Pretreatment Dose Verification Research Articles

Assessment of a 2D electronic portal imaging devices-based dosimetry algorithm for pretreatment and in-vivo midplane dose verification.

Background:The use of electronic portal imaging devices (EPIDs) is a method for the dosimetric verification of radiotherapy plans, both pretreatment and in vivo. The aim of this study is to test a 2D EPID-based dosimetry algorithm for dose verification of some plans inside a homogenous and anthropomorphic phantom and in vivo as well.Materials and Methods:Dose distributions were reconstructed from EPID images using a 2D EPID dosimetry algorithm inside a homogenous slab phantom for a simple 10 × 10 cm2 box technique, 3D conformal (prostate, head-and-neck, and lung), and intensity-modulated radiation therapy (IMRT) prostate plans inside an anthropomorphic (Alderson) phantom and in the patients (one fraction in vivo) for 3D conformal plans (prostate, head-and-neck and lung).Results:The planned and EPID dose difference at the isocenter, on an average, was 1.7% for pretreatment verification and less than 3% for all in vivo plans, except for head-and-neck, which was 3.6%. The mean γ values for a seven-field prostate IMRT plan delivered to the Alderson phantom varied from 0.28 to 0.65. For 3D conformal plans applied for the Alderson phantom, all γ1% values were within the tolerance level for all plans and in both anteroposterior and posteroanterior (AP-PA) beams.Conclusion:The 2D EPID-based dosimetry algorithm provides an accurate method to verify the dose of a simple 10 × 10 cm2 field, in two dimensions, inside a homogenous slab phantom and an IMRT prostate plan, as well as in 3D conformal plans (prostate, head-and-neck, and lung plans) applied using an anthropomorphic phantom and in vivo. However, further investigation to improve the 2D EPID dosimetry algorithm for a head-and-neck case, is necessary.

Pretreatment Dose Verification for Squamous Cell carcinoma of The Tongue

The aim of this study is to assess and evaluate the significant of performing patient's specification quality assurance (QA) for patients diagnosed with squamous cell carcinoma of the tongue (SCC) whom treated with intensity modulated radiation therapy (IMRT). The study was done in twelve pre- treatment's plans that been prepared for patients. All the ten selected plans are going to be treated with split-field (SF) technique for intensity modulated radiation therapy (IMRT) planning using 10 MV beams and a prescribed dose between 66Gy and 74 Gy. For quality assurance protocol we are using the two-dimensional ionization-chamber array. The study showed that an agreement between the measured dose and the pre-planned dose using TPS. All the plans passed >95% Gamma with the pixels within 5% distance to agreement 5 mm for IMRT patient-specific quality assurance (QA). It concludes that intensity modulated radiation therapy (IMRT) has the ability to deliver a highly conformal dose distribution to the planning target volume while sparing the organs at risk. However, the result showed a very good agreement between measurements dose and calculations dose which lead and proved that the planning using IMRT treatment are accurately done.

Noncoplanar verification: a feasibility study using Philips' Pinnacle3 treatment planning system.

Noncoplanar fields are normally used to improve the dose conformity of the target while sparing organs at risk. One of the methods to verify the dose distribution from the noncoplanar fields is by comparing their planar dose distributions from the treatment planning system (TPS) and the measured ones; for example, using film or electronic portal imaging devices (EPID). The planar dose distributions of the measurement tools, that are normally perpendicular to the central axis of the beam, can be calculated by creating special structures to mimic them in the TPS. With TPS commercially available today, however, it is not easy to create these special structures, especially in the noncoplanar configuration. For this work, we have written in‐house scripts in the Pinnacle3 TPS that can create the structures and define them as virtual planes. These virtual planes can be generated for any arbitrary gantry and couch angles, as well as source to planar distance, so that the planar dose maps at these planes can be computed. Two independent quality assurance (QA) tools were used to validate the planar dose distributions computed using the scripts for three open fields and one IMRT field at several different couch angles. The absolute planar dose patterns measured by the QA tools for all fields at all couch angles were found to be in good agreement, more than 95% (gamma criteria 3% delta dose and 3 mm distance to agreement), with the calculated ones by TPS. The results of this feasibility study can be valuable either for pretreatment dose verification or for in vivo dosimetry that directly implements the planar dose distributions from the TPS, particularly for the noncoplanar fields.PACS numbers: 87.55.de, 87.55.Qr, 87.56.Fc

Incorporation of gantry angle correction for 3D dose prediction in intensity-modulated radiation therapy.

