Intensity Modulated Radiation Therapy Verification Research Articles

Clinically evaluating directional dependence of 2D seven29 ion-chamber array with different IMRT plans

Purpose: This study aims to clinically evaluate the directional dependence of a 2D seven29 ion-chamber array with different intensity-modulated radiotherapy (IMRT) plans. Methods: Twenty-five patients who had already been treated with IMRT plans were selected for the study. Verification plans were created in an Eclipse treatment planning system (TPS) for each treatment plan. The verification plans were executed twice for each patient. The first IMRT plan used a true gantry angle (plan-related approach), and the second plan used a 0° gantry angle (field-related approach). Measurements were performed using a Varian Clinac 2100 iX linear accelerator. The fluence was measured for all the delivered plans and analyzed using Verisoft software. A comparison of the fluence was performed between IMRT with a static gantry (0° gantry angle) and real gantry angles. Results: The results indicate that the Gamma average was 98.8 % for IMRT with a 0° gantry angle and 96.616% for IMRT with a true gantry angle. Average percent difference of normalized doses for IMRT delivered with zero degree gantry angle and IMRT with actual gantry angles is 0.15 and 0.88 respectively. Conclusion: The ion chamber of the 2D array used in IMRT verification has angular dependence, reducing the verification accuracy when the 2D array is used for measuring the actual beams of the treatment plan.

SU-E-T-256: Development of a Monte Carlo-Based Dose-Calculation System in a Cloud Environment for IMRT and VMAT Dosimetric Verification

Purpose: Intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) are techniques that are widely used for treating cancer due to better target coverage and critical structure sparing. The increasing complexity of IMRT and VMAT plans leads to decreases in dose calculation accuracy. Monte Carlo simulations are the most accurate method for the determination of dose distributions in patients. However, the simulation settings for modeling an accurate treatment head are very complex and time consuming. The purpose of this work is to report our implementation of a simple Monte Carlo simulation system in a cloud-computing environment for dosimetric verification of IMRT and VMAT plans. Methods: Monte Carlo simulations of a Varian Clinac linear accelerator were performed using the BEAMnrc code, and dose distributions were calculated using the DOSXYZnrc code. Input files for the simulations were automatically generated from DICOM RT files by the developed web application. We therefore must only upload the DICOM RT files through the web interface, and the simulations are run in the cloud. The calculated dose distributions were exported to RT Dose files that can be downloaded through the web interface. The accuracy of the calculated dose distribution was verified by dose measurements. Results: IMRT and VMAT simulations were performed and good agreement results were observed for measured and MC dose comparison. Gamma analysis with a 3% dose and 3 mm DTA criteria shows a mean gamma index value of 95% for the studied cases. Conclusion: A Monte Carlo-based dose calculation system has been successfully implemented in a cloud environment. The developed system can be used for independent dose verification of IMRT and VMAT plans in routine clinical practice. The system will also be helpful for improving accuracy in beam modeling and dose calculation in treatment planning systems. This work was supported by JSPS KAKENHI Grant Number 25861057.

SU‐E‐T‐194: Commissioning of Monaco Treatment Planning System On An Elekta VersaHD Linear Accelerator

