZOR silicon diode has better dose response than the CC13 ionization chamber for measuring the PDD and the OCR in small photon beams.

VMAT Based TBI Prior to BMT in Malignant & Non-malignant Disorders – Single Institution Experience

The purpose of this paper is to report the feasibility of Total Body Irradiation (TBI) by multiple Iso-centric Volumetric Modulated Arc Therapy (VMAT) technique before Bone Marrow Transplant (BMT) in malignant & Non-malignant, hematological & non-hematological disorders. Total of 5 patients, 3 with ALL, 1each with Hurlor syndrome and Thalasemia Major were recruited for the TBI in our hospital from April 2018 to May 2019 before going for BMT. Immobilizations were done with thermoplastic masks and vaclocs for all patients in SBRT Multi-board. HFS&FFS two sets of CT scans done for 4 patients having more than 120 cm in heights with dosimetric reference marker in junction and only HFS scan done for a patient less than 120 cm in height. Dose prescribed to PTV excluding lung and kidneys was 12Gy in 6 fractions over 3 days (2 fractions per day) for ALL, single dose of 3 Gy for Hurler syndrome and 4 Gy in 2 fractions for Thalasemia Major. Treatment planning was done using VMAT technique in a treatment planning system. It consisted of average of 13 arcs for ALL and 8 arcs for other two patients with multiple isocenters and small static fields used to enhance the dose coverage. The best treatment plans were selected ensuring 90% PTV dose coverage and acceptable dose to OARs. Treatment executed with Linear accelerator equipped with OBI, CBCT used as Image Guided Radiation Therapy (IGRT) for each VMAT delivery. Quality assurance performed with mini-phantom and point dose measurements done with slab phantom and CC13 ion chamber followed by in vivo dosimetry performed with Optically Stimulated Luminescence Dosimeter nano chips. Contouring and planning time took 15-20 hours and quality assurance took 4-5 hours for each patient. PTV coverage of 90% achieved with PTV Dmean of 12.54 Gy for ALL patients and 3.02&4.32 respectively in other two cases. OAR average doses of lungs, kidneys and liver reduced to 9.8, 9.5 & 10.3 Gy respectively and lens restricted to 6 Gy for ALL patients and no OAR constraints for single and dual fraction low dose prescription patients. On couch treatment time on day one was 2.5 hours including setup and mounting of OSLD chips and irradiation for all cases. Further fractions time reduced to 1.5 hrs. 3 sets of CBCT performed for head first with average setup error of ±0.5 mm and two CBCT scans performed for feet first with average setup error of ±0.3 mm. OSLD dosimeter readings showed the results of within ±5% deviation. TBI by VMAT technique is feasible and provides satisfactory 3D dose distribution within PTV and reducing the dose to the critical OARs. VMAT provides a homogenous dose to the whole body, verified & confirmed by in vivo dosimetry. VMAT-TBI has advantage over extended SSD based conventional TBI methods in resource limited community as it can be used in regular radiotherapy department without requiring dedicated accessories.

Validation of Dosimetric Commissioning Accuracy of IMRT and Rapidarc for Halcyon Linear Accelerator Using AAPM TG-119 Protocol

The purpose of the study is to evaluate the end-to-end commissioning accuracy of IMRT and RapidArc for Halcyon linear accelerator based on test cases developed by AAPM Task Group 119 protocol. IMRT dynamic (DMLC) and RapidArc plans were created for TG119 test cases using phantom with contour structure that downloaded from AAPM website. All the plans were generated using a treatment planning system (TPS) for halcyon linear accelerator. All plans were designed using 7-9 beams for IMRT and 2-4 arcs for RapidArc with beam energy of 6MV-FFF (FFF, Flattening filter free). The prescription and planning goals were as kept as per TG119. For point dose measurement CC13 (0.13cc) ion chamber and MVP (Mobius Verification Phantom) were used and measurements were carried out as per TG119 specified points in high gradient regions. Point dose difference was calculated as ratio of difference between measured and planned dose with prescription dose. Similarly, for composite planar dose measurement an independent quality assurance tool was used for measurement. The per-field relative gamma was measured using EPID (electronic portal imaging device) in a way similar to the routine pretreatment patient-specific quality assurance. Planned and measured dose planes were compared based on γ-criteria of 3%/3mm-G (global normalization), and a more stringent 2%/2mm-L (local normalization). All measurements were transferred with the original gantry angle. Confidence limit calculation was done as specified in TG119. At high dose point measurement mean dose differences averaged and corresponding confidence limit were -0.003±0.011 and 0.025 for the IMRT plans, -0.012±0.006 and 0.024 for the RapidArc plans respectively. Planar dose measurement using an independent quality assurance tool gamma passing rate averaged was 99.72%±0.24 for 3%/3mm criteria and 96.80%±1.17 for 2%/2mm criteria for IMRT plans, 99.70%±0.45 for 3%/3mm criteria and 95.24%±2.83 for 2%/2mm criteria for RapidArc plans, respectively. Confidence limit were 0.75 and 1.19 for 3%/3mm and 5.49 and 10.31 for 2%/2mm criteria for IMRT and RapidArc plans. The Confidence limit using 2%/2 mm gamma criteria using electronic portal imaging device were 98.80%±0.01 and 96.60%±0.02 for IMRT and RapidArc plans, respectively. TG 119 report have successfully been used to Validation of dosimetric commissioning accuracy of IMRT and RapidArc for Halcyon linear accelerator. The Confidence limit value of the study could be used as baseline for future patient specific quality assurance. From the obtained data in this study, we conclude that the commissioning of IMRT and VMAT delivery were found within the limits of TG-119.