Pretreatment dose verification with beam-by-beam analysis for intensity-modulated radiation therapy (IMRT) is commonly performed with a gantry angle of 0° using a 2D diode detector array. Any changes in multileaf collimator (MLC) position between the actual treatment gantry angle and 0° may result in deviations from the planned dose. We evaluated the effects of MLC positioning errors between the actual treatment gantry angles and nominal gantry angles. A gantry angle correction (GAC) factor was generated by performing a non-gap test at various gantry angles using an electronic portal imaging device (EPID). To convert pixel intensity to dose at the MLC abutment positions, a non-gap test was performed using an EPID and a film at 0° gantry angle. We then assessed the correlations between pixel intensities and doses. Beam-by-beam analyses for 15 prostate IMRT cases as patient-specific quality assurance were performed with a 2D diode detector array at 0° gantry angle to determine the relative dose error for each beam. The resulting relative dose error with or without GAC was added back to the original dose grid for each beam. We compared the predicted dose distributions with or without GAC for film measurements to validate GAC effects. A gamma pass rate with a tolerance of 2%/2 mm was used to evaluate these dose distributions. The gamma pass rate with GAC was higher than that without GAC (P = 0.01). The predicted dose distribution improved with GAC, although the dosimetric effect to a patient was minimal.

Clinical implementation and rapid commissioning of an EPID based in-vivo dosimetry system

Using an Electronic Portal Imaging Device (EPID) to perform in-vivo dosimetry is one of the most effective and efficient methods of verifying the safe delivery of complex radiotherapy treatments. Previous work has detailed the development of an EPID based in-vivo dosimetry system that was subsequently used to replace pre-treatment dose verification of IMRT and VMAT plans. Here we show that this system can be readily implemented on a commercial megavoltage imaging platform without modification to EPID hardware and without impacting standard imaging procedures. The accuracy and practicality of the EPID in-vivo dosimetry system was confirmed through a comparison with traditional TLD in-vivo measurements performed on five prostate patients.The commissioning time required for the EPID in-vivo dosimetry system was initially prohibitive at approximately 10 h per linac. Here we present a method of calculating linac specific EPID dosimetry correction factors that allow a single energy specific commissioning model to be applied to EPID data from multiple linacs. Using this method reduced the required per linac commissioning time to approximately 30 min.The validity of this commissioning method has been tested by analysing in-vivo dosimetry results of 1220 patients acquired on seven linacs over a period of 5 years. The average deviation between EPID based isocentre dose and expected isocentre dose for these patients was (−0.7 ± 3.2)%.EPID based in-vivo dosimetry is now the primary in-vivo dosimetry tool used at our centre and has replaced nearly all pre-treatment dose verification of IMRT treatments.

Time dependent pre-treatment EPID dosimetry for standard and FFF VMAT

Methods to calibrate Megavoltage electronic portal imaging devices (EPIDs) for dosimetry have been previously documented for dynamic treatments such as intensity modulated radiotherapy (IMRT) using flattened beams and typically using integrated fields. While these methods verify the accumulated field shape and dose, the dose rate and differential fields remain unverified. The aim of this work is to provide an accurate calibration model for time dependent pre-treatment dose verification using amorphous silicon (a-Si) EPIDs in volumetric modulated arc therapy (VMAT) for both flattened and flattening filter free (FFF) beams.A general calibration model was created using a Varian TrueBeam accelerator, equipped with an aS1000 EPID, for each photon spectrum 6 MV, 10 MV, 6 MV-FFF, 10 MV-FFF. As planned VMAT treatments use control points (CPs) for optimization, measured images are separated into corresponding time intervals for direct comparison with predictions. The accuracy of the calibration model was determined for a range of treatment conditions. Measured and predicted CP dose images were compared using a time dependent gamma evaluation using criteria (3%, 3 mm, 0.5 sec).Time dependent pre-treatment dose verification is possible without an additional measurement device or phantom, using the on-board EPID. Sufficient data is present in trajectory log files and EPID frame headers to reliably synchronize and resample portal images. For the VMAT plans tested, significantly more deviation is observed when analysed in a time dependent manner for FFF and non-FFF plans than when analysed using only the integrated field.We show EPID-based pre-treatment dose verification can be performed on a CP basis for VMAT plans. This model can measure pre-treatment doses for both flattened and unflattened beams in a time dependent manner which highlights deviations that are missed in integrated field verifications.