Purpose:The Monaco treatment planning system (TPS) uses a Monte‐Carlo algorithm based dose computation engine to model the photon beams of a linear accelerator. The aim is to perform verification of Monaco TPS beam modeling of a Elekta VersaHD linac with 6MV, 6MV FFF, 10 MV, 10MV FFF, 18MV photon beams and 160 multileaf collimators (MLC) with a projected width of 5‐mm at the isocenter.Methods:A series of dosimetric tests were performed to validate Monaco calculated beams including point dose measurement in water with and without heterogeneity and 2‐dimensional dose distributions on a Delta4 bi‐planar diode dosimeter array (Scandidos, Uppsala, Sweden). 3D conformal beams of different field sizes, source‐to‐surface distances, wedges, and gantry angles were delivered onto a phantom consisting of several plastic water and Styrofoam slabs. Point dose measurements were verified with a PTW 31013 Semiflex 0.3 cc ionization chamber (PTW, Freiburg, Germany). In addition, 8 step and shoot intensity modulated radiotherapy (IMRT) and volumetric modulated arc radiotherapy (VMAT) beams included in the Monaco TPS commissioning suite were verified against measurements on Delta4 to test and fine tune parameters in the beam model. IMRT verification was computed using gamma analysis with dose difference and distance‐to‐agreement criteria of 3%/3mm with a dose threshold of 10%.Results:Point dose measurements agreed within 2% in the homogeneous phantom and within 3% in the heterogeneous phantom for all photon energies. IMRT beams yielded a passing percentage of 99.1±1.1% in the gamma analysis which is well above the institutional passing threshold of 90%.Conclusion:Monaco TPS commissioning was successfully performed for all the photon energies on the Elekta VersaHD linac prior to clinical usage.

SU‐E‐T‐483: In Vivo Dosimetry of Conventional and Rotational Intensity Modulated Radiotherapy Using Integral Quality Monitor (IQM)

Purpose:To investigate the accuracy, sensitivity and constancy of integral quality monitor (IQM), a new system for in vivo dosimetry of conventional intensity modulated radiation therapy (IMRT) or rotational volumetric modulated arc therapy (VMAT)Methods:A beta‐version IQM system was commissioned on an Elekta Infinity LINAC equipped with 160‐MLCs Agility head. The stationary and rotational dosimetric constancy of IQM was evaluated, using five‐field IMRT and single‐or double‐arc VMAT plans for prostate and head‐and‐neck (H&N) patients. The plans were delivered three times over three days to assess the constancy of IQM response. Picket fence (PF) fields were used to evaluate the sensitivity of detecting MLC leaf errors. A single leaf offset was intentionally introduced during delivery of various PF fields with segment apertures of 3×1, 5×1, 10×1, and 24×1cm2. Both 2mm and 5mm decrease in the field width were used.Results:Repeated IQM measurements of prostate and H&N IMRT deliveries showed 0.4 and 0.5% average standard deviation (SD) for segment‐by‐segment comparison and 0.1 and 0.2% for cumulative comparison. The corresponding SDs for VMAT deliveries were 6.5, 9.4% and 0.7, 1.3%, respectively. Statistical analysis indicates that the dosimetric differences detected by IQM were significant (p < 0.05) in all PF test deliveries. The largest average IQM signal response of a 2 mm leaf error was found to be 2.1% and 5.1% by a 5mm leaf error for 3×1 cm2 field size. The same error in 24×1 cm2 generates a 0.7% and 1.4% difference in the signal.Conclusion:IQM provides an effective means for real‐time dosimetric verification of IMRT/ VMAT treatment delivery. For VMAT delivery, the cumulative dosimetry of IQM needs to be used in clinical practice.

A novel method for dose distribution registration using fiducial marks made by a megavoltage beam in film dosimetry for intensity-modulated radiation therapy quality assurance

PurposePhotographic film is widely used for the dose distribution verification of intensity-modulated radiation therapy (IMRT). However, analysis for verification of the results is subjective. We present a novel method for marking the isocenter using irradiation from a megavoltage (MV) beam transmitted through slits in a multi-leaf collimator (MLC). MethodsWe evaluated the effect of the marking irradiation at 500 monitor units (MU) on the total transmission through the MLC using an ionization chamber and Radiochromic Film. Film dosimetry was performed for quality assurance (QA) of IMRT plans. Three methods of registration were used for each film: marking by irradiating with an MV beam through slits in the MLC (MLC-IC); marking with a fabricated phantom (Phantom-IC); and a subjective method based on isodose lines (Manual). Each method was subjected to local γ-analysis. ResultsThe effect of the marking irradiation on the total transmission was 0.16%, as measured by a ionization chamber at a 10-cm depth in a solid phantom, while the inter-leaf transmission was 0.3%, determined from the film. The mean pass rates for each registration method agreed within ±1% when the criteria used were a distance-to-agreement (DTA) of 3 mm and a dose difference (DD) of 3%. For DTA/DD criteria of 2mm/3%, the pass rates in the sagittal plane were 96.09 ± 0.631% (MLC-IC), 96.27 ± 0.399% (Phantom-IC), and 95.62 ± 0.988% (Manual). ConclusionThe present method is a versatile and useful method of improving the objectivity of film dosimetry for IMRT QA.

