IMRT Phantom Research Articles

Characterizing the interplay of treatment parameters and complexity and their impact on performance on an IROC IMRT phantom using machine learning

Aim of the StudyTo elucidate the important factors and their interplay that drive performance on IMRT phantoms from the Imaging and Radiation Oncology Core (IROC). MethodsIROC’s IMRT head and neck phantom contains two targets and an organ at risk. Point and 2D dose are measured by TLDs and film, respectively. 1,542 irradiations between 2012–2020 were retrospectively analyzed based on output parameters, complexity metrics, and treatment parameters. Univariate analysis compared parameters based on pass/fail, and random forest modeling was used to predict output parameters and determine the underlying importance of the variables. ResultsThe average phantom pass rate was 92% and has not significantly improved over time. The step-and-shoot irradiation technique had significantly lower pass rates that significantly affected other treatment parameters’ pass rates. The complexity of plans has significantly increased with time, and all aperture-based complexity metrics (except MCS) were associated with the probability of failure. Random forest-based prediction of failure had an accuracy of 98% on held-out test data not used in model training. While complexity metrics were the most important contributors, the specific metric depended on the set of treatment parameters used during the irradiation. ConclusionWith the prevalence of errors in radiotherapy, understanding which parameters affect treatment delivery is vital to improve patient treatment. Complexity metrics were strongly predictive of irradiation failure; however, they are dependent on the specific treatment parameters. In addition, the use of one complexity metric is insufficient to monitor all aspects of the treatment plan.

Assessing the out-of-field dose calculation accuracy by eclipse treatment planning system in sliding window IMRT of prostate cancer patients

AimThe objective of this study was to evaluate out-of-field dose distribution calculation accuracy by the Anisotropic Analytical Algorithm (AAA), version 13.0.26, in Eclipse TPS, (Varian Medical Systems, Palo Alto, Ca, USA) for sliding window IMRT delivery technique in prostate cancer patients. Materials and methodsProstate IMRT plans with nine coplanar were calculated with the AAA Eclipse treatment planning system. To assess the accuracy of dose calculation predicted by the Eclipse in normal tissue and OARs located out of radiation field areas, including the rectum, bladder, right and left head of the femur, absolute organ dose value, and dose distribution were measured using the Delta4+ IMRT phantom. ResultsIn the out-of-field areas, underestimation of −0.66% in organs near the field edge to −39.63% in organs far from the field edge (2.5 and 7.3 cm respectively) occurred in the TPS calculations. The percentage of dose deviation for the femoral heads was 95.7 on average while for the organ closer to the target (rectum) it was 79.81. ConclusionsAAA dosimetry algorithm (used in Eclipse TPS) showed poor dose calculation in areas beyond the treatment fields border where underestimation varies with the distance from the field edges. A significant underestimation was found for the AAA algorithm in the sliding window IMRT technique (P-value > 0.05).

Modeling the Agility MLC in the Monaco treatment planning system.

We investigate the relationship between the various parameters in the Monaco MLC model and dose calculation accuracy for an Elekta Agility MLC. The vendor‐provided MLC modeling procedure — completed first with external vendor participation and then exclusively in‐house — was used in combination with our own procedures to investigate several sets of MLC modeling parameters to determine their effect on dose distributions and point‐dose measurements. Simple plans provided in the vendor procedure were used to elucidate specific mechanical characteristics of the MLC, while ten complex treatment plans — five IMRT and five VMAT — created using TG‐119‐based structure sets were used to test clinical dosimetric effects of particular parameter choices. EDR2 film was used for the vendor fields to give high spatial resolution, while a combination of MapCHECK and ion chambers were used for the in‐house TG‐119‐based procedures. The vendor‐determined parameter set provided a reasonable starting point for the MLC model and largely delivered acceptable gamma pass rates for clinical plans — including a passing external evaluation using the IROC H&N phantom. However, the vendor model did not provide point‐dose accuracy consistent with that seen in other treatment systems at our center. Through further internal testing it was found that there existed many sets of MLC parameters, often at opposite ends of their allowable ranges, that provided similar dosimetric characteristics and good agreement with planar and point‐dose measurements. In particular, the leaf offset and tip leakage parameters compensated for one another if adjusted in opposite directions, which provided a level curve of acceptable parameter sets across all plans. Interestingly, gamma pass rates of the plans were less dependent upon parameter choices than point‐dose measurements, suggesting that MLC modeling using only gamma evaluation may be generally an insufficient approach. It was also found that exploring all parameters of the very robust MLC model to find the best match to the vendor‐provided QA fields can reduce the pass rates of the TG‐119‐based clinical distributions as compared to simpler models. A wide variety of parameter sets produced MLC models capable of meeting RPC passing criteria for their H&N IMRT phantom. The most accurate models were achievable using a combination of vendor‐provided and in‐house procedures. The potential existed for an over‐modeling of the Agility MLC in an effort to obtain the fine structure of certain quality assurance fields, which led to a reduction in agreement between calculation and measurement of more typical clinical dose distributions.PACS number(s): 87.56.nk, 87.53.Kn, 87.55.km, 87.55.Qr