Sci-Sat AM: Stereo - 01: 3D Pre-treatment Dose Verification for Stereotactic Body Radiation Therapy Patients

Radical treatment techniques such as stereotactic body radiation therapy (SBRT) are becoming popular and they involve delivery of large doses in fewer fractions. Due to this feature of SBRT, a high-resolution, pre-treatment dose verification method that makes use of a 3D patient representation would be appropriate. Such a technique will provide additional information about dose delivered to the target volume(s) and organs-at-risk (OARs) in the patient volume compared to 2D verification methods. In this work, we investigate an electronic portal imaging device (EPID) based pre-treatment QA method which provides an accurate reconstruction of the 3D-dose distribution in the patient model. Customized patient plans are delivered ‘in air’ and the portal images are collected using the EPID in cine mode. The images are then analysed to determine an estimate of the incident energy fluence. This is then passed to a collapsed-cone convolution dose algorithm which reconstructs a 3D patient dose estimate on the CT imaging dataset. To date, the method has been applied to 5 SBRT patient plans. Reconstructed doses were compared to those calculated by the TPS. Reconstructed mean doses were mostly within 3% of those in the TPS. DVHs of target volumes and OARs compared well. The Chi pass rates using 3%/3mm in the high dose region are greater than 97% in all cases. These initial results demonstrate clinical feasibility and utility of a robust, efficient, effective and convenient pre-treatment QA method using EPID. Research sponsored in part by Varian Medical Systems.

Analysis of patient specific dosimetry quality assurance measurements in intensity modulated radiotherapy: a multi centre study.

Statistical analysis of pre-treatment dose verification of intensity-modulated radiotherapy (IMRT) fields to assess the quality of the IMRT practice at different radiotherapy centers. The dose verification data acquired by the institutional physicist of 10 different hospitals for various types of patients were collected and analyzed for mean, median, standard deviation (SD), range, minimum and maximum % deviation. The percentage of cases having positive and negative dose differences as well dose differences within ± 3% were also determined. The mean values of percentage variation in difference between treatment planning systems calculated dose and difference between measured dose (D(TPS) and D(Meas)) are found to be from -1.79 to 1.48 and median from -1.79 to 1.51. The SDs are found to be from 0.76 to 3.70. The range of variation at these centers varies from 3.99 to 16.45 while minimum and maximum values of percentage variation in difference between D(TPS) and D(Meas) ranges from -10.33 to 13.38. The percentage of cases having positive dose difference ranges from 8 to 94 and cases having negative dose difference ranges from 6 to 92. The percentage of cases having dose difference within ± 3% varies from 57 to 100. IMRT centers are having random and biased (skewed towards over or under dose) distribution of the percentage variation in difference between measured and planned doses. The analysis of results of the IMRT pre-treatment dose verification reveals that there are systematic errors in the chain of IMRT treatment process at a few centers. The dosimetry quality audit prior to commissioning of IMRT may play an important role in avoiding such discrepancies.

Pre-Treatment Dose Verification of Imrt Using Gafchromic Ebt3 Film and 2DArray

Purpose: The present work deals with pre-treatment dosimetric verification of 10 prostate intensity-modulated radiotherapy (IMRT) plans obtained by Gafchromic EBT3 film and 2D-Array seven29 to establish quality control protocol for the delivery of prostate IMRT in our clinic. Methods: 10 prostate patients were irradiated according to IMRT techniques with 6 MV photon beams produced by a Varian DHX linear accelerator. Dose plans were computed using Eclipse 8.9 (version8.9, Varian, Palo Alto, CA, United States) treatment planning system (TPS). Pre-treatment dosimetric verification was carried out on field-perfield and total IMRT plan basis measuring 2D dose distributions in RW3 solid water phantom (PTW-Freiburg, Germany). For 10 patient plans, the dose distribution was re-calculated with the phantom CT scan and delivered to the phantom with the original and 0 degrees gantry, collimator and table angles. Thus IMRT quality control (QC) plans were generated. Dose distribution measurements were measured with Gafchromic EBT3 film and 2D-Array seven29 (PTW-Freiburg, Germany) two dimensional ionization chamber system. Verification measurements of total IMRT plans and each individual beams were compared with expected dose maps, and differences were evaluated by Verisoft program (PTW-Freiburg, Germany). To provide comparisons of multidimensional dose distributions, dose comparison tools such as gamma dose distribution, distance- to- agreement (DTA) and dose difference (DD) have been developed. The gamma dose distribution tool was used in our study. Three different gamma criteria of dose difference (DD) and distance to agreement (DTA) (3%/3 mm, 4%/4 mm and 5%/5 mm DD / DTA) are selected. These criteria are evaluated while suppressing the dose of 5% from dose distribution. Criterion validity accepted as section with gamma value less than or equal to 1 (γ ≤ 1) to be 90%. Results: In the comparison of dose distributions obtained from TPS and the results of PTW 2D-Array seven29 and Gafchromic EBT3 film dosimetry systems, it was showed that the compatibility of both methods were above 90% with respect to 3% DD and 3mm DTA criteria. PTW 2D-Array seven29 results compared to the results of the film was closer to the TPS data and this difference was statistically meaningful (p 0.05). Conclusions: The measurements performed with PTW 2D-Array seven29 were closer to the TPS data than film measurements and it takes less time during clinical use, so it could be preferred for routine use. The present results suggest the gamma criteria of 3%/3 mm as the most suitable criteria for prostate IMRT quality assurance.