Technical Note: Dosimetric characterization of the dynamic beam flattening MLC sequence on a ring shaped, Jawless Linear Accelerator with double stacked MLC.

To characterize the dosimetric features and limitations of the dynamic beam flattening (DBF) on the Halcyon 2.0 linear accelerator (Varian Medical Systems). A predefined multi-leaf collimator (MLC) sequence was introduced and used to flatten the 6MV flattening filter free (FFF) beam on the Halcyon 2.0. Dosimetric characterizations of the flattened beams, including beam flatness, symmetry, percent depth dose (PDD), output factor and MU linearity, were investigated. Flatness and symmetry were obtained from profile measurements with both radiographic films (EDR2) and a two dimensional ion-chamber array (IC Profiler, Sun Nuclear Corporation). MU linearity, output factors, and PDDs were measured in a water tank with a CC13 ion chamber (Scanditronix Wellhöfer, Nuremburg, Germany). In addition, the effect of the DBF sequence on 3D plan quality was evaluated by creating DBF plans for a 4-field box rectum and an AP/PA spine plan. Patient specific QA was performed on these plans. At 100cm SSD and 10cm depth, a flatness of <3% was observed on both transversal and radial profiles for all square field sizes ≥10cm with DBF. For both larger and smaller field sizes the flatness showed a tendency to increase as the fields got bigger or smaller, respectively. Similar trends in flatness were observed at all depths measured. All measured output factors for square field sizes ≥5cm were within 1% of the TPS prediction. Linearity was ≤2.02% for all measurements. For both treatment sites, the MD judged the plans created for the Halcyon without the use of DBF not to be clinically acceptable, however considered both the TrueBeam plan and the Halcyon plan with the DBF sequence to be clinically acceptable. The DBF sequence on the Halcyon and its characteristics were investigated. The analysis indicates that the DBF sequence can be used on the Halcyon to generate clinically acceptable 3D treatment plans.

Wedge angle confirmation in computer controlled wedge field

Aim: The use of computer controlled wedge system is an important segment of radiotherapy and increases the uniformity of dose in the target volume. The aim of this study is to verify the virtual wedge angles from the machine setup angles in Siemens ONCOR Linear accelerator (Linac) and compare with published data of different linear accelerators as a function of beam energy and field sizes. Method and material: This experiment was carried out on Siemens ONCOR impression linear accelerator (Linac). The doses at different depth were measured by using CC13 ion chamber. During our work the source to surface distance was kept 100 cm. The square field sizes on which we worked were 10 cm2, 15cm2 and 20 cm2.The selected Virtual wedge angles for our study are 15°, 30°, 45° and 60°.This work is carried out for both photon energies 15 MV and 6 MV, tissue equivalent water phantom IBA blue water phantom inside which all the observations were taken. The LDA 99 detector for virtual wedge profile was used. The wedge angle were calculated for the Siemen’s given formula. The variation in wedge angle from machine setup angle and published data as a function of beam energy and field sizes were analyzed. Results: The variation increases with field size and wedge angle but decreases with beam energy. Conclusion: Deviations are under 3% which are acceptable before treatment planning.

Design, Fabrication, and Validation of a Polymethyl Methacrylate Head Phantom for Dosimetric Verification of Cranial Radiotherapy Treatment Plans.