SU‐E‐T‐179: Clinical Impact of IMRT Failure Modes at Or Near TG‐142 Tolerance Criteria Levels

Purpose:Quantitatively assess the clinical impact of 11 critical IMRT dose delivery failure modes.Methods:Eleven step‐and‐shoot IMRT failure modes (FMs) were introduced into twelve Pinnacle v9.8 treatment plans. One standard and one highly modulated plan on the IROC IMRT phantom and ten previous H&N patient treatment plans were used. FMs included physics components covered by basic QA near tolerance criteria levels (TG‐142) such as beam energy, MLC positioning, and MLC modeling. Resultant DVHs were compared to those of failure‐free plans and the severity of plan degradation was assessed considering PTV coverage and OAR and normal tissue tolerances and used for FMEA severity scoring. Six of these FMs were physically simulated and phantom irradiations performed. TLD and radiochromic film results are used for comparison to treatment planning studies.Results:Based on treatment planning studies, the largest clinical impact from the phantom cases was induced by 2 mm systematic MLC shift in one bank with the combination of a D95% target under dose near 16% and OAR overdose near 8%. Cord overdoses of 5%–11% occurred with gantry angle, collimator angle, couch angle, MLC leaf end modeling, and MLC transmission and leakage modeling FMs. PTV coverage and/or OAR sparing was compromised in all FMs introduced in phantom plans with the exception of CT number to electron density tables, MU linearity, and MLC tongue‐and‐groove modeling. Physical measurements did not entirely agree with treatment planning results. For example, symmetry errors resulted in the largest physically measured discrepancies of up to 3% in the PTVs while a maximum of 0.5% deviation was seen in the treatment planning studies. Patient treatment plan study results are under analysis.Conclusion:Even in the simplistic anatomy of the IROC phantom, some basic physics FMs, just outside of TG‐142 tolerance criteria, appear to have the potential for large clinical implications.

A dosimetric study of deep skin electron therapy with overlapping stationary radiation fields – A case report

Background : For radiotherapy of total skin including sub-cutaneousssue up to a depth of 3 cm on the enre leleg of an adult (Angiosarcoma skin), a complex treatment with mulple staonary electron fi elds was planned at our clinic. The details of dosimetry, clinical dose measurements are presented. Materials and Methods : The treatment planned with 6 overlapping 9 MeV electron fields in Clinac 2300CD linac. With 25×25 cm cone, a cut-out insert provided 56 × 30 cm field at FSD 213 cm, while paent lying on the floor. Dose distribuons were checked using Kodak V densitometric film in cylindrical plasc can phantom. The calibraon was carried out using solid water phantom, water equivalent IMRT phantom and water can leg phantom. A dose of 45 Gy in 23 fracons at 5 fracons/week was prescribed. 6 field overlapping field factor was measured by the method described for total body electron irradiaon (AAPM). Skin doses were esmated at random selected points using TLD chips and semiconductor diodes. Results : Measured absorbed doses by three methods were 0.174 cGy/MU, 0.166 cGy/MU and 0.162 cGy/MU agreed well with the calculated value 0.163 cGy/MU. 6 field overlap factor was 2.315. Clinical dose esmates of mean skin dose was 246.0 + 14 cGy (n=18), delivering higher dose by 23%. The gonad dose esmate under shield was <5%. The excess dose to skin delivered in first 14 fracons was adjusted in following 9 fracons. Conclusion : It appears that the excess dose in real situaon may be due to either floor backscaDer or non uniform overlap of dose from adjacent fields.