3D EPID-based in vivo dosimetry for IMRT and VMAT

In this paper the various approaches of EPID-based in vivo IMRT and VMAT dose verification, and their clinical implementation, are described. It will be shown that EPID-based in vivo dosimetry plays an important role in the total chain of verification procedures in a radiotherapy department. EPID-based dosimetry, in combination with in-room imaging, is a fast and accurate tool for 3D in vivo verification of VMAT delivery. EPID-based in vivo dosimetry provides clinically more useful information and is less time consuming than patient-specific pre-treatment dose verification. In addition to accurate 3D dose verification, in vivo EPID-based dosimetry will also detect major errors in the dose received by individual patients, and provides a safety net for advanced treatments such as IMRT and VMAT.

Dosimetry Investigation of MOSFET for Clinical IMRT Dose Verification

In IMRT, patient-specific dose verification is followed regularly at each centre. Simple and efficient dosimetry techniques play a very important role in routine clinical dosimetry QA. The MOSFET dosimeter offers several advantages over the conventional dosimeters such as its small detector size, immediate readout, immediate reuse, multiple point dose measurements. To use the MOSFET as routine clinical dosimetry system for pre-treatment dose verification in IMRT, a comprehensive set of experiments has been conducted, to investigate its linearity, reproducibility, dose rate effect and angular dependence for 6 MV x-ray beam. The MOSFETs shows a linear response with linearity coefficient of 0.992 for a dose range of 35 cGy to 427 cGy. The reproducibility of the MOSFET was measured by irradiating the MOSFET for ten consecutive irradiations in the dose range of 35 cGy to 427 cGy. The measured reproducibility of MOSFET was found to be within 4% up to 70 cGy and within 1.4% above 70 cGy. The dose rate effect on the MOSFET was investigated in the dose rate range 100 MU/min to 600 MU/min. The response of the MOSFET varies from -1.7% to 2.1%. The angular responses of the MOSFETs were measured at 10 degrees intervals from 90 to 270 degrees in an anticlockwise direction and normalized at gantry angle zero and it was found to be in the range of 0.98 ± 0.014 to 1.01 ± 0.014. The MOSFETs were calibrated in a phantom which was later used for IMRT verification. The measured calibration coefficients were found to be 1 mV/cGy and 2.995 mV/cGy in standard and high sensitivity mode respectively. The MOSFETs were used for pre-treatment dose verification in IMRT. Nine dosimeters were used for each patient to measure the dose in different plane. The average variation between calculated and measured dose at any location was within 3%. Dose verification using MOSFET and IMRT phantom was found to quick and efficient and well suited for a busy radiotherapy department.

