Neck Plans Research Articles

WE‐C‐BRB‐08: Independent Dose Calculation Followed by Dynalog File Analysis as an Alternative for Measurement Based IMRT QA

Purpose: Experimental methods are commonly used for patient‐specific IMRT verification. The aim of this work is to investigate the possibility of substituting the time and labor intensive IMRT QA with calculation based dose verification and log file analysis. Method: 11 IMRT plans (5 head and neck plans, 3 rectum plans, 3 breast plans and 5 prostate plans) made using Pinnacle were recalculated using a QA phantom. All the plans were verified by ICs and 2D MapCheck measurements and compared to the independent dose calculation using Eclipse. The deviations between dosimetric measurements and independent dose calculation in a QA phantom were evaluated. The plans with patient anatomy were imported to Eclipse for 3D dose calculations. The DVHs and point doses calculated by Pinnacle and Eclipse are evaluated for PTV and critical structures. To verify data transfer and performance of the delivery unit dynalog file analysis was performed.Results: Average deviation between IC measurements and point dose calculations with Pinnalce and Eclipse for Head and Neck were 1.1±1.3% and 1.4±1.6%, respectively. For MapCheck measurement, the mean gamma value with 3% dose difference and 3 mm distance‐to‐agreement were within 1.5% for most plans. The mean 3D dose differences calculated from two different treatment systems are with 3 % for Head‐and‐Neck cases and are within 2% for other plans. The dynalog analysis shows that the gantry angle, jaw position, collimator angle and MLC positions have the passing rate of above 97%. The independent dose calculation with the dynalog analysis takes only 37 minutes, while experimental approach, which takes up to 3 hours. Conclusion: Independent dose calculation followed by dynalog file analysis is a reliable tool to verify the IMRT treatment. With the IMRT QA program becoming more mature, independent dose calculations and dynalog analysis may be used to replace experimental based verification methods.

SU‐D‐214‐04: Accuracy of the Dose Calculation Algorithm of a Commercially Availale Fluence Based IMRT QA Software

Purpose: In this study we attempt to evaluate the accuracy of the dose calculation algorithm available in the DosimetryCheck quality control system used for patient specific IMRT QA. Methods: Slab geometries including inhomogeneities and ten (n=10) lung and head and neck patientsˈ plans from Pinnacle3 (ver. 9.0) and Eclipse (ver. 10.0) treatment planning systems (TPS) were evaluated. Fluence measurements acquired using an Varian aSi1000 electronic portal imaging device (EPID). The measurements consisted of open fields of various size and photon energy for the slab geometry plans and of integrated images for each patient. The EPID images were imported in the Dosimetry Check software. The corresponding plans including CT images, structures and doses were exported to the Dosimetry Check software. The software then used the imported measured fluence of each field to calculate the dose to the patient. The same fluence was also used in Pinnacle3 and Eclipse to calculate the dose to the patient. Results: Discrepancies were observed between the Dosimetry Check and the TPS EPID based calculation for the slab geometry tests with inhomogeneities. The highest discrepancies were observed, as expected, for small fields of high energy. The gamma index values between the TPS fluence based calculations and the Dosimetry Check ranged from 85–98% for the patient cases. Point dose differences of 2–15% were observed for both lung and head and neck plans. Conclusions: Fluence based dose reconstruction using patient geometry is an invaluable tool. However, it is known that the pencil beam algorithm is not able to predict the dose accurately in the presence of inhomogeneities as in the case of head neck and lung and users are advised to use caution when evaluating IMRT QA plans with inhomogeneities. On the other hand, the effect of inhomogeneities is much less profound for pelvic, abdominal and brain cases.