SU-E-T-105: Development of 3D Dose Verification System for Volumetric Modulated Arc Therapy Using Improved Polyacrylamide-Based Gel Dosimeter

Purpose: The aim of this dosimetric study was to develop 3D dose verification system for volumetric modulated arc therapy (VMAT) using polyacrylamide-based gel (PAGAT) dosimeter improved the sensitivity by magnesium chloride (MgCl2). Methods: PAGAT gel containing MgCl2 as a sensitizer was prepared in this study. Methacrylic-acid-based gel (MAGAT) was also prepared to compare the dosimetric characteristics with PAGAT gel. The cylindrical glass vials (4 cm diameter, 12 cm length) filled with each polymer gel were irradiated with 6 MV photon beam using Novalis Tx linear accelerator (Varian/BrainLAB). The irradiated polymer gel dosimeters were scanned with Signa 1.5 T MRI system (GE), and dose calibration curves were obtained using T2 relaxation rate (R2 = 1/T2). Dose rate (100-600 MU min−1) and fractionation (1-8 fractions) were varied. In addition, a cubic acrylic phantom (10 × 10 × 10 cm3) filled with improved PAGAT gel inserted into the IMRT phantom (IBA) was irradiated with VMAT (RapidArc). C-shape structure was used for the VMAT planning by the Varian Eclipse treatment planning system (TPS). The dose comparison of TPS and measurements with the polymer gel dosimeter was accomplished by the gamma index analysis, overlaying the dose profiles for a set of data on selected planes using in-house developed software. Results: Dose rate and fractionation dependence of improved PAGAT gel were smaller than MAGAT gel. A high similarity was found by overlaying the dose profiles measured with improved PAGAT gel dosimeter and the TPS dose, and the mean pass rate of the gamma index analysis using 3%/3 mm criteria was achieved 90% on orthogonal planes for VMAT using improved PAGAT gel dosimeter. Conclusion: In-house developed 3D dose verification system using improved polyacrylamide-based gel dosimeter had a potential as an effective tool for VMAT QA.

SU‐E‐T‐378: IMRT Severity Scoring for TG‐100: Do You Really Know?

Purpose: Failure modes and effects analysis (FMEA) as defined in TG‐100 has become a popular concept throughout the radiotherapy community. This risk mitigation technique involves detailed process mapping, analysis, and ranking of potential errors by means of a subjective, ordinal scoring system. This study aims to reduce the subjectivity of FMEA severity scoring for IMRT delivery by providing quantitative values. Methods: First we created an IMRT delivery process map for physics‐applicable processes and identified 11 physical failure modes (FMs). To determine the magnitude of dose delivery errors for several of the physical FMs (i.e., the severity of the FM), FMs were induced and dosimetry measurements were performed on a Varian Clinac accelerator going out of clinical service. Treatment planning studies to simulate remaining FMs are to follow. The quantitative severity scores will be compared to recommended FMEA subjective scores. Results: We identified the following physical FMs to investigate: photon beam energy, symmetry, MLC position, MLC transmission and leakage, MLC rounded‐end and leaf offset, MLC tongue‐and‐groove, CT look‐up table, gantry angle, collimator angle, couch angle and displacement, and MU linearity. Within the PTV of a H&amp;N IMRT phantom, the maximum dose delivery error of 3% absolute dose was seen for 2 mm systematic MLC offsets, 4% for 1.1% increased energy, and 4% for 3.5% symmetry error. Measurements will be compared to computational study results and then used for quantitative severity scoring determination. Conclusion: Current FMEA practice for radiotherapy requires quantitative data in order to make accurate assessments associated with clinical QA programs. This study has shown examples of error magnitudes induced by IMRT physical FMs that can be used to quantitate and rank FMEA severity scoring. Work supported by grants CA10953 and CA81647 (NCI, DHHS).

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.