Purpose:The present study aims to design and fabricate a novel, versatile, and cost-effective Polymethyl Methacrylate (PMMA) head phantom for the dosimetric pretreatment verification of radiotherapy (RT) treatment plans.Materials and Methods:The head phantom designing involves slice-wise modeling of an adult head using PMMA. The phantom has provisions to hold detectors such as ionization chambers of different sizes, Gafchromic films, gel dosimeter, and optically stimulated luminescence dosimeter. For the point dose verification purpose, 15 volumetric modulated arc therapy patient plans were selected, and doses were measured using a CC13 ionization chamber. The percentage gamma passing rate was calculated for acceptance criteria 3%/3 mm and 2%/2 mm using OmniPro I’mRT film QA software, and Gafchromic EBT3 films were used for 2D planar dose verification.Results:Treatment planning system calculated, and the measured point doses showed a percentage deviation ranged from 0.26 to 1.92. The planar dose fluence measurements, for set acceptance criteria of 3%/3 mm and 2%/2 mm, percentages of points having gamma value <1 were in the range of 99.17 ± 0.25 to 99.88 ± 0.15 and 93.16 ± 0.38 to 98.89 ± 0.23, respectively. Measured dose verification indices were within the acceptable limit.Conclusions:The dosimetric study reveals that head phantom can be used for routine pretreatment verification for the cranial RT, especially for stereotactic radiosurgery/RT as a part of patient-specific quality assurance. The presently fabricated and validated phantom is novel, versatile, and cost-effective, and many institutes can afford it.

Evaluation of the Differences Between Measurements in Multiple Institutions and Calculation Modeled by Representative Beam Data in Prostate VMAT Plan.

This study aimed to investigate the potential differences between multi-institutional measurements and treatment planning system (TPS) calculation modeled by representative beam data for patient-specific quality assurance (QA), including multi-leaf collimator (MLC) parameters. Eleven TrueBeam from nine institutions were used in this study. Volumetric arc therapy (VMAT) plan for verification was created using Eclipse. The point dose of the CC13 ionization chamber and the dose distribution of the GAFCHROMIC EBT3 film were measured and analyzed. Point dose differences in patient-specific QA provided a mean±standard deviation of 1.0%±0.6%. Mean gamma pass rates of dose distribution were in excess of 99% and 96% for 3%/2 mm and 2%/2 mm gamma criteria, respectively. There was good agreement between measurements and calculations, indicating the small influence of complex VMAT in the underlying processes. Therefore, implementation of the same MLC parameters on TPS among different institutions with the same planning policy should be considered to ensure consistency and efficiency in radiation treatment processes.

Feasibility of a GATE Monte Carlo platform in a clinical pretreatment QA system for VMAT treatment plans using TrueBeam with an HD120 multileaf collimator.

PurposeTo evaluate the quality of patient‐specific complicated treatment plans, including commercialized treatment planning systems (TPS) and commissioned beam data, we developed a process of quality assurance (QA) using a Monte Carlo (MC) platform. Specifically, we constructed an interface system that automatically converts treatment plan and dose matrix data in digital imaging and communications in medicine to an MC dose‐calculation engine. The clinical feasibility of the system was evaluated.Materials and MethodsA dose‐calculation engine based on GATE v8.1 was embedded in our QA system and in a parallel computing system to significantly reduce the computation time. The QA system automatically converts parameters in volumetric‐modulated arc therapy (VMAT) plans to files for dose calculation using GATE. The system then calculates dose maps. Energies of 6 MV, 10 MV, 6 MV flattening filter free (FFF), and 10 MV FFF from a TrueBeam with HD120 were modeled and commissioned. To evaluate the beam models, percentage depth dose (PDD) values, MC calculation profiles, and measured beam data were compared at various depths (Dmax, 5 cm, 10 cm, and 20 cm), field sizes, and energies. To evaluate the feasibility of the QA system for clinical use, doses measured for clinical VMAT plans using films were compared to dose maps calculated using our MC‐based QA system.ResultsA LINAC QA system was analyzed by PDD and profile according to the secondary collimator and multileaf collimator (MLC). Values for MC calculations and TPS beam data obtained using CC13 ion chamber (IBA Dosimetry, Germany) were consistent within 1.0%. Clinical validation using a gamma index was performed for VMAT treatment plans using a solid water phantom and arbitrary patient data. The gamma evaluation results (with criteria of 3%/3 mm) were 98.1%, 99.1%, 99.2%, and 97.1% for energies of 6 MV, 10 MV, 6 MV FFF, and 10 MV FFF, respectively.ConclusionsWe constructed an MC‐based QA system for evaluating patient treatment plans and evaluated its feasibility in clinical practice. We observed robust agreement between dose calculations from our QA system and measurements for VMAT plans. Our QA system could be useful in other clinical settings, such as small‐field SRS procedures or analyses of secondary cancer risk, for which dose calculations using TPS are difficult to verify.