EP-1141: Analisys of pre-treatment dose verification; 1.5 years experience. are they still necessary? yes

EP-1141: Analisys of pre-treatment dose verification; 1.5 years experience. are they still necessary? yes

Bidimensional silicon dosimeter: Development and characterization

Abstract Clinical dosimetry in radiotherapy is a well known matter but high conformal radiotherapy modalities (Intensity Modulated Radiation Therapy (IMRT), stereotactic treatments with photons and protons, Intensity Modulated Proton Therapy (IMPT)) possess problems due to small radiation fields with high dose gradients, variation in space and time of the dose rate and variation in space and time of the beam energy spectrum. A modular dosimetric detector, adequate for 2D pre-treatment dose verifications, has been developed in the framework of the European Integrated project MAESTRO. The detector is a monolithic segmented sensor obtained by n-type implantation on a 50 μthick epitaxial silicon p-type layer; this is later used to guarantee improved radiation hardness of the device against the accumulated dose. The detector is composed of a matrix of 21×21 pixels with a size of 2×2 mm 2 each and a 3 mm center-to-center distance. A full dosimetric characterization of the detector was performed with photon and proton beams and with gammas from a 60 Co unit. Results have been compared with those obtained with a Farmer and a CC13 Scanditronix/Welhoffer ion chamber as well as with the silicon matrix MapCHECK™. The first application of the MAESTRO prototype in the dosimetric verification of a clinical IMRT field is also reported. Results show that our modular detector represents a valuable tool for quality assurance in IMRT dose delivery and for high precision radiotherapy techniques.

SU-E-T-213: Preliminary Testing of a 2-D Fluence Measurement Prototype Device (Delta4-AT) for In-Vivo Patient Verification Dosimetry

Purpose: To perform preliminary sensitivity testing on the performance of a prototype device (Delta4-AT) that measures the 2D linear accelerator photon fluence during IMRT treatments. The measured fluence is then projected onto the Delta4 phantom to assess the dosimetric impact. Methods: Two Smart-Arc patient treatment plans were delivered on the Delta4 for pretreatment dose (PT) verification with the At-treatment (AT) Delta4 prototype. The measurement process of Delta4-AT is the same as the Delta4-PT. Delta4-AT acquires the fluence data and reconstructs a semi-measured dose in the Delta4 geometry based on daily changes such as MLC position and output. To assess the sensitivity, MLC patterns for the two patients were modified to introduce two sets of known errors: 1) Systematic error—all leaves moved relative to isocenter, and 2) Random error— selected leaves shifted. Data was collected and an inter-comparison of the TPS calculated dose, AT semi-measured, and PT semi-measured dose was performed. All data was analyzed using the Delta4 software. Results: Measurements showed a good gamma index (GI) agreement of 90.0% (3%/3mm) between the Delta4-AT and Delta4-PT when small discrepancies were introduced. When larger errors were present, a larger discrepancy existed between both measurement devices partly due to the decreased resolution of the prototype Delta4-AT. Relative to the TPS dose, both showed good agreement within the GI criteria. Conclusions: The Delta4-AT prototype together with the Delta4-PT is a promising solution for the quantification of the dosimetric impact of daily variations in linear accelerators during IMRT delivery. The Delta4-PT can serve as a vital, addon tool for patient dose verification.

Patient-Specific 3D Pretreatment and Potential 3D Online Dose Verification of Monte Carlo–Calculated IMRT Prostate Treatment Plans

Fast and reliable comprehensive quality assurance tools are required to validate the safety and accuracy of complex intensity-modulated radiotherapy (IMRT) plans for prostate treatment. In this study, we evaluated the performance of the COMPASS system for both off-line and potential online procedures for the verification of IMRT treatment plans. COMPASS has a dedicated beam model and dose engine, it can reconstruct three-dimensional dose distributions on the patient anatomy based on measured fluences using either the MatriXX two-dimensional (2D) array (offline) or a 2D transmission detector (T2D) (online). For benchmarking the COMPASS dose calculation, various dose-volume indices were compared against Monte Carlo-calculated dose distributions for five prostate patient treatment plans. Gamma index evaluation and absolute point dose measurements were also performed in an inhomogeneous pelvis phantom using extended dose range films and ion chamber for five additional treatment plans. MatriXX-based dose reconstruction showed excellent agreement with the ion chamber (<0.5%, except for one treatment plan, which showed 1.5%), film (∼100% pixels passing gamma criteria 3%/3 mm) and mean dose-volume indices (<2%). The T2D based dose reconstruction showed good agreement as well with ion chamber (<2%), film (∼99% pixels passing gamma criteria 3%/3 mm), and mean dose-volume indices (<5.5%). The COMPASS system qualifies for routine prostate IMRT pretreatment verification with the MatriXX detector and has the potential for on-line verification of treatment delivery using T2D.