SU-E-T-109: Calypso RF Interference On Portal Images and a Physical Filter Solution

Purpose: Portal measurement is becoming an important tool for in vivo dosimetric verification, and Calypso provides real‐time tracking capability; however, when both work simultaneously, large interference arises for portal measurement. The purpose of this study is to investigate the interference of Calypso on portal measurement and mitigation of the interference by applying aluminum shielding over portal panels. Methods: For the same IMRT field and phantom setup, we acquired portal measurements at every 15 degree gantry angle, without and with Calypso, and for those measurements with Calypso, we also acquired portal measurements without and with aluminum shielding over the portal panels. The aluminum shielding consists of a layer of aluminum foil of 0.1 mm thickness covering the portal panel. The measurements without Calypso and without aluminum shielding were regarded as the reference images. All other measurements were regarded as the test images. We measured the deviation of the test images from the reference images by the amplitude difference and using the Gamma Index (3%, 3 mm). Results: With Calypso interference and without aluminum shielding, the signals are larger than the reference, and in some unfavorable gantry angles, the signals can be as much as 15% larger. With aluminum shielding, the interference was much reduced to ∼3% for those unfavorable angles, and the Gamma passing rate achieves 95% for most of the angles. Conclusion: The Calypso interference on portal measurements is gantry angle dependent due to panel orientation and proximity with respect to the Calypso transducer. Shielding on the portal panel can largely reduce electronic interference, and it is anticipated that with an improved complete shielding over the entire portal panel, the interference could be further reduced.

Imaging and radiation therapy: Gate Monte Carlo simulation of a MV-CBCT flat panel with specific application in head and neck cancer

Purpose In radiotherapy treatment, it is necessary to position the patient before each session to ensure that the beams target the tumor while sparing the surrounding healthy tissue. This step is made possible via different imaging techniques like the ‘‘electronic portal imaging device” (EPID) which is the most common approach. In this configuration, the imaging source is the same as the treatment source, while a flat panel detector is automatically placed under the patient via an articulated arm. In this context, our study was focused on the Monte Carlo modeling of such an EPID in order to simulate both planar and 3D CBCT images. Methods The work presented here follows a preliminary study in which the treatment part of a clinical LINAC (Siemens Oncor) was modeled in GATE v6.2. The objective was to calculate the dose delivered by complex treatments like IMRT. The objective of the study is to extend this model by incorporating the imaging part of the LINAC, which consists of a flat panel detector (Perkin Elmer XRD1640). For this purpose, geometrical and physical data provided by the manufacturer were first accurately modeled in GATE. Assessments of portal imaging were performed by performing dose calculations in a CQ IMRT phantom with GATE for two measurement points with protocols of 8, 15 and 60 UM and comparing the doses with clinical measurements. And with performing 2D CT projections over a 200° arc, 1° increments, and 41 cm × 41 cm field of view for homogeneous and heterogeneous structures (introduction of a patient head and neck CT patient). Results For the calculation of the deposit dose at the isocentre of the CQ IMRT phantom, respectively GATE gave doses of 5.734 ± 0.169, 11.008 ± 0.777 and 46.006 ± 0.711 cGy compared with measurements are: 6.821, 12,469 and 49.600 cGy protocols for 8, 15 and 60 UM. From the obtained series of 2D projections a 3D reconstruction was performed in each case, and the dimensions of the reconstructed structures were compared with actual objects. Conclusion We have successfully developed within the GATE platform a complete and detailed model of a 160 MLC LINAC with its MV-CBCT flat panel. Future work will consist in simulating epidbased transit dosimetry.

SU‐E‐T‐464: A Patient Specific Quality Assurance Study on Absolute Dose Verification Using Ionization Chambers of Different Volumes in RapidArc Treatments