SU-E-T-92: The Field Size Dependence of the ArcCheck 4D Diode Array and Its Correction Strategy

Purpose: The ArcCheck 4D diode array was designed specifically for rotational therapy QA and records data every 50 milliseconds. This work quantifies its field size dependence and designs strategy for its correction. Methods: The field size dependence was measured using a 6 MV photon beam of a linac and compared with ion chamber measurements in the same geometry. Square field sizes of 4×4 cm2 to 30×30 cm2 as well as rectangular fields of 6×4 to 6×30 cm2 elongated in both the X and Y directions were measured. To correct for the field size dependence, the field size for each signal update was automatically detected and a correction factor that corresponded to the equivalent square was applied to the raw data. Five fixed-gantry IMRT plans (18 fields) that covered a variety of sites were used to test the correction strategy. Treatment plans were generated using the Pinnacle3 (v9.0) with a density override of 1.05 g/cc for the entire cylinder. Only beams that did not go through the treatment couch were used for the comparison to eliminate the uncertainty associated with couch transmission. Results: Normalized to a field size of 10×10 cm2, the diodes over responded by 2.1% at 30×30 cm2 and under responded by 2.0% at 4×4 cm2. The biggest improvement in passing rates was seen in the head and neck plan which exhibited the largest treatment fields, with the average passing rates improved from 79.5% and 90.4% for 2%/2mm and 3%/3mm, respectively, when no field size dependence was corrected, to 95.7% and 98.7%, respectively, with field size dependence correction. Conclusions: The ArcCheck diodes exhibit a measurable field size dependence which can be corrected using its 4D raw data. This correction is important for large and small treatment fields which allows for the use of more stringent criteria.

SU-E-T-476: Volumetric Modulated Arc Therapy (VMAT) Plan Validation Using MatriXX Measurements and Monte Carlo Calculation

Purpose: To establish confidence in VMAT plan delivery for recently launched VMAT with Varianˈs RapidArc technique at our clinic, a sophisticated plan validation has been performed by comparison of ion- chamber measurements, planning system calculations, and independent Monte Carlo calculations. Methods: Although MatriXX has been used as a reliable device for validations of intensity modulate radiation therapy (IMRT) treatment plans, it was challenging to use it for VMAT plan validations because of its angle-dependent dose response. A new version of MatriXX Evolution with a gantry angle sensor is able to correct the angle dependency and promises accurate measurements while the gantry rotates. To evaluate the performance of MatriXX for RapidArc plan validations, Monte Carlo calculations using XVMC was used as an independent method to our IMRT validation program. DICOM plans and CT images from Eclipse planning system were translated by an in-house code for XVMC input. Measured dose planes were compared with XVMC dose planes as well as Eclipse dose planes for 12 pelvis plans, 19 prostate plans, and 5 head and neck (H&N) plans. Construction of a scatterplot by taking differences of calculations from measurements displayed a positive correlation between them. Results: Eclipse calculations were off from measurements by 2.64%, 1.89%, and 1.84% in quadratic mean for pelvis, prostate, and H&N plans, respectively. XVMC calculations were off from the measurement by 2.18%, 1.73%, and 2.30% in quadric mean for pelvis, prostate, and HN plans, respectively. Passing percent of Gamma test for Eclipse calculations was over 97.0% and that for XVMC calculations was about 95.7% due to inherent statistical uncertainty of Monte Carlo calculation. Eclipse and XVMC calculations agreed to each other within 4 % Conclusions: The present study shows that Eclipse calculations and measurements using MatriXX evolution can be used for validation of VMAT plans using Rapid Arc delivery technique.