SU-GG-T-513: Patient-Specific Independent GammaPlan Quality Assurance for Gamma Knife Perfexion

Purpose: One of the most important aspects of Quality Assurance (QA) in radiation therapy is redundancy of patient treatment dose calculation. This work is focused on the patient-specific time and 3D dose treatment plan verification for stereotactic radiosurgery using Lexell Gamma Knife® Perfexion™ (LGK PFX). Method and Materials: The virtual model of LGK PFX was built in Matlab™, based on the physical dimensions of this modality supplied by the manufacturer. However, in our model, the ring-specific linear attenuation coefficients (LAC) and output factors (OFs) reported by the manufacturer were replaced by the measurement-based collimator size-specific OFs and a single LAC=0.0065 mm−1, as the manufacturer-supplied parameters resulted in 6–9% lower absolute dose in all cases. Depths for each LGK PFX shot was obtained by ray-tracing technique, and the dose calculation formalism was identical to the one used by GammaPlan treatment planning software. The architecture of the QA process was based on the in-house on-line database search code that retrieved the LGK PFX plan-specific information. A series of QA phantom plans was examined to verify geometric and dosimetric accuracy of our software. This was followed by calculation of a series of patient plans, which aimed at establishing action limits for our QA process. Results: The shot time/focus point dose verification for each shot takes less than 1sec/shot with 3Disodose verification taking about 30sec/shot, database access time being less than 0.05sec. The geometric accuracy (location of the point of maximum dose) of the phantom and patient plan was dependant on the resolution of the original dose matrix and was of the order of 1–1.5pixel. Dosimetric accuracy of our calculated phantom and patient point dose was within 3–3.5% from the GammaPlan with the mean=2.3% and SD=1.1%. Conclusion: The process for independent pre-treatment patient-specific Gamma Knife Perfexion time and dose verification was created and validated.

A Survey on the Quality Assurance Procedures Used in Intensity Modulated Radiation Therapy (IMRT) at Indian Hospitals

A national survey to obtain information about the Quality Assurance (QA) procedures and methods being followed at Indian radiotherapy centers for intensity modulated radiation therapy (IMRT) was conducted. A questionnaire containing parameters relevant to IMRT QA was evolved to collect the information pertaining to the QA of IMRT delivery system, QA of IMRT treatment planning system, and patient specific IMRT QA. The questionnaire was circulated to 40 hospitals in the country and responses of 31 centers were received. Survey results showed that 71% centers are having adequate machine specifi c IMRT QA programme, 19% centers have inadequate machine specific IMRT QA programme and 9% centers have irrelevant machine specific IMRT QA programme. No specific answer for question of QA tests of TPS specific to IMRT were received from the user. Almost all the centers have programme of setup verification of the patient by means of EPID/DRR/OBI. However, 91% of centers could not provide any information about the QA methodology of the devices used for setup verification. For patient specific dosimetric QA, almost all the hospitals have the program of pre-treatment dose verification using calibrated ionization chambers of sensitive volumes in the range of 0.01 to 0.65 cc. Dosimetric verification is performed by combining dose from all gantry angles to a single gantry angle. Two dimensional (2D) dosimetry systems such as radiographic and radiochromic films, 2D array of ionization chambers/ semiconductor diodes and EPID are also used in patient specific dosimetry verifications. Majority of the centers (about 48%) accept the plan with 3% dose difference and 3 mm dose to distance agreement criteria with gamma index less than unity. However, a number of other acceptance criteria specific to institution and tumor site are being also followed. This survey reveals that a variety of IMRT QA program is being followed at the Indian hospitals. This study has brought into focus the need to evolve a national protocol for IMRT QA so that treatment outcomes of all the IMRT centers of country can be compared.

Acrylonitrile Butadiene Styrene (ABS) plastic‐based low cost tissue equivalent phantom for verification dosimetry in IMRT