Purpose: To evaluate and compare five different ionization chamber ‐ phantom combinations for verification in routine patient specific quality assurance of RapidArc treatments. Methods: Fifteen different RapidArc plans conforming to the clinical standards were selected for the study. Verification plan was subsequently created for each treatment plans with different chamber‐phantom combinations CT scanned. This includes Medtec‐ IMRT phantom with micro ionization chamber (0.007cm3) and pinpoint chamber (0.015cm3), PTW‐Octavius phantom with semiflex chamber (0.125cm3) and 2D array (0.125cm3) and indigenously made Circular wax phantom with 0.6 cm3 chamber. The measured isocentre absolute dose was compared with the TPS planned. Results: Micro ionization chamber shows more deviations when compared to semiflex and 0.6 cm3 with a maximum variation of −4.76%,−1.49% and 2.23% for micro ionization, semiflex and farmer chambers respectively. The positive variations indicate that the chamber with larger volume slightly overestimates. Farmer chamber shows higher deviation when compared to 0.125cm3. In general the deviation was found to be less than 1% with semiflex and farmer chamber. A maximum variation of 2% between the plans was observed for 0.007cm3 ionization chamber, except for few cases. Pinpoint chamber underestimates the calculated isocentre dose by maximum 4%. Conclusions: Absolute dose measurements using semiflex ionization chamber with intermediate volume (0.125cm3) shows good agreement with the TPS calculated among the detectors used in this study. Positioning is very important when using smaller volume chambers as they are more sensitive to geometrical errors within the treatment fields. Also it is suggested to average the dose over the sensitive volume for larger volume chambers. The ionization chamber‐phantom combinations used in this study can be used interchangeably for routine RapidArc patient specific quality assurance with a satisfactory accuracy for clinical practices.

SU-E-T-49: A Method of 3D Dose Reconstruction Using MapCHECK Measurement for IMRT QA

Purpose: A method to reconstruct point dose in an IMRT phantom using 2D dose measurement to replace ion chamber measurement for routine prostate IMRT QA.Methods: IMRT treatment plans for a cohort of 46 patients with prostate cancer were generated by Pinnacle V8.0. The planar dose for each IMRT field was measured with MapCHECK 2 mounted at an isocentric jig positioned at SAD of 100.4 cm with water equivalent build up of 5 cm. The 2D MapCHECK measurements with corresponding gantry and collimator angles were subsequently imported into an in‐house code for 3D dose reconstruction in a virtual water equivalent cylindrical phantom (diameter: 20cm, length: 18 cm). The dose deposition to the cylindrical phantom was calculated with ray tracing with percent depth dose (PDD) applied based on the MapCHECK measurements. Separately, a volumetric ion chamber (PTW TN30011‐1) measurement in low dose‐gradient region was made in a water‐equivalent cylindrical phantom of the same size for each IMRT plan. The cumulative dose values measured by the ion chamber were compared with the calculated doses with the proposed method.Results: The cumulative dose was compared among the three different methods. The mean and standard deviation (SD) of the differences were found to be 0.10±1.22% in the comparison of Pinnacle vs ion chamber, 0.10±1.22% in Pinnacle vs proposed method, and 0.26±1.24% in ion chamber vs proposed method, respectively. Bland‐Altman analysis in the cumulative dose between ion chamber and proposed method suggested that there was negligible systematic bias (mean difference: 0.57 cGy) between the two methods and the 95% limits of agreement (−5.42 to 6.57 cGy) was acceptable Conclusions: The study showed the feasibility of replacement of a point dose measurement by ion chamber with the proposed 3D dose reconstruction method for prostate IMRT QA.

WE‐C‐BRB‐09: Volumetric Modulated Arc Therapy Based Total Marrow Irradiation and Dosimetry Verification