SU‐E‐T‐181: IMRT Quality Assurance ‐Evolution from 2D to 3D

Purpose: We evaluated the performance of a 3D diode device ArcCHECK (Sun Nuclear) for IMRT patient plan verification and compared the results with our current IMRT QA 2D diode array device MapCHECK (Sun Nuclear). Methods: The ArcCHECK QA device consists of 1386 diodes of 0.8×0.8 mm2 size geometrically arranged in 21 adjacent rings spaced 1 cm apart. An acrylic insert with an ionization chamber holder was plugged into ArcCHECK to form a 26.6 cm diameter cylindrical phantom. A compact ion chamber was placed inside the holder to measure dose. Seven IMRT patient plans generated using Pinnacle 9.0 were evaluated for this study, including 3 Head & Neck, 3 prostate and one whole pelvis. Actual beam angles were used for ArcCHECK QA plans, but set to 0 degrees for MapCHECK. Gamma criteria of 3%/3mm were used for analysis. Results: Results using our gamma criteria fall between 90.4% to 96.8% for ArcCHECK and 96% to 100% for MapCHECK. For 3 Head & Neck plans ArcCHECK achieved 96% or above pass rate using our gamma criteria, very similar to our MapCHECK result. 90.4% of ArcCHECK measurements passed the gamma analysis for the whole pelvis while 100% of MapCHECK measurements passed. The gamma analysis for three prostate plans was 91.5%, 95% and 96.7% using Arc Check measurements and 98.2%, 96% and 99.2% using MapCHECK. Doses between ion chamber measurement and calculation were in good agreement (0.89%∼2.44%) for all 7 plans. Conclusions: ArcCHECK achieved above 90% pass rate using gamma criteria of 3%/3mm. ArcCHECK measurements produced very similar results to MapCHECK for most plans. Using ArcCHECK in the clinic for IMRT QA can mimic patient dose delivery and examine gantry angle and couch transmission effects.

SU‐GG‐T‐555: Dosimetric Effect of Couch Position Variation for Intensity Modulated Arc Therapy

Purpose: To investigate the dosimetric effect of variation in treatment couch positioning relative to isocenter for intensity modulated arc therapy. Method and Materials: 5 RapidArc (Varian Medical Systems, Palo Alto, CA) treatment plans (2 head and neck, 2 prostate, and a lung) were selected that were calculated including the treatment couch with the support rails in the lateral position. Calculations were done for the couch positioned ±3 cm relative to isocenter and for the support rails in the center position. Dose‐volume histograms were compared for the targets and critical structures. Results: For ±3 cm shift, the largest deviation of the minimum, maximum, and mean target doses was 0.2%. For the critical structures, the largest deviation of the minimum, maximum, and mean doses was 0.6%, 0.4%, and 0.4%, respectively. The largest deviations occurred for the head and neck plans. Excluding the head and neck plans, the largest deviation of the minimum, maximum, and mean critical structure doses was 0.2%. For the support rails in the center, largest deviation of the minimum, maximum, and mean target doses from the planned lateral rail position was 1.6%, 1.6%, and 1.1%, respectively. For the critical structures, the largest deviation of the minimum, maximum, and mean doses from the planned lateral rail position plan was 0.9%, 3.8%, and 1.2%, respectively. Conclusion: The effect of couch shifts of up to 3 cm relative to isocenter is small. For positioning systems indexed to the treatment couch, the daily variation of the couch position can be safely ignored. However, for positioning systems that are independent of the couch, care must be taken to limit the daily deviation. The support rail positions have a more significant effect than couch position relative to isocenter.Conflict of Interest: First author has a sponsored research agreement with Varian.

SU‐GG‐T‐199: Quality Assurace for Large Field IMRT Using EPID Images

Purpose: Quality assurance of Large Field IMRT (LFIMRT) treatments present a unique problem for users of portal dosimetry because LFIMRT calculations are not currently supported in the portal dosimetry calculation module. However, EPID images of LFIMRT delivery can be acquired in the normal manner used for portal dosimetry. This study reports a technique for the dosimetric analysis of LFIMRT delivery using integrated EPID images and IMRT analysis software. Method and Materials: LFIMRT plans are calculated using the ECLIPSE treatment planning system from Varian Medical Systems. Treatments are delivered and images are acquired using a Varian Trilogy linear accelerator and an as 1000 imager. For comparison with the acquired EPID images we use the AAA algorithm in Eclipse to calculate dose in a plane at dmax in a water phantom simulating the imager geometry. A single scaling factor is applied to the Dicom image to allow conversion to absolute dose. The calculated and measured images are then analyzed using the RIT113 IMRT analysis software. Results: Analysis of a 7 field Head and Neck plan was performed utilizing the gamma analysis with 3%/3mm criteria and a dose threshold of 10%. Gamma passing rates ranged from 98.6% to 99.7% with an average passing rate of 99.3% .Conclusion: Quality assurance of LFIMRT delivery can be performed with integrated EPID images and standard IMRT analysis software utilizing dose calculations at dmax in a water phantom with excellent results.