A novel IMRT phantom was designed and fabricated using Acrylonitrile Butadiene Styrene (ABS) plastic. Physical properties of ABS plastic related to radiation interaction and dosimetry were compared with commonly available phantom materials for dose measurements in radiotherapy. The ABS IMRT phantom has provisions to hold various types of detectors such as ion chambers, radiographic/radiochromic films, TLDs, MOSFETs, and gel dosimeters. The measurements related to pretreatment dose verification in IMRT of carcinoma prostate were carried out using ABS and Scanditronix‐Wellhofer RW3 IMRT phantoms for five different cases. Point dose data were acquired using ionization chamber and TLD discs, while Gafchromic EBT and radiographic EDR2 films were used for generating 2D dose distributions. Treatment planning system (TPS) calculated and measured doses in ABS plastic and RW3 IMRT phantom were in agreement within ± 2%. The dose values at a point in a given patient acquired using ABS and RW3 phantoms were found comparable within 1%. Fluence maps and dose distributions of these patients generated by TPS and measured in ABS IMRT phantom were also found comparable both numerically and spatially. This study indicates that ABS plastic IMRT phantom is a tissue‐equivalent phantom and, dosimetrically, it is similar to solid/plastic water IMRT phantoms. Although this material is demonstrated for IMRT dose verification, it can also be used as a tissue‐equivalent phantom material for other dosimetry purposes in radiotherapy.PACS number: 87.53Kn, 87.55Qr, 87.53Bn and 87.55Km

Measurement‐based Monte Carlo dose calculation system for IMRT pretreatment and on‐line transit dose verifications

The aim of this study was to develop a dose simulation system based on portal dosimetry measurements and the BEAM Monte Carlo code for intensity-modulated (IM) radiotherapy dose verification. This measurement-based Monte Carlo (MBMC) system can perform, within one systematic calculation, both pretreatment and on-line transit dose verifications. BEAMnrc and DOSXYZnrc 2006 were used to simulate radiation transport from the treatment head, through the patient, to the plane of the aS500 electronic portal imaging device (EPID). In order to represent the nonuniform fluence distribution of an IM field within the MBMC simulation, an EPID-measured efficiency map was used to redistribute particle weightings of the simulated phase space distribution of an open field at a plane above a patient/phantom. This efficiency map was obtained by dividing the measured energy fluence distribution of an IM field to that of an open field at the EPID plane. The simulated dose distribution at the midplane of a homogeneous polystyrene phantom was compared to the corresponding distribution obtained from the Eclipse treatment planning system (TPS) for pretreatment verification. It also generated a simulated transit dose distribution to serve as the on-line verification reference for comparison to that measured by the EPID. Two head-and-neck (NPC1 and NPC2) and one prostate cancer fields were tested in this study. To validate the accuracy of the MBMC system, film dosimetry was performed and served as the dosimetry reference. Excellent agreement between the film dosimetry and the MBMC simulation was obtained for pretreatment verification. For all three cases tested, gamma evaluation with 3%/3 mm criteria showed a high pass percentage (> 99.7%) within the area in which the dose was greater than 30% of the maximum dose. In contrast to the TPS, the MBMC system was able to preserve multileaf collimator delivery effects such as the tongue-and-groove effect and interleaf leakage. In the NPC1 field, the TPS showed 16.5% overdose due to the tongue-and-groove effect and 14.6% overdose due to improper leaf stepping. Similarly, in the NPC2 field, the TPS showed 14.1% overdose due to the tongue-and-groove effect and 8.9% overdose due to improper leaf stepping. In the prostate cancer field, the TPS showed 6.8% overdose due to improper leaf stepping. No tongue-and-groove effect was observed for this field. For transit dose verification, agreements among the EPID measurement, the film dosimetry, and the MBMC system were also excellent with a minimum gamma pass percentage of 99.6%.

Dosimetric characterization with 62 MeV protons of a silicon-segmented detector for 2D dose verifications in radiotherapy

Due to the features of the modern radiotherapy techniques, namely intensity modulated radiation therapy and proton therapy, where high spatial dose gradients are often present, detectors to be employed for 2D dose verifications have to satisfy very narrow requirements. In particular they have to show high spatial resolution. In the framework of the European Integrated Project—Methods and Advanced Equipment for Simulation and Treatment in Radio-Oncology (MAESTRO, no. LSHC-CT-2004-503564), a dosimetric detector adequate for 2D pre-treatment dose verifications was developed. It is a modular detector, based on a monolithic silicon-segmented sensor, with an n-type implantation on an epitaxial p-type layer. Each pixel element is 2×2 mm 2 and the distance center-to-center is 3 mm. The sensor is composed of 21×21 pixels. In this paper, we report the dosimetric characterization of the system with a proton beam. The sensor was irradiated with 62 MeV protons for clinical treatments at INFN-Laboratori Nazionali del Sud (LNS) Catania. The studied parameters were repeatability of a same pixel, response linearity versus absorbed dose, and dose rate and dependence on field size. The obtained results are promising since the performances are within the project specifications.