An investigation of PRESAGE® 3D dosimetry for IMRT and VMAT radiation therapy treatment verification

The purpose of this work was to characterize three formulations of PRESAGE® dosimeters (DEA-1, DEA-2, and DX) and to identify optimal readout timing and procedures for accurate in-house 3D dosimetry. The optimal formulation and procedure was then applied for the verification of an intensity modulated radiation therapy (IMRT) and a volumetric modulated arc therapy (VMAT) treatment technique.PRESAGE® formulations were studied for their temporal stability post-irradiation, sensitivity, and linearity of dose response. Dosimeters were read out using a high-resolution optical-CT scanner. Small volumes of PRESAGE® were irradiated to investigate possible differences in sensitivity for large and small volumes (‘volume effect’). The optimal formulation and read-out technique was applied to the verification of two patient treatments: an IMRT plan and a VMAT plan.A gradual decrease in post-irradiation optical-density was observed in all formulations with DEA-1 exhibiting the best temporal stability with less than 4% variation between 2–22 h post-irradiation. A linear dose response at the 4 h time point was observed for all formulations with an R2 value >0.99. A large volume effect was observed for DEA-1 with sensitivity of the large dosimeter being ~63% less than the sensitivity of the cuvettes. For the IMRT and VMAT treatments, the 3D gamma passing rates for 3%/3 mm criteria using absolute measured dose were 99.6 and 94.5% for the IMRT and VMAT treatments, respectively.In summary, this work shows that accurate 3D dosimetry is possible with all three PRESAGE® formulations. The optimal imaging windows post-irradiation were 3–24 h, 2–6 h, and immediately for the DEA-1, DEA-2, and DX formulations, respectively. Because of the large volume effect, small volume cuvettes are not yet a reliable method for calibration of larger dosimeters to absolute dose. Finally, PRESAGE® is observed to be a useful method of 3D verification when careful consideration is given to the temporal stability and imaging protocols for the specific formulation used.

3D Secondary Verifications for VMAT Prostate Treatments Based in DVH-Metrics and 3D Gamma Analysis