SU‐GG‐T‐293: Development of National Radiotherapy Audit in the UK

Purpose: The use of audit within the radiotherapy community in the UK has developed over the last 20 years. A National Interdepartmental Audit Network was set up which comprised eight regional groups covering the 66 departments. Recently, national audits have been introduced, to set a baseline for all departments to achieve a dosimetry standard. Method and Materials: Standard photon and electron audits were developed which could be performed by departments within each regional group. The aim was to measure a clinically relevant situation using both host and auditors' equipment. The use of a spreadsheet meant that the results were available instantly to exclude set up errors, and enable investigation of any unexpected results. The uptake of IMRT has been slow in the UK so an audit method was developed in order to increase confidence in centers starting IMRT programs. The method required all institutions to export a plan on their treatment planning system (TPS) and then irradiate dosimetry film in a water equivalent phantom at a standard depth. In addition, a set of alanine dosemeters was used to measure absolute delivered dose at a defined point. The TPS calculated dose grid was exported in a standard format and films and alanine were processed centrally. Results: 33/58 centres in the IMRT audit have been analysed. 49/55 alanine measurements were within 3% of predicted dose. 20/21 prostate plans achieved better than 96% pixels passing a 3%/3mm gamma evaluation and 17/18 head and neck plans at 4%/4mm. Conclusion: Recent reporting of radiotherapy incidents in the press have shown that confidence needs to be established in the whole radiotherapy process and audit has a major part to play in achieving this. Participating in and developing new forms of audit which reflect the modern methods used in 21st century radiotherapy should be a major goal.

Dose verification of helical tomotherapy intensity modulated radiation therapy planning using 2D-array ion chambers

Purpose:To investigate the clinical usage of dose verification of Helical Tomotherapy plans by using 2D-array ion chambers, and to develop an efficient way to validate the dose delivered for the patients during treatments.Materials and Methods:A pixel-segmented ionisation chamber device, IMRT MatriXX™ and Multicube™ phantom from IBA were used on ten selected Tomotherapy IMRT/IGRT head and neck plans in this study. The combined phantom was set up to measure the dose distribution from coronal and sagittal planes. The setup of phantom was guided for verifying the correction position by pre-treatment Tomotherapy MVCT images. After the irradiation, the measured dose distributions of coronal and sagittal planes were compared with those from calculation by the planning system for cross verification. The results were evaluated by the absolute and relative doses as well as Gamma (γ) function. The feasibility of the different measuring methods was studied for this rotational treatment technique.Results:The dose distributions measured by the MatriXX 2D array were in good agreements with plans calculated by Tomotherapy planning system. The discrepancy between the measured dose and predicted dose in the selected points was within ±3%. In the comparison of the pixel-segmented ionisation chamber versus treatment planning system using the 3 mm/3% γ-function criteria, the mean passing rates of 2 mm dose grid with γ-parameter ≤1 were 97.37% and 96.91%, in two orthogonal planes (coronal and sagittal directions), respectively.Conclusion:MatriXX with Multicube is a new system created for rotational delivery quality assurance (QA) and found to be reliable to measure both absolute dose and relative dose distributions, simultaneously. It achieves the goal of an efficient and accurate dosimetry validation method of the helical delivery pattern for the Helical Tomotherapy IMRT planning.