The effect of SSD, Field size, Energy and Detector type for Relative Output Factor measurement in small photon beams as compared with Monte Carlo simulation

Abstract Introduction: Small fields photon dosimetry is associated with many problems. Using the right detector for measurement plays a fundamental role. This study investigated the measurement of relative output for small photon fields with different detectors. It was investigated for three-photon beam energies at SSDs of 90, 95, 100 and 110 cm. As a benchmark, the Monte Carlo simulation was done to calculate the relative output of these small photon beams for the dose in water. Materials and Methods: 6, 10 and 15 MV beams were delivered from a Synergy LINAC equipped with an Agility 160 multileaf collimator (MLC). A CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film, and EDR2 radiographic film were used to measure the relative output of the linac. Measurements were taken in water for the CC01 ion chamber, EFD-3G diode, and the PTW60019. Films were measured in water equivalent RW3 phantom slabs. Measurements were made for 1 × 1, 2 × 2, 3 × 3, 4 × 4, 5 × 5 and a reference field of 10 × 10 cm2. Field sizes were defined at 100cm SSD. Relative output factors were also compared with Monte Carlo (MC) simulation of the LINAC and a water phantom model. The influence of voxel size was also investigated for relative output measurement. Results and Discussion: The relative output factor (ROF) increased with energy for all fields large enough to have lateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due to the onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gave the highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs. Conclusion: These are results compared to MC simulation, higher density detectors give higher ROF values. Relative to water, the ROF measured with the air-chamber remained virtually unchanged. The ROFs, as measured in this study showed little variation due to increased SSDs. The effect of voxel size for the Monte Carlo calculations in water does not lead to significant ROF variation over the small fields studied.

A method of measuring the field output factor based on daisy–chaining

Objective improve the accuracy of the measurement results by using the field output factor measurement method based on daisy-chaining. Methods The Varian Edge Accelerator 6 MV X-ray data were measured using the IBA CC13 ionization chamber, IBA CC01 ionization chamber, IBA Razor semiconductor detector, IBA EFD semiconductor detector and Gafchromic EBT3 film, respectively. Results Compared with the daisy-chaining measurement method, the results obtained by the conventional measurement method using CC13 were smaller. The deviation value was 16.71% in the 1 cm × 1 cm field. The measurement results in a larger field via CC01 were bigger with a deviation of 8.39% in the 40 cm × 40 cm filed. The measurement results via Razor in a larger field were larger with a deviation of 9.40% in the 40 cm × 40 cm field. The measurement results were similar between EFD and Razor with a deviation of 9.14% in the 40 cm × 40 cm field. The results of the film measurement were equivalent to those obtained from the daisy-chaining method in a field of> 1 cm × 1 cm with a deviation within 1.60%, whereas the deviation was increased to 3.13% in the 1 cm × 1 cm field. The results were consistent with daisy-chaining measurement if the 3 cm × 3 cm or 4 cm × 4 cm fields were selected as the intermediate fields with the maximum deviation of 0.29%. Conclusions For the detectors with changing response along with the field size, daisy-chaining measurement method can be utilized to extend the measurement range and improve the accuracy of the measurement results. Key words: Daisy-Chaining; Output factor; Measurement method

The evaluation of change in attenuation coefficient of cerrobend block used for radiation protection of healthy tissues in megavoltage photon radiation therapy after multiple melting

Introduction: Protecting the vital organs in radiation therapy is one of the most important issues. Cerrobend alloy, the most common material used as the shield, is melted several times and used for different patients. In this study changes in attenuation coefficient of cerrobend due to melting was investigated. Materials and Methods: In melting furnace, cerrobend was melted up to nine times and irradiated by Varian accelerator at radiotherapy department of Golestan hospital and the dose was measured by CC-13 ionization chamber with and without cerrobend blocks. The attenuation coefficients of cerrobend blocks were measured for irradiation fields of 6 × 6 cm2 and 10×10 cm2 at the photon energies of 6 mv and 18mv and analyzed by statistical methods. Results: Results of regression analysis with P<0.05, indicated that there was a significant relationship between the attenuation coefficient and frequencies of cerrobend melting: the attenuation coefficient increases with the increasing frequency of cerrobend melting. Conclusion: Continuous increasing in the frequency of cerrobend melting leads to increase in the attenuation coefficient of cerrobend block, thus cerrobend blocks can be used for patients, safely.