Purpose: This study develops treatment planning and a comprehensive dosimetry verification procedure for volumetric modulated arc therapy(VMAT) based total marrow irradiation(TMI), a form of targeted total body irradiation(TBI). Methods: The simulation CT of 3 patients treated with TBI were used. Planning was done with Eclipse treatment planning system(Varian Medical Systems, CA). Planning volume(PTV) was defined as skeletal bone, rib, and sternum, with 3mm margin, from head to mid‐femur. In total marrow and lymphoid irradiation(TMLI), PTV is expanded to include spleen and major lymph nodes. The mandible, maxillary bones and forearms were excluded. The dose prescription was 12Gy in 6 fractions. The PTV is divided into head‐and‐neck(H&amp;N), chest and pelvis regions. Correspondingly, there is separate plan for each region, composed of 2∼3 arcs/fields with multiple isocenters along patient axial direction. The 2D dosimetry verification was conducted with EBT2 Gafchromic film(International Specialty Products, NJ) for gamma evaluation and ion chamber for absolute point dose measurement, using a IMRT phantom(Computerized Imaging Reference Systems, VA) and RIT software(Radiological Imaging Technology, CO), with emphasis of junction area between neighboring plans for hot/cold spots. We further conduct volumetric dose verification with electronic portal imaging devices and DosimetryCheck program(Math resolutions, MD). The gamma evaluation criteria were 5% absolute point dose difference and 3mm distance to agreement. Results: More than 90% of PTV is covered by 100% of prescribed dose. Compared to TBI, average dose reduction on critical organs are: lung(10%), kidney(53%), liver(48%), Eyes(69%), and heart(51%). Junction areas show average 98% gamma passing rate and 3% absolute dose difference; volumetric portal dosimetry show average 91% gamma passing rate and 2% absolute dose difference, compared to plan dose. Conclusions: Adequate target dose coverage and reduced normal organ dose are observed. A comprehensive dosimetry verification procedure is developed. The initial results are encouraging and warrant further investigation towards clinical implementation.

SU‐E‐T‐444: Feasibility Study of In‐Vivo Transit Dosimetry for IMRT Patients

Purpose: The ultimate goal of IMRT QA should be to verify that the correct dose has been delivered to the patient. The purpose of this study is to evaluate for feasibility and reliability of volumetric transit dosimetry as a routine in‐vivo dosimetry method during the actual patient treatment for different sites. Methods: To evaluate volumetric transit dosimetry for IMRT delivery, prostate, H&amp;N and lung IMRT cases were selected. During patient treatment, the EPID imager was extended to collect transit radiation through the patient. Collected transit dose images are imported and reconstructed as a volumetric dose distribution using the Dosimetry Check (Math Resolutions LLC, Maryland, USA) Exit Dose module. Dosimetry Check program to compare to the reconstructed volumetric dose including tissue heterogeneous correction from the transit EPID images. Also, to investigate the influence of daily setup or anatomy change, multiple transit dosimetry measurements were performed on different treatment days up to 5 fractions for prostate case. In addition, patientˈs verification plan was generated on a homogeneous CIRS IMRT phantom to verify the absolute dose and transit dosimetry algorithm check. Collected QA results were evaluated with 1) gamma passing rate (3 mm DTA and 3% dose) comparing calculated dose from exit dosimetry and treatment planning system (TPS) dose prediction, 2) difference of absolute point dose from exit dosimetry module compared to ion chamber measurements in the phantom. Results: The current implementation shows promise at predicting point doses from exit dose images acquired during patient treatment. However, 3D dose distributions calculated from exit dose images are not consistent in their agreement with the planned dose distribution. Conclusions: Further study is needed to explain the differences. Areas to consider include offsets in imager position, differences in dose calculation algorithm, couch attenuation effects and anatomy changes.

SU‐GG‐T‐200: Comparison of a Novel Transmission Detector to a Standard Measurement Technique for Patient IMRT Quality Assurance

Purpose: A new measurement‐based, pre‐treatment IMRT composite quality assurance (QA) procedure was compared to a standard QA method. The standard QA method consisted of delivering the treatment to a cuboidal IMRT phantom containing dose measuring media. This standard measurement employed Gaf‐chromic film sheets for relative dosimetry and an ion chamber for absolute dosimetry. The new method examined here used a novel transmission detector measurement (IBA Dosimetry) designed for in vivo patient dosimetry. However, before in vivo application, we compare to our standard QA method, which is the focus of this work. Method and Materials: Treatment plans for ten previously‐treated, head and neck IMRT patients were used. All treatment plans were QA'd with the standard method at the treatment beam angles. Measurements with the transmission detector (an array of 1600 IC's) were also performed at the treatment beam angles. The COMPASS software (IBA Dosimetry) converts these 2D‐array measurements to incident fluence, and then forward calculates dose in a patient model (ie. CT data). In this work the patient model was a cuboidal shaped solid‐water IMRT phantom (MedTec). Absolute doses were compared using isocentre location for the COMPASS/planning system, and a low dose gradient point location for the IC/planning system comparison (near isocentre, but not necessarily due to alignment needs during measurement). Results: On average, the standard QA method results for absolute dosimetry were different by 1.4±1.8% compared to the planning system (measured above planning system). On average the transmission detector based QA method results for absolute dosimetry were different by 0.3±1.6% compared to the planning system (measured above planning system). Conclusion: QA results using the COMPASS software with the transmission detector were comparable to the standard QA method.Conflict of Interest: CancerCare Manitoba has a collaborative research agreement with IBA Dosimetry, although there is not direct financial support.