Analysis of RapidArc optimization strategies using objective function values and dose‐volume histograms

RapidArc is a novel treatment planning and delivery system that has recently been made available for clinical use. Included within the Eclipse treatment planning system are a number of different optimization strategies that can be employed to improve the quality of the final treatment plan. The purpose of this study is to systematically assess three categories of strategies for four phantoms, and then apply proven strategies to clinical head and neck cases. Four phantoms were created within Eclipse with varying shapes and locations for the planning target volumes and organs at risk. A baseline optimization consisting of a single 359.8° arc with collimator at 45° was applied to all phantoms. Three categories of strategies were assessed and compared to the baseline strategy. They include changing the initialization parameters, increasing the total number of control points, and increasing the total optimization time. Optimization log files were extracted from the treatment planning system along with final dose‐volume histograms for plan assessment. Treatment plans were also generated for four head and neck patients to determine whether the results for phantom plans can be extended to clinical plans. The strategies that resulted in a significant difference from baseline were: changing the maximum leaf speed prior to optimization (p<0.05), increasing the total number of segments by adding an arc (p<0.05), and increasing the total optimization time by either continuing the optimization (p<0.01) or adding time to the optimization by pausing the optimization (p<0.01). The reductions in objective function values correlated with improvements in the dose‐volume histogram (DVH). The addition of arcs and pausing strategies were applied to head and neck cancer cases, which demonstrated similar benefits with respect to the final objective function value and DVH. Analysis of the optimization log files is a useful way to intercompare treatment plans that have the same dose‐volume objectives and importance values. The results for clinical head and neck plans were consistent with phantom plans.PACS number: 87.55.x, 87.55.D, 87.55.de 87.55.dk

An integral quality monitoring system for real‐time verification of intensity modulated radiation therapy

To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy. The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22 x 22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX. For a 10 x 10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3 x 3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3 X 3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%. The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.

Evaluation criteria for film based intensity modulated radiation therapy quality assurance

The aim of this study was to use different gamma histogram criteria for the comparison of planned dose with irradiated dose distribution and find that what percent of pixels passing a certain criteria imitate a good quality plan. The dose was calculated for 156 patients by inverse planning optimization using the Corvus treatment planning system. Gafchromic films in combination with 2571 0.6 cm 3 Farmer type ionization chamber and Farmer 2570/1 electrometer from NE Technology were used to measure the delivered dose in solid water phantom. All the measurements were performed on Varian CL21EX linear accelerator (Varian Medical Systems, Palo Alto, CA) fitted with a Millennium 120 leaf collimator. In this study the mean value of the percent of passing pixels within the region of interest under the criterion of 3% DD and 3 mm DTA is 90.2 ± 7.1% for head and neck cases and 92.2 ± 5.8% for non-head and neck cases. If we choose the criteria of 3% DD and 3 mm DTA then 96.3% head and neck plans have the percent of passing pixels ≥ 75% and 95.1% non-head and neck plans have the percent of passing pixels ≥ 80%. It is evident from the results of this study that the criterion of 5% DD and 3 mm DTA with the percent of passing pixels ≥ 90 for non-head and neck cases while the percent of passing pixels ≥ 85 for head and neck cases endorse that a plan is good. The results of this study may be useful for other institutions which use verification software and EBT films for patient specific IMRT QA.

SU-FF-T-166: Determination of Minimum Number of Segments for Direct Machine Parameter Optimization (DMPO) for IMRT Plans and Impact On Treatment Delivery Time

Purpose: Direct machine parameter optimization in IMRT plans offers flexibility in controlling the number of segments. The objective was to device a method to determine the DMPO optimal minimum number of segments and to evaluate its impact on treatment delivery time. Method and Materials:IMRT plans for 10 different prostate patients and 10 different head and neck patients were generated using conventional IMRT planning, where intensity maps are optimized prior to conversion into deliverable MLC apertures. Converted plans were then copied and re‐optimized with DMPO using differing numbers of segments, each time maintaining the same prescription objectives and constraints. The process was repeated until a significant difference in the isodose distributions and Dose‐Volume Histograms (DVHs) between the DMPO and the conventional IMRT plans occurred. The ratio of the minimum DMPO number of segments to the conventional IMRT number of segments was calculated. Three plans from each treatment site were performed using DMPO starting from scratch and the treatment planning time evaluated. Results: The maximum target and organ doses with DMPO were found to be about 3% higher than the conventional IMRT maximum doses, resulting into higher hot spots and slightly less dose homogeneity. DMPO reduced the number of segments by 45% for prostate cases and 40% for head and neck cases. DMPO took an average of 3 hours longer to plan from scratch. DMPO provided an average of 50% decrease in treatment delivery time. Conclusion: It is easier and faster to do a conventional IMRT plan first and then apply DMPO to the resultant plan. An acceptable DMPO would need about 55% of the total number of segments of a conventional IMRT prostate plan, and 60% of a conventional head and neck plan. We did not find DMPO to offer any benefit over conventional plans with less than 60 segments.

