Monitor Unit Calculation Research Articles

TH-C-19A-07: Output Factor Dependence On Range and Modulation for a New Proton Therapy System

Purpose: Proton treatment planning systems are not able to accurately predict output factors and do not calculate monitor units (MU) for proton fields. Output factors (cGy/MU) for patient-specific fields are usually measured in phantoms or modeled empirically. The purpose of this study is to predict the output factors (OFs) for a given proton (R90) and modulation width (Mod) for the first Mevion S250 proton therapy system. Methods: Using water phantoms and a calibrated ionization chamber-electrometer, over 100 OFs were measured for various R90 and Mod combinations for 24 different options. OFs were measured at the center of the Mod, which coincided with the isocenter. The measured OFs were fitted using an analytic model developed by Kooy (Phys.Med.Biol. 50, 2005) for each option and a derived universal empirical-based polynomial as a function of R90 and Mod for all options. Options are devised for ranges of R90 and Mod. The predicted OFs from both models were compared to measurements. Results: Using the empirical-based model, the values could be predicted to within 3% for at least 90% of measurements and within 5% for 98% of the measurements. Using the analytic model to fit each option with the same effective source position, the predictionmore » is much more accurate. The maximal uncertainty between measured and predicted is within 2% and the averaged root-mean-square is 1.5%. Conclusion: Although the measured data was not exhaustive, both models predicted OFs within acceptable uncertainty. Both models are currently used for a sanity check of our continual patient field OF measurements. As we acquire more patient-field OFs, the model will be refined with an ultimate goal of eliminating the time-consuming patient-specific OF measurements.« less

SU-E-T-100: Designing a QA Tool for Enhance Dynamic Wedges Based On Dynalog Files

Purpose: A robust quality assurance (QA) program for computer controlled enhanced dynamic wedge (EDW) has been designed and tested. Calculations to perform such QA test is based upon the EDW dynamic log files generated during dose delivery. Methods: Varian record and verify system generates dynamic log (dynalog) files during dynamic dose delivery. The system generated dynalog files contain information such as date and time of treatment, energy, monitor units, wedge orientation, and type of treatment. It also contains the expected calculated segmented treatment tables (STT) and the actual delivered STT for the treatment delivery as a verification record. These files can be used to assess the integrity and precision of the treatment plan delivery. The plans were delivered with a 6 MV beam from a Varian linear accelerator. For available EDW angles (10°, 15°, 20°, 25°, 30°, 45°, and 60°) Varian STT values were used to manually calculate monitor units for each segment. It can also be used to calculate the EDW factors. Independent verification of fractional MUs per segment was performed against those generated from dynalog files. The EDW factors used to calculate MUs in TPS were dosimetrically verified in solid water phantom with semiflex chamber on central axis.more » Results: EDW factors were generated from the STT provided by Varian and verified against practical measurements. The measurements were in agreement of the order of 1 % to the calculated EDW data. Variation between the MUs per segment obtained from dynalog files and those manually calculated was found to be less than 2%. Conclusion: An efficient and easy tool to perform routine QA procedure of EDW is suggested. The method can be easily implemented in any institution without a need for expensive QA equipment. An error of the order of ≥2% can be easily detected.« less

TH‐E‐BRE‐10: Independent Verification of Monitor Units for Volumetric Modulated Arc Therapy (VMAT)