Independent monitor unit (MU) verification of intensity-modulated radiation therapy (IMRT) treatments is traditionally performed with Monte Carlo calculations or comparing point-dose calculations in homogeneous media. Recent verifications systems permit to perform secondary checks of treatment plans calculating dose on patient CT images. Single-point MU check is replaced with 3D treatment plan verification, incorporating DVH-based metrics with clinical relevance to the QA process. This work shows the 3D dosimetric verification of several volumetric modulated arc therapy (VMAT) prostate treatment plans by means of dose-volume parameters comparison with a new 3D treatment plan QA software. VMAT prostate plans are generated and delivered with a single VMAT arc technique. DICOM RT information (CT, RT Struct, RT Plan and RT Dose) is transferred from TPS to verification system. This software uses a new independently developed collapsed cone algorithm to calculate dose distributions in CT. A stock reference beam model is used for calculations. Beam data can be fitted with key parameters (PDDs, OFs, OARs). A dosimetric leaf gap of -2 mm is needed to scale the results. 25 prostate plans have been verified using this solution, comparing DVH parameters (Dmed, D90 and eventually Dmax) and performing 3D gamma tests for PTVs (prostate, seminal vesicles, pelvic lymph nodes) and OARs (rectum, bladder, femoral heads, penile bulb). Mean relative deviations (Dmed/D90) are (-0.22±0.97/-0.62±1.20)% for prostate PTV, (0.29±0.90/0.56±0.90)% for seminal vesicles PTV and (1.14±0.53/0.90±0.58)% for pelvic lymph nodes PTV. 3D gamma passing rate values for PTVs are (97.99±3.12)%, (99.23±1.90)% and (97.54±2.90)%, respectively. Mean deviation and gamma passing rate value (Dmed/% of points) is (-0.00±0.81/99.50±0.71)% for rectum, (0.80±0.90/99.60±1.21)% for bladder and (2.84±2.28/100.00±0.00)% for penile bulb. Mean deviations and gamma passing rates for right and left femoral heads (Dmed/Dmax/% of points) are (0.03±0.43/-1.27±2.35/100.00±0.00)% and (0.05±0.38/-1.49±2.24/100.00±0.00)%, respectively. Deviations are below 1.5% for all parameters and volumes, excepting penile bulb (due to the limited number of contoured slices). 3D gamma passing rates are above 97.5% for all structures, reaching 100.0% in some cases. The tested system is a quick, effective and reliable tool to perform independent TPS-quality dose calculations of complex treatments, such as those used in VMAT techniques.

Poster — Thur Eve — 39: Feasibility of Commissioning HybridArc with the Delta 4 two plane diode phantom: comparisons with Gafchromic Film.

HybridArc is a relatively novel radiation therapy technique which combines optimized dynamic conformai arcs (DCA) and intensity modulated radiation therapy (IMRT). HybridArc has possible dosimetry and efficiency advantages over stand alone DCA and IMRT treatments and can be readily implemented on any linac capable of DCA and IMRT, giving strong motivation to commission the modality. The Delta4 phantom (Scandidos, Uppsala, Sweden) has been used for IMRT and VMAT clinical dosimetric verification making it a candidate for HybridArc commissioning. However the HybridArc modality makes use of several non co‐planar arcs which creates setup issues due to the geometry of the Delta4, resulting in possible phantom gantry collisions for plans with non‐zero couch angles. An analysis was done determining the feasibility of using the Delta4 fixed at 0° couch angle compared with results obtained using Gafchromic ETB2 film (Ashland, Covington Kentucky) in an anthropomorphic phantom at the planned couch angles. A gamma index analysis of the measured and planned dose distributions was done using Delta4 and DoseLab Pro (Mobius Medical Systems, Houston Texas) software. For both arc and IMRT sub‐fields there is reasonable correlation between the gamma index found from the Delta4 and Gafchromic film. All results show the feasibility of using the Delta4 for HybridArc commissioning.

TH-C-17A-03: Dynamic Visualization and Dosimetry of IMRT and VMAT Treatment Plans by Video-Rate Imaging of Cherenkov Radiation in Pure Water

Purpose: A novel optical dosimetry technique for the QA and verification of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) radiotherapy plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. Methods: An intensified CCD camera (ICCD) was used to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank. The ICCD acquisition was gated to the Linac, operated for single pulse imaging, and binned to a resolution of 512×512 pixels. The resulting videos were analyzed temporally for regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR) and summed to obtain an overall light distribution, which was compared to the expected dose distribution from the TPS using a gammaindex analysis. Results: The chosen camera settings resulted in data at 23.5 frames per second. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.2% and 95.6% agreement between the light distribution and expected TPS dose distribution based upon a 3% / 3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans respectively. Conclusion: The results from this initial study demonstrate the first documented use of Cherenkov radiation for optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications. NIH R01CA109558 and R21EB017559.