SU-GG-T-144: Dosimetric Effect of Using Non-Flat Photon Beam to Create Conventional Open Fields and Standard Phantom IMRT Plans

Purpose: Removing the flattening filter from a photon beam creates a forward peaked intensity profile with a higher dose rate at the central axis, which can be an attractive feature for SBRT and IMRT. The purpose of this project is to understand the impact of using unflattened photon beams to create a conventional square field and various standard IMRT phantom plans by comparing those plans using a flattened beam. Method and Materials: A conventional 10×10 cm2 square field was created by a non‐flat beam with multiple segments. Furthermore, five standard phantom structure sets from the AAPM TG 119 were downloaded for this study, including multitarget, prostate, head and neck, easy C‐Shape, and hard C‐Shape. Planning constraints from TG119 were used as a guide for planning, using a 6MV non‐flat beam from a Siemens Oncor and a 6MV flat beam from a Siemens Artiste. Results: The 10×10 cm2 open field was matched using a non‐flat beam with three segments and 21.7% more monitor units. Using both flat and non‐flat beams, all IMRT plans met the plan acceptance criteria of the target coverage and organ at risk sparring. The plan conformality indices were comparable while keeping complexity of the plans (measured by the number of segments) the same. However, non‐flat beam plans increased monitor units by 0.5% to 32.0% when compared to the corresponding plans with flat beams. The largest increase in monitor units was observed in the plan with a large “plain” target (multitarget). Plans for a concave “irregular” target (C‐Shape) result in similar monitor units. Conclusion: A non‐flat beam can produce equivalent dose distributions to a traditional flat beam. The increase in monitor units is needed when the pointed dose profile is projected onto a flat target. Conflict of Interest: This research is partly supported by Siemens Medical Solutions

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

The Comparison of Two Optimization Algorithms for Step-and-shoot IMRT Planning

There are two optimization algorithms for step-and-shoot IMRT planning: Beamlet-based inverse planning (BBIP) and Aperture-based inverse planning (ABIP). BBIP works with small number of fields, high number of segments, and constant collimator angle. Segments are subordinated automatically according to MLC position for time optimization (sequencer). ABIP is characterized by pre-defined segments, variable collimator angle for each segment, large number of fields, and low number of segments. The purpose of the study is to compare the optimization algorithms in routine clinical praxis. For comparison of both optimization procedures, we used planning systems Plato Sunrise ITP version 1.2.3 (Nucletron) and PrecisePLAN version 2.11 (Elekta) for BBIP and ABIP optimization, respectively. We made 8 pairs of plans in eight patients with locally advanced head and neck cancer. The plans were designed as the first phase of radiotherapy (PTV1) because of high number of critical structures around the target volume in this region. The treatment plans had to be fully comparable regarding the parameter V90 (target volume included in the 90 per cent isodose) for PTV1, mean dose for both parotids and maximum dose for spinal cord. For plan verification we used classical film dosimetry together with measuring of absolute dose in a defined point within the IMRT phantom. Radiation treatment was performed on a linear accelerator Elekta Synergy with 6 MV photons. The plans created on PrecisePlan were used mainly for higher verification precision. For each treatment plan, we compared number of segments, number of monitoring units and treatment time for a single fraction. Statistical analysis was made using paired t-test with two-tailed p values. Mean number of segments produced by PrecisePlan (ABIP) and Plato (BBIP) was 41 and 98, respectively (p < 0,001). Similarly significant difference was observed comparing the mean number of monitor units - 448 for PrecisePlan, and 1591 for Plato (p < 0,001). The number of MU is in fact the real irradiation time (“beam on”), thus the difference might be important for the operating life of radiation-generating parts of the linear accelerator. The number of MU, however, does not represent the real fraction time. To obtain this, we have to include the time necessary for gantry and collimator rotation as well as lamellae movement. The mean fraction times were 15 min. for PrecisePlan and 19 min. for Plato (p = 0,001). The optimization algorithm ABIP for step-and-shoot IMRT produces fewer segments, fewer monitor units and significantly shortens the total fraction time in comparison to the BBIP algorithm.