Purpose:A challenging problem in VMAT is how to independently validate the monitor units (MUs) of the treatment plan for a patient generated by the treatment planning system (TPS). In this work we establish a novel framework for independent MU verification of VMAT.Methods:The proposed MU verification technique is based on the relationship between the absorbed dose and the weighed sequence of station points. In VMAT, the dose at a point can be expressed as a sum of contributions from all the station points (or control points). To proceed, we first obtain the matrix elements which characterize the dosimetric contribution of the involved station points by computing the doses at a series of voxels, typically on the prescription surface of the VMAT treatment plan, with unit MU setting for all the station points. This dose matrix is calculated using an in‐house Monte Carlo (MC) software. The MUs of the station points are then derived by minimizing the least‐squares difference between doses computed by the TPS and that of the MC for the selected set of voxels. The technique is applied to sixteen clinical cases with a variety of energies, disease sites and TPS dose calculation algorithms.Results:For cases without tissue density inhomogeneity, the independently computed MUs agree with that of TPS to within 2.7% for all the station points. For lung cases, the MC‐calculated MUs differ significantly from that of the treatment plan computed using AAA. However, the discrepancies are reduced to within 3% when the TPS dose calculation algorithm is switched to a transport equation‐based technique (AcurosTM).Conclusion:An independent MU calculation method is developed, for the first time, for patient specific VMAT treatment plan QA.

Implementation of an improved dose-per-MU model for double-scattered proton beams to address interbeamline modulation width variability.

Because treatment planning systems (TPSs) generally do not provide monitor units (MUs) for double‐scattered proton plans, models to predict MUs as a function of the range and the nominal modulation width requested of the beam delivery system, such as the one developed by the MGH group, have been proposed. For a given nominal modulation width, however, the measured modulation width depends on the accuracy of the vendor's calibration process and may differ from this nominal value, and also from one beamline to the next. Although such a difference can be replicated in our TPS, the output dependence on range and modulation width for each beam option or suboption has to be modeled separately for each beamline in order to achieve maximal 3% inaccuracy. As a consequence, the MGH output model may not be directly transferable. This work, therefore, serves to extend the model to more general clinic situations. In this paper, a parameterized linear‐quadratic transformation is introduced to convert the nominal modulation width to the measured modulation width for each beam option or suboption on a per‐beamline basis. Fit parameters are derived for each beamline from measurements of 60 reference beams spanning the minimum and maximum ranges, and modulation widths from 2 cm to full range per option or suboption. Using the modeled modulation width, we extract the MGH parameters for the output dependence on range and modulation width. Our method has been tested with 1784 patient‐specific fields delivered across three different beamlines at our facility. For these fields, all measured outputs fall within 3%, and 64.4% fall within 1%, of our model. Using a parameterized linear‐quadratic modulation width, MU calculation models can be established on a per‐beamline basis for each double scattering beam option or suboption.PACS number: 87.53.Qc

Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71.

A protocol is presented for the calculation of monitor units (MU) for photon and electron beams, delivered with and without beam modifiers, for constant source-surface distance (SSD) and source-axis distance (SAD) setups. This protocol was written by Task Group 71 of the Therapy Physics Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol defines the nomenclature for the dosimetric quantities used in these calculations, along with instructions for their determination and measurement. Calculations are made using the dose per MU under normalization conditions, D'0, that is determined for each user's photon and electron beams. For electron beams, the depth of normalization is taken to be the depth of maximum dose along the central axis for the same field incident on a water phantom at the same SSD, where D'0 = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent D'0 ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of a normalization depth of dm, with D'0 = 1 cGy/MU, although both systems are acceptable within the current protocol. For photon beams, the formalism includes the use of blocked fields, physical or dynamic wedges, and (static) multileaf collimation. No formalism is provided for intensity modulated radiation therapy calculations, although some general considerations and a review of current calculation techniques are included. For electron beams, the formalism provides for calculations at the standard and extended SSDs using either an effective SSD or an air-gap correction factor. Example tables and problems are included to illustrate the basic concepts within the presented formalism.

A simple formula to determine the bolus thickness on an oblique surface

For certain clinical situations such as breast treatments, a bolus is used and must be accounted for in monitor unit calculations. Because of the oblique incidence involved, the thickness of bolus traversed by the radiation beam, as opposed to its nominal thickness, must be used. Using principles of geometry, a simple method is derived to calculate the thickness of bolus for these calculations.