Acceptance and Commissioning of a Treatment Planning System Based on Monte Carlo Calculations

The Monaco Treatment Planning System (TPS), based on a virtual energy fluence model of the photon beam head components of the linac and a dose computation engine made with Monte Carlo (MC) algorithm X-Ray Voxel MC (XVMC), has been tested before being put into clinical use. An Elekta Synergy with 6 MV was characterized using routine equipment. After the machine's model was installed, a set of functionality, geometric, dosimetric and data transfer tests were performed. The dosimetric tests included dose calculations in water, heterogeneous phantoms and Intensity Modulated Radiation Therapy (IMRT) verifications. Data transfer tests were run for every imaging device, TPS and the electronic medical record linked to Monaco. Functionality and geometric tests were run properly. Dose calculations in water were in accordance with measurements so that, in 95% of cases, differences were up to 1.9%. Dose calculation in heterogeneous media showed expected results found in the literature. IMRT verification results with an ionization chamber led to dose differences lower than 2.5% for points inside a standard gradient. When an 2-D array was used, all the fields passed the g (3%, 3 mm) test with a percentage of succeeding points between 90% and 95%, of which the majority of the mentioned fields had a percentage of succeeding points between 95% and 100%. Data transfer caused problems that had to be solved by means of changing our workflow. In general, tests led to satisfactory results. Monaco performance complied with published international recommendations and scored highly in the dosimetric ambit. However, the problems detected when the TPS was put to work together with our current equipment showed that this kind of product must be completely commissioned, without neglecting data workflow, before treating the first patient.

Practical approach for pretreatment verification of IMRT with flattening filter free(FFF) beams using Varian Portal Dosimetry.

Patient‐specific pretreatment verification of intensity‐modulated radiation therapy (IMRT) or volumetric‐modulated arc therapy (VMAT) is strongly recommended for all patients in order to detect any potential errors in treatment planning process and machine deliverability, and is thus performed routinely in many clinics. Portal dosimetry is an effective method for this purpose because of its prompt setup, easy data acquisition, and high spatial resolution. However, portal dosimetry cannot be applied to IMRT or VMAT with flattening filter‐free (FFF) beams because of the high dose‐rate saturation effect of the electronic portal imaging device (EPID). In our current report, we suggest a practical QA method of expanding the conventional portal dosimetry to FFF beams with a QA plan generated by the following three steps: 1) replace the FFF beams with flattening filtered (FF) beams of the same nominal energy; 2) reduce the dose rate to avoid the saturation effect of the EPID detector; and 3) adjust the total MU to match the gantry and MLC leaf motions. Two RapidArc plans with 6 and 10 MV FFF beams were selected, and QA plans were created by the aforementioned steps and delivered. The trajectory log files of TrueBeam obtained during the treatment and during the delivery of QA plan were analyzed and compared. The maximum discrepancies in the expected trajectories between the treatment and QA plans were within 0.002 MU for the MU, 0.06° for the motion of gantry rotation, and 0.006 mm for the positions of the MLC leaves, indicating much higher levels of accuracy compared to the mechanical specifications of the machine. For further validation of the method, direct comparisons of the delivered QA FF beam to the treatment FFF beam were performed using film dosimetry and show that gamma passing rates under 2%/2 mm criteria are 99.0%–100% for the all four arc beams. This method can be used on RapidArc plans with FFF beams without any additional procedure or modifications on the conventional portal dosimetry of IMRT and is, therefore, a practical option for routine clinical use.PACS numbers: 87.53.Kn, 87.55.T‐, 87.56.bd, 87.59.‐e

Analyse two kinds of intensity-modulated radiotherapy verification methods comparatively by using the MatriXX