Commissioning of a System to Correlate Dose Measurements to Patients Anatomy

Complex treatment plans, especially for intensity modulated radiotherapy (IMRT), need to be verified with at least one independent method. No matter which QA method is chosen (ionization chamber, film, or 2D arrays) it is difficult, to correlate a potentially occurring discrepancy between the calculated and measured dose distribution in the phantom to patients anatomy, to estimate its impact on the patient’s organs. We present here a system that can show 3D dose distribution on the patient’s organs based on fluence measurements. Introduction R. Boggula1, F. Lorenz1, H. Chen1, C. Gansemer1, H. Wertz1, A. Giuliacci2, C. Brusasco2, L.Mueller2, F. Wenz1 1Klinikum Mannheim, Mannheim, Germany, 2IBA Dosimetry, Schwarzenbruck, Germany

Poster - Thurs Eve-07: The dosimetric consequences of MLC position inaccuracy in IMRT.

The Multileaf Collimator (MLC), the most widely used means of intensity modulating beams for IMRT, is subject to random and systematic errors in leaf positions that may compromise the treatment quality. This work is a simulation study of the effect of random and systematic errors in leaf position on delivered dose distributions. The dosimetric effects of random errors of up to 2 mm and systematic errors (±1mm in 2 banks, ±0.5mm in 2 banks and 2mm in 1 bank of leaves) were analysed for a typical head and neck IMRT plan and a typical prostate IMRT plan. Dose Volume Histograms and Equivalent Uniform Doses (EUD) of the target volumes, bladder and rectum for the prostate plan and brainstem, larynx, parotids and spinal cord for the head and neck plan were calculated with and without MLC positioning errors and compared. The results show that if we adopt a 2% change in EUD of the target and 2 Gy for the OARs as acceptable levels of uncertainty in dose due to MLC effects only, then random errors of up to 2mm may be tolerated but systematic errors in leaf position will need to be limited to 0.5mm. Our study provides guidance, based on a surrogate of clinical outcome, for the development of quality control standards for multileaf collimators.

SU‐GG‐T‐143: Comparisons of a Monte Carlo IMRT Plan Recalculation Results with the Pinnacle Treatment Planning System

Purpose: We developed a Monte Carlo based IMRT recalculation tool and determined its parameters for Varian Clinac 2100C 6 MV and 18 MV photon beams. We report our comparisons with prostate and head and neck IMRT Pinnacle treatment plans. Method and Materials: Our source model components include: a primary photon point source, an extended extra‐focal source, and contamination electrons. One unique feature of the system is that it is fluence‐based, not a segment based calculation. A modified composite fluence map for each beam is built by summing the MLC segments and modifying for the effects of leakage, and rounded leaf edges. Model parameters are automatically determined by fitting to measurements. We re‐computed two 6 MV head &amp; neck, and three 18 MV prostate, IMRT plans created by Pinnacle. For head &amp; neck plans, we compared the DVHs of PTV, brainstem and parotid glands, as well as the mean dose to parotid glands. For the prostate plans, the DVHs for PTV, rectum, and bladder, as well as the D50, D98, and minimum doses for PTV are compared. Results: We found that our dose calculation system is comparable with Pinnacle for prostate IMRT plans, with small differences. For the prostate tests, the D50 for the PTV agrees within 0.7% with Pinnacle. DVHs for rectum and bladder all agree closely. The model predicts more pronounced dose inhomogeneity inside PTV in head and neck cases: the average reduction in the D98 value for the primary PTV was 5.5%. Conclusion: As expected, prostate IMRT recalculations agree well with the Pinnacle results. However, differences in head and neck results may be due to improved physics in the Monte Carlo system. The results support the use of the Monte Carlo tool as a treatment planning QA tool. Conflict of Interest: Work partially supported by grant PHS CA010953.