Dose calculation accuracy using cone-beam CT (CBCT) for pelvic adaptive radiotherapy

This study is to evaluate the dose calculation accuracy using Varian's cone-beam CT (CBCT) for pelvic adaptive radiotherapy. We first calibrated the Hounsfield Unit (HU) to electron density (ED) for CBCT using a mini CT QC phantom embedded into an IMRT QA phantom. We then used a Catphan 500 with an annulus around it to check the calibration. The combined CT QC and IMRT phantom provided correct HU calibration, but not Catphan with an annulus. For the latter, not only was the Teflon an incorrect substitute for bone, but the inserts were also too small to provide correct HUs for air and bone. For the former, three different scan ranges (6 cm, 12 cm and 20.8 cm) were used to investigate the HU dependence on the amount of scatter. To evaluate the dose calculation accuracy, CBCT and plan-CT for a pelvic phantom were acquired and registered. The single field plan, 3D conformal and IMRT plans were created on both CT sets. Without inhomogeneity correction, the two CT generated nearly the same plan. With inhomogeneity correction, the dosimetric difference between the two CT was mainly from the HU calibration difference. The dosimetric difference for 6 MV was found to be the largest for the single lateral field plan (maximum 6.7%), less for the 3D conformal plan (maximum 3.3%) and the least for the IMRT plan (maximum 2.5%). Differences for 18 MV were generally 1–2% less. For a single lateral field, calibration with 20.8 cm achieved the minimum dosimetric difference. For 3D and IMRT plans, calibration with a 12 cm range resulted in better accuracy. Because Catphan is the standard QA phantom for the on-board imager (OBI) device, we specifically recommend not using it for the HU calibration of CBCT.

Investigation of the feasibility of relative 3D dosimetry in the Radiologic Physics Center Head and Neck IMRT phantom using Presage/optical‐CT

This study presents the application of the Presage/optical-CT 3D dosimetry system for relative dosimetry in the Radiologic Physics Center (RPC) Head and Neck (H&N) IMRT phantom. Performance of the system was evaluated by comparison with the "gold-standard" RPC credentialing test. A modified Presage cylindrical insert was created that extended the capability of the RPC H&N phantom to 3D dosimetry. The RPC phantom was taken through the entire treatment planning procedure with both the standard RPC insert and the modified Presage insert. An IMRT plan was created to match the desired dose constraints of the credentialing test. This plan was delivered twice to the RPC phantom: first containing the standard insert, and then again containing the Presage insert. After irradiation, the standard insert was sent for routine credentialing analysis; including point dose measurements (TLD) and planar Gafchromic EBT film measurement. The 3D dose distribution from Presage was read out at Duke using the OCTOPUS 5X optical-CT scanner. The Presage distribution was compared with gold-standard EBT measurement (determined by the RPC) and the calculated Eclipse distribution. The agreement between the normalized EBT, Presage, and Eclipse distributions, in the central axial plane was evaluated using profiles and gamma-map comparisons (4% dose difference and 3 mm distance to agreement). Profiles showed good agreement between EBT, Presage, and Eclipse distributions. 2D gamma-map comparisons between all three modalities showed at least 98% pass rate. The excellent agreement between Presage and EBT in the central plane established Presage as a standard against which to evaluate the accuracy of the 3D calculated Eclipse distribution. A gamma comparison between normalized Presage and Eclipse 3D distributions gave an overall pass rate of approximately 94%. In conclusion, the Presage/optical-CT system was found to be feasible for relative 3D dosimetry in the RPC IMRT H&N phantom. The potential to extend the RPC IMRT credentialing procedure to 3D may be feasible provided accurate calibration to dose (Gy) and robustness to shipping stress are demonstrated.