Objective To explore the MatriXX measurements the dose distributions for each beam in intensity-modulated radiotherapy (IMRT) plans were measured with 0 degree gantry angle and actual gantry position respectively.To discuss whether the two multi-angle synthetic pass rate from the two methods has statistics differences.Methods The dose distributions for each beam in IMRT plans were measured with 0 degree gantry angle and actual gantry position for twelve patients with head and neck tumor respectively.The γ pass rates (according to 3％/3 mm) for each beam under each angle condition was obtained by the comparison between the measured and the calculated dose distributions from the treatment planning system which was treated as the reference distribution.Use the t-test to analyse the actual gantry angle method and use the one factor analysis of variance to analyze the two multi-angle synthetic pass rate from the two methods.Results The γ pass rates of actual gantry angle was found generally declined seemingly compared with 0 degree gantry angle,but differentγ pass rates showed only in 80 °,120°and 240° with98.71％,93.59％(t=2.10,P=0.000),98.15％,93.17％ (t=2.10,P=0.000) and 98.94％,92.85％ (t =2.10,P =0.000) respectively.The γpass rate of multi-angle synthetic was seemingly between methods (98.27％,94.63 ％,F =0.50,P =0.134).Conclusions Two kinds of IMRT verification mode are from two position to validated the IMRT plans dose accuracy,comparatively analysing the conclusions drawn from the two methods can protect accuracy of IMRT plans more comprehensively. Key words: MatriXX Ionization chamber array; Dose verification; Intensity-modulated radiotherapy

Independent calculation-based verification of IMRT plans using a 3D dose-calculation engine

Independent monitor unit verification of intensity-modulated radiation therapy (IMRT) plans requires detailed 3-dimensional (3D) dose verification. The aim of this study was to investigate using a 3D dose engine in a second commercial treatment planning system (TPS) for this task, facilitated by in-house software. Our department has XiO and Pinnacle TPSs, both with IMRT planning capability and modeled for an Elekta-Synergy 6MV photon beam. These systems allow the transfer of computed tomography (CT) data and RT structures between them but do not allow IMRT plans to be transferred. To provide this connectivity, an in-house computer programme was developed to convert radiation therapy prescription (RTP) files as generated by many planning systems into either XiO or Pinnacle IMRT file formats. Utilization of the technique and software was assessed by transferring 14 IMRT plans from XiO and Pinnacle onto the other system and performing 3D dose verification. The accuracy of the conversion process was checked by comparing the 3D dose matrices and dose volume histograms (DVHs) of structures for the recalculated plan on the same system. The developed software successfully transferred IMRT plans generated by 1 planning system into the other. Comparison of planning target volume (TV) DVHs for the original and recalculated plans showed good agreement; a maximum difference of 2% in mean dose, − 2.5% in D95, and 2.9% in V95 was observed. Similarly, a DVH comparison of organs at risk showed a maximum difference of +7.7% between the original and recalculated plans for structures in both high- and medium-dose regions. However, for structures in low-dose regions (less than 15% of prescription dose) a difference in mean dose up to +21.1% was observed between XiO and Pinnacle calculations. A dose matrix comparison of original and recalculated plans in XiO and Pinnacle TPSs was performed using gamma analysis with 3%/3mm criteria. The mean and standard deviation of pixels passing gamma tolerance for XiO-generated IMRT plans was 96.1 ± 1.3, 96.6 ± 1.2, and 96.0 ± 1.5 in axial, coronal, and sagittal planes respectively. Corresponding results for Pinnacle-generated IMRT plans were 97.1 ± 1.5, 96.4 ± 1.2, and 96.5 ± 1.3 in axial, coronal, and sagittal planes respectively.

SU‐E‐T‐201: Dosimetric Comparison of Patient Specific IMRT QA Using Varian DynaLog Files and TPS Calculations in a Phantom Geometry