Effect of Acuros XB algorithm on monitor units for stereotactic body radiotherapy planning of lung cancer

Stereotactic body radiotherapy (SBRT) is a curative regimen that uses hypofractionated radiation-absorbed dose to achieve a high degree of local control in early stage non–small cell lung cancer (NSCLC). In the presence of heterogeneities, the dose calculation for the lungs becomes challenging. We have evaluated the dosimetric effect of the recently introduced advanced dose-calculation algorithm, Acuros XB (AXB), for SBRT of NSCLC. A total of 97 patients with early-stage lung cancer who underwent SBRT at our cancer center during last 4 years were included. Initial clinical plans were created in Aria Eclipse version 8.9 or prior, using 6 to 10 fields with 6-MV beams, and dose was calculated using the anisotropic analytic algorithm (AAA) as implemented in Eclipse treatment planning system. The clinical plans were recalculated in Aria Eclipse 11.0.21 using both AAA and AXB algorithms. Both sets of plans were normalized to the same prescription point at the center of mass of the target. A secondary monitor unit (MU) calculation was performed using commercial program RadCalc for all of the fields. For the planning target volumes ranging from 19 to 375cm3, a comparison of MUs was performed for both set of algorithms on field and plan basis. In total, variation of MUs for 677 treatment fields was investigated in terms of equivalent depth and the equivalent square of the field. Overall, MUs required by AXB to deliver the prescribed dose are on an average 2% higher than AAA. Using a 2-tailed paired t-test, the MUs from the 2 algorithms were found to be significantly different (p < 0.001). The secondary independent MU calculator RadCalc underestimates the required MUs (on an average by 4% to 5%) in the lung relative to either of the 2 dose algorithms.

MapCHECK vs MU Check for QA of FinF Breast Techniques

This project proposes introducing a new QA technique for performing patient specific forward-planned IMRT verification focusing on breast patients, with a view to increasing department efficiency in UPMC Whitfield Cancer Centre. Since the introduction of the field-in-field (FinF) forward planned technique, patient specific QA has been to irradiate a 2D diode array (MapCHECK™) with the patient plan (recalculated on a phantom representing MapCHECK) and then to compare the resulting dose distribution to the patient plan. We propose that a monitor unit (MU) calculation could be performed for each segment, as is routine for static fields. This may lead to increased efficiency through a reduction in machine IMRT QA time which could be redirected to machine QA and departmental projects. This project involved a retrospective analysis of fifty breast patients from 2012, with varying types of breast cancer treatments. There were 142 open tangent fields; 136 of these fields were 6 MV and 6 were 18 MV. The average number of FinF segments was 2 per open field, with a maximum of 4 segments. All 142 fields had passed a MapCHECK QA test with an average pass rate of 99.5%. Of the 142 fields evaluated in this study, 141 passed the MU calculation. It was found that MU check is a valid QA method for FinF breast treatments. This led to increased departmental efficiency with an average of 10 minutes QA time saved per patient – which is approximately 33 hours per year for breast patients alone.

Dosimetric Advantages and Superior Treatment Delivery Efficiency of RapidArc over Conventional Intensity-modulated Radiotherapy in High-risk Prostate Cancer Involving Seminal Vesicles and Pelvic Nodes