SU‐GG‐T‐351: Using Dose Mass Histograms (DMH) for the Evaluation of Head and Neck IMRT Plans Calculated by Monte Carlo

Purpose: Dose volume histograms (DVH) represent an important tool for the clinical evaluation of radiotherapy treatment plans. For head and neck regions, however, the presence of air cavities makes the DVH tool less adequate for Monte Carlo calculated IMRT plans. The air cavities may introduce high dose uncertainties in both air cavities and surrounding tissues (interface effects). Since the dose to air is clinically irrelevant DVH becomes less clinically representative in the presence of large air cavities, and hence is no longer a good parameter for plan evaluation. In this work we assess the limitations of DVH for head and neck plan evaluation and investigate the dose mass histogram (DMH) as an alternative to overcome those limitations. Method and Materials: Seven IMRT head and neck cases were included in this study. The Monte Carlo simulation geometry (2mm voxels) was built from patient CT data. Patient dose calculations were performed using the EGS4 based MCSIM code and photon source models for 6 and 10MV beams. Isodose distributions and DVH (including air) were generated for these plans. A new feature was created in order to calculate DMH for both the target (PTV) and critical structures. In addition, DVH excluding air were also generated. Results: Our results for the 7 patients show significant differences (up to 10%) between the DVH calculated including and excluding air for the PTV and critical structures. DMH, on the other hand, eliminates the effect of large Monte Carlo statistical uncertainties in air cavities and is a better parameter than DVH for evaluating head and neck IMRT treatment plans. Conclusion: DMH is a better parameter than DVH for evaluating treatment plans calculated by Monte Carlo simulations that may have large statistical uncertainties in low‐density regions such as air cavities and lung tissues.

Performance of a hybrid MC dose algorithm for IMRT optimization dose evaluation

This paper presents a hybrid intensity modulated radiation therapy (IMRT) optimization strategy which combines the speed of pencil beam (PB) and the accuracy of Monte Carlo (MC) dose calculations. After an initial deliverable-based optimization using a PB algorithm, doses are recomputed using the VMC++ MC code to determine dose correction factors, which are then utilized during further PB-based optimization. The hybrid method is benchmarked with respect to full MC deliverable-based optimization for ten prostate and ten head-and-neck IMRT plans. Final optimized plans are compared in terms of dose-volume indices used for the plan optimization. Dose prediction errors (DPEs) and optimization convergence errors (OCEs) at intermediate steps of the hybrid sequence are evaluated. The hybrid method is found to produce optimized plans that are clinically equivalent to full MC-based optimization, yet requires only 40% of the number of MC dose calculations. With the hybrid strategy presented here, MC-based optimization results are achieved in 35 min or less on a modest computing cluster. While the initial PB-deliverable-based optimization is found to have DPEs and OCEs of up to 3 Gy relative to the 65-73 Gy prescription doses, application of the first MC correction reduces the average DPEs to less than 0.3 Gy for the prostate plans and less than 0.06 Gy for the head and neck plans. The maximum observed DPE or OCE is 0.7 Gy after 1 MC dose correction, indicating that a single MC dose calculation correction might be sufficient for IMRT optimization.

Treatment verification in the presence of inhomogeneities using EPID‐based three‐dimensional dose reconstruction

Treatment verification is a prerequisite for the verification of complex treatments, checking both the treatment planning process and the actual beam delivery. Pretreatment verification can detect errors introduced by the treatment planning system (TPS) or differences between planned and delivered dose distributions. In a previous paper we described the reconstruction of three-dimensional (3-D) dose distributions in homogeneous phantoms using an in-house developed model based on the beams delivered by the linear accelerator measured with an amorphous silicon electronic portal imaging device (EPID), and a dose calculation engine using the Monte Carlo code XVMC. The aim of the present study is to extend the method to situations in which tissue inhomogeneities are present and to make a comparison with the dose distributions calculated by the TPS. Dose distributions in inhomogeneous phantoms, calculated using the fast-Fourier transform convolution (FFTC) and multigrid superposition (MGS) algorithms present in the TPS, were verified using the EPID-based dose reconstruction method and compared to film and ionization chamber measurements. Differences between dose distributions were evaluated using the gamma-evaluation method (3%/3 mm) and expressed as a mean gamma and the percentage of points with gamma> 1 (P(gamma>1)). For rectangular inhomogeneous phantoms containing a low-density region, the differences between film and reconstructed dose distributions were smaller than 3%. In low-density regions there was an overestimation of the planned dose using the FFTC and MGS algorithms of the TPS up to 20% and 8%, respectively, for a 10 MV photon beam and a 3 x 3 cm2 field. For lower energies and larger fields (6 MV, 5 x 5 cm2), these differences reduced to 6% and 3%, respectively. Dose reconstruction performed in an anthropomorphic thoracic phantom for a 3-D conformal and an IMRT plan, showed good agreement between film data and reconstructed dose values (P(gamma>1) <6%). The algorithms of the TPS underestimated the dose in the low-dose regions outside the treatment field, due to an implementation error of the jaws and multileaf collimator of the linac in the TPS. The FFTC algorithm of the TPS showed differences up to 6% or 6 mm at the interface between lung and breast. Two intensity-modulated radiation therapy head and neck plans, reconstructed in a commercial phantom having a bone-equivalent insert and an air cavity, showed good agreement between film measurement, reconstructed and planned dose distributions using the FFTC and MGS algorithm, except in the bone-equivalent regions where both TPS algorithms underestimated the dose with 4%. Absolute dose verification was performed at the isocenter where both planned and reconstructed dose were within 2% of the measured dose. Reproducibility for the EPID measurements was assessed and found to be of negligible influence on the reconstructed dose distribution. Our 3-D dose verification approach is based on the actual dose measured with an EPID in combination with a Monte Carlo dose engine, and therefore independent of a TPS. Because dose values are reconstructed in 3-D, isodose surfaces and dose-volume histograms can be used to detect dose differences in target volume and normal tissues. Using our method, the combined planning and treatment delivery process is verified, offering an easy to use tool for the verification of complex treatments.

SU-FF-T-215: Estimation of Whole Body Dose During the Delivery of Step-And-Shoot Intensity Modulated Radiation Therapy

Purpose: To investigate whether the increase in the number of monitor units required to deliverIMRT results in a significant increase in the whole body dose to the patient. Method and Materials:IMRT and conformal plans were constructed using Nucletron MasterPlan for head and neck and prostate tumours. The anthropomorphic phantom RandoMan was used as a dummy patient. TLD 100's were placed along the central axis of the phantom in 5cm intervals and the whole body dose was then measured for all plans. In addition, the effective whole body dose was calculated for the head and neck plans using NCRP report 116. Results: The number of monitor units required to deliver a cGy to the isocenter was up to three times greater for the IMRT plans in comparison to the conventional conformal plans. A significant increase in whole body dose was observed when comparing 15MV IMRT prostate plans with the conformal prostate plan. The 6MV prostate plans showed no significant increase in whole body dose due to the lack of neutron production at this energy. The calculated whole body effective dose per Gy delivered to the isocenter for the head and neck plans was 6.23mSv, 5.12mSv and 4.27mSv for the nine field IMRT, seven field IMRT and conventional conformal plans respectively. Conclusion: The 15MV prostate plans should be avoided where possible as they lead to a significant increase in whole body dose. Due to the larger treatment field with the conformal head and neck plan, the scatter leakage along the patient is greater for the conventional plan than the IMRT plans up to 40cm from the isocenter, beyond this the head leakage becomes dominant and the dose from IMRT plans is greater. The whole body effective dose is lower for the IMRT head and neck plans than the conformal plans.