Purpose: To evaluate significance of gamma index between the planned 2D doses by Eclipse treatment planning system and the calculated 2D doses based on DMLC Dynalog information for patient specific IMRT QA. Methods: DMLC (Dynamic Multi‐Leaf Collimator) of IMRT (Intensity Modulated Radio Therapy) delivery information recorded in log files during delivery. The Dynalog file includes amongst other, the beam on and off status, segment dose index, and individual leaf positions. In this study, previous treated IMRT patient plans are selected and delivered using a Varian CLINAC 600 C/D linear accelerator equipped with Millennium 120 MLC. IMRT verification plans were generated on PTW OCTAVIUS phantom using Eclipse TPS. Dynalog file data were recorded during delivery and were converted to fluence maps using an in‐house Matlab code. The fluence maps were imported to Elipse TPS and dose distributions on the OCTAVIUS phantom. PTW VeriSoft was used to compare the planned and recalculated IMRT plans. Gamma index, profiles and isodose distributions were used to evaluate the dosimetric difference. Results: The comparison of the planned and the recalculated dose distribution showed that the gamma index passing rate was 90% or greater for all plans using a 3%/3mm criteria. The analysis of the results based on the complete 3D dose distributions is under investigation. Conclusion: Our preliminary results show that the dynalog files can be an alternative method for patient specific IMRT QA. Good agreement between measurements and calculations was achieved in all cases tested and further investigation for calculations using patient geometry and for VMAT is in progress.

A calibration method for patient specific IMRT QA using a single therapy verification film

AimThe aim of the present study is to develop and verify the single film calibration procedure used in intensity-modulated radiation therapy (IMRT) quality assurance. BackgroundRadiographic films have been regularly used in routine commissioning of treatment modalities and verification of treatment planning system (TPS). The radiation dosimetery based on radiographic films has ability to give absolute two-dimension dose distribution and prefer for the IMRT quality assurance. However, the single therapy verification film gives a quick and significant reliable method for IMRT verification. Materials and methodsA single extended dose rate (EDR 2) film was used to generate the sensitometric curve of film optical density and radiation dose. EDR 2 film was exposed with nine 6cm×6cm fields of 6MV photon beam obtained from a medical linear accelerator at 5-cm depth in solid water phantom. The nine regions of single film were exposed with radiation doses raging from 10 to 362cGy. The actual dose measurements inside the field regions were performed using 0.6cm3 ionization chamber. The exposed film was processed after irradiation using a VIDAR film scanner and the value of optical density was noted for each region. Ten IMRT plans of head and neck carcinoma were used for verification using a dynamic IMRT technique, and evaluated using the gamma index method with TPS calculated dose distribution. ResultsSensitometric curve has been generated using a single film exposed at nine field region to check quantitative dose verifications of IMRT treatments. The radiation scattered factor was observed to decrease exponentially with the increase in the distance from the centre of each field region. The IMRT plans based on calibration curve were verified using the gamma index method and found to be within acceptable criteria. ConclusionThe single film method proved to be superior to the traditional calibration method and produce fast daily film calibration for highly accurate IMRT verification.

Dosimetric properties and clinical application of an a-Si EPID for dynamic IMRT quality assurance

Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for verification of intensity-modulated radiation therapy (IMRT) were investigated as a replacement for conventional verification tools. The portal dosimetry system of Varian's EPID (aS1000) has an integrated image mode for portal dosimetry (PD). The source-to-imager distance was 105 cm, and there were no extra buildup materials on the surface of the EPID in this study. Several dosimetric properties were examined. For clinical dosimetry, the dose distributions of dynamic IMRT beams for prostate cancer (19 patients, 97 beams) were measured by EPID and compared with the results of ionization chamber (IC) measurements. In addition, pretreatment measurements for prostate IMRT (50 patients, 309 beams) were performed by EPID and were evaluated by the gamma method (criterion: 3 mm/3 %). The signal-to-monitor unit ratio of PD showed dose dependence, indicating ghosting effects. Tongue-and-groove effects were observed as a result of the dose difference in the measured EPID images. The results of PD for clinical IMRT beams were in good agreement with the predicted dose image with average values of 1.37 and 0.25 for γ (max) and γ (ave), respectively. The point doses of PD were slightly, but significantly, higher than the results of IC measurements (p < 0.05 paired t test). However, this small difference seems clinically acceptable. This portal dosimetry system is useful as a rapid and convenient verification tool for dynamic IMRT.