AimsTo compare the dosimetry and treatment delivery efficiency of RapidArc with conventional intensity-modulated radiotherapy (IMRT) in the treatment of high-risk prostate cancer. Materials and methodsFifteen patients with high-risk localised prostate cancer were studied. Sequential treatment was used. The initial planning target volume (PTV-L) included the prostate, seminal vesicles and pelvic lymphatics, whereas the prostate boost PTV (PTV-P) included the prostate and seminal vesicles only. The total prescription dose was 76 Gy (44 Gy to PTV-L, 32 Gy to PTV-P; 2 Gy/fraction). Two separate planning techniques were generated for each patient: seven static-field IMRT versus two-arc RapidArc. Dose–volume parameters for the organs at risk, conformity index and homogeneity index for the PTVs, the calculated monitor units and treatment delivery time for both techniques were compared. ResultsRapidArc gave more conformal plans than IMRT for both PTVs. RapidArc gave a higher homogeneity index to the PTV-P and a similar homogeneity index to the PTV-L. The two techniques gave similar dosimetric results for the rectum, bladder and femoral heads. The mean dose (Dmean) and the maximum dose (Dmax) of the bowel space were reduced by 3.06 and 2.83%, respectively, with RapidArc. The V20 Gy, V30 Gy and V40 Gy for healthy tissues were reduced by 7.77, 14.25 and 17.55%, respectively, with RapidArc. The calculated treatment delivery time and monitor units were reduced by 74.09%/60.93% and 68.32%/48.06% for the PTV-L/PTV-P, respectively, with RapidArc. ConclusionRapidArc is better than conventional IMRT in terms of dosimetry and delivery efficiency for high-risk prostate cancer.

Pre-treatment quality assurance for stereotactic cranial plans

Objective To report the Results of Quality Assurance (QA) dose planning for stereotactic cranial treatments delivered with Dynamic Conformal Arcs (DCA) or Volumetric Modulated Arc Therapy (VMAT) techniques. Material and methods A total of 26 dose plans, 19 patients with DCA from Iplan® (Brainlab) and 7 patients with VMAT from EclipseTM (Varian), were verified with different QA methods. Plans were delivered on a Novalis TX unit. Each treatment field was checked for absolute isocenter dose using a Lucy® 3D QA Phantom (Standard Imaging) and a Diamond chamber (PTW). GafchromicTM EBT3 film dosimetry (ISP) was selected to verify total dose plans in the 2D axial planes. A flatbed RGB scanner (Epson Expression 10,000 XL) was used to digitize the films and the RIT software to analyze the data (gamma criteria 1 mm and 3%). Relative dose analysis was performed normalized at isocenter. Same criteria were used for Portal dosimetry (PD) verifications of VMAT plans. Finally for DCA plans, manual independent Monitor Units Calculation (MUC) number was implemented. Results Absolute isocenter dose measurements: the mean dose ratio (measured/expected) was for DCA (total of 102 fields, dose range 0.3–6 Gy), and for VMAT treatments (a total of 22 treatment fields, and dose range 0.3–1.3 Gy) 1.014 (SD 0.017) and 1.034 (SD 0.031) respectively. Films: the mean gamma agreement index (GAI) was for DCA plans (3 patients) and VMAT plans (3 patients) 99.9% (SD 0.1%) and 93.0 (SD 3.9%) respectively. PD: for VMAT plans (22 fields) the mean GAI was 93.4% (SD 3.4%). MUC: the mean relative difference over 102 treatment fields was 0.02% (SD 1.64%). Conclusion DCA QA results were excellent. VMAT, chosen to better spare organ at risk with its modulation, showed worst results likely for this last feature. HybridArc will next be tested. To report the Results of Quality Assurance (QA) dose planning for stereotactic cranial treatments delivered with Dynamic Conformal Arcs (DCA) or Volumetric Modulated Arc Therapy (VMAT) techniques. A total of 26 dose plans, 19 patients with DCA from Iplan® (Brainlab) and 7 patients with VMAT from EclipseTM (Varian), were verified with different QA methods. Plans were delivered on a Novalis TX unit. Each treatment field was checked for absolute isocenter dose using a Lucy® 3D QA Phantom (Standard Imaging) and a Diamond chamber (PTW). GafchromicTM EBT3 film dosimetry (ISP) was selected to verify total dose plans in the 2D axial planes. A flatbed RGB scanner (Epson Expression 10,000 XL) was used to digitize the films and the RIT software to analyze the data (gamma criteria 1 mm and 3%). Relative dose analysis was performed normalized at isocenter. Same criteria were used for Portal dosimetry (PD) verifications of VMAT plans. Finally for DCA plans, manual independent Monitor Units Calculation (MUC) number was implemented. Absolute isocenter dose measurements: the mean dose ratio (measured/expected) was for DCA (total of 102 fields, dose range 0.3–6 Gy), and for VMAT treatments (a total of 22 treatment fields, and dose range 0.3–1.3 Gy) 1.014 (SD 0.017) and 1.034 (SD 0.031) respectively. Films: the mean gamma agreement index (GAI) was for DCA plans (3 patients) and VMAT plans (3 patients) 99.9% (SD 0.1%) and 93.0 (SD 3.9%) respectively. PD: for VMAT plans (22 fields) the mean GAI was 93.4% (SD 3.4%). MUC: the mean relative difference over 102 treatment fields was 0.02% (SD 1.64%). DCA QA results were excellent. VMAT, chosen to better spare organ at risk with its modulation, showed worst results likely for this last feature. HybridArc will next be tested.

SU‐E‐T‐12: Clinical Implementation of the Dedicated Program for MU Calculation in Proton Beam Therapy

Purpose: The purpose of this study is to develop the dedicated in‐house program for calculation of the monitor unit (MU) of proton therapy beam and to verify the feasibility of its clinical implementation in proton beam therapy with a combination of range modulation wheel (RMW).Materials and Methods: The MU calculation for the proton beam was suggested by Dr.Sahoo in 2008 and given by the formalism using the multiplication of several factors with regard to the structure of the proton beam system. Following his theory, we implemented eight factors into the formalism for MU calculation as follows: Relative output factor (ROF), Spread‐out Bragg peak factor (SOBPF), Range shifter factor (RSF), SOBP off‐center factor (SOBPOCF), Off‐center ratio (OCR), Field size factor (FSF), Inverse square factor (ISF), and Compensator and patient scatter factor (CPSF). Our program was built by Microsoft Visual Basic language. The MU could be calculated by division of the prescribed dose with (d/MU). After building the program, the calculated doses by the formalism were compared with the measured dose. Results: The (d/MU) was calculated by the multiplication using eight factors by use of our program. Once the energy and the settings were input to the program, these factors were estimated from the measurement data. Then MU calculation was done immediately. It was simply and satisfactory fast for the user to complete calculating (d/MU). As the results of the comparison between the measured and calculated doses, the discrepancies in all referential settings were within 2%. Conclusions: The in‐house program provides the fast MU calculation for proton beams. The accuracy of the dose calculation is good in the referential settings. However, further improvement is required for the complicated condition.

SU‐E‐T‐186: Treatment Planning Dose Accuracy of Whole Breast Irradiation Using Field‐In‐Field Technique

Purpose: To evaluate the planned dose accuracy of whole breast irradiation using field‐in‐field technique by comparing with 2D array detector measurements and Monte Carlo simulations. Methods: The ion chamber based Octavius 2D‐array detector (PTW, Freiburg, Germany) was used to measure the absolute dose and dose distributions of tangential field‐in‐field breast irradiation. A commercially available treatment planning system (Eclipse V10.0, Varian, Palo Alto CA USA) was used to calculate the absolute dose and dose distributions. Phantom geometry was simplified for the measurements and for Monte Carlo simulations, but its size and shape were similar to typical breast. The dose accuracy of the treatment planning system was evaluated and the desirable points for monitor unit calculation were recommended based on the results of the study. Results: The discrepancy between measured and planned dose distribution depended on the location of the monitor unit calculation point. The differences were larger when monitor unit calculation point was closer to the irradiated edges of phantom. The doses calculated by the treatment planning system near the center of the irradiated field were about 2% higher than measured doses. There was good agreement between the measured doses and those predicted by Monte Carlo methods. Conclusion: Because of inadequate treatment of scatter in the irradiation of breast with tangential ports, the dose predictions by a treatment planning system do not always agree with the actual measurements. The treatment planning algorithm evaluated in this study over‐compensates for the loss of scatter in tangential ports glancing the sloping