Pre-treatment verification of intensity-modulated radiation therapy in paediatric patients: adequate estimation for tolerance limits

Objective The objective of this work was to establish adequate tolerance limits based on a certain defined institutional indices and generate published data presenting our results to the radiotherapy community.

In-phantom dose verification of prostate IMRT and VMAT deliveries using plastic scintillation detectors

The goal of this work was to demonstrate the feasibility of using a plastic scintillation detector (PSD) incorporated into a prostate immobilization device to verify doses in vivo delivered during intensity-modulated radiation therapy (IMRT) and volumetric modulated-arc therapy (VMAT) for prostate cancer. The treatment plans for both modalities had been developed for a patient undergoing prostate radiation therapy. First, a study was performed to test the dependence, if any, of PSD accuracy on the number and type of calibration conditions. This study included PSD measurements of each treatment plan being delivered under quality assurance (QA) conditions using a rigid QA phantom. PSD results obtained under these conditions were compared to ionization chamber measurements. After an optimal set of calibration factors had been found, the PSD was combined with a commercial endorectal balloon used for rectal distension and prostate immobilization during external beam radiotherapy. This PSD-enhanced endorectal balloon was placed inside of a deformable anthropomorphic phantom designed to simulate male pelvic anatomy. PSD results obtained under these so-called "simulated treatment conditions" were compared to doses calculated by the treatment planning system (TPS). With the PSD still inserted in the pelvic phantom, each plan was delivered once again after applying a shift of 1 cm anterior to the original isocenter to simulate a treatment setup error.The mean total accumulated dose measured using the PSD differed the TPS-calculated doses by less than 1% for both treatment modalities simulated treatment conditions using the pelvic phantom. When the isocenter was shifted, the PSD results differed from the TPS calculations of mean dose by 1.2% (for IMRT) and 10.1% (for VMAT); in both cases, the doses were within the dose range calculated over the detector volume for these regions of steep dose gradient. Our results suggest that the system could benefit prostate cancer patient treatment by providing accurate in vivo dose reports during treatment and verify in real-time whether treatments are being delivered according to the prescribed plan.

TU-F-BRCD-01: Tolerance Levels and Methodologies for IMRT Verification QA.

1. To discuss commonly employed IMRT measurement methods and discuss the pros and cons of each method. 2. To review methodologies for absolute dose verification (single small-volume, 1D, 2D methods), and review dose-difference, DTA, and Gamma analysis techniques including the variability of vendors implementation 3. To review IMRT QA passing rates for given tolerances and action levels, and discuss the clinical relevance of failed IMRT QA.

An empirical calibration method for an a-Si portal imaging device: applications in pretreatment verification of IMRT

A new calibration method for an amorphous silicon (a-Si) electronic portal imaging device (EPID) used for dose measurements in pretreatment verification (field-related) of intensity-modulated radiation therapy (IMRT) with sliding-window technique. The method is independent of data contained in the multileaf collimator (MLC) leaf-motion files and of any calculations made by the treatment planning system (TPS). Sensitivity of the EPID is dependent on radiation energy. For fluence-modulated fields, different dose/reading calibration factors are associated with each pixel of the image acquired by calculating equivalent areas representing the exact ratio between primary and scatter components. The dose measured in the detector plane was compared with that calculated with TPS by using gamma-analysis. Each calibration factor was compared with that calculated by considering the individual contributions of primary and secondary radiation obtained using the convolution method with analytical kernel for homogeneous media. In 837/854 (98%) of the clinical fields analysed, the proportion of irradiated area in which the gamma-index was <1.0 exceeded 95%. The overall average gamma-index was 0.39. There was good agreement between the dose/reading calibration factors obtained with the empirical algorithm and with the convolution method. The proposed calibration method is suitable for routine clinical pretreatment verification in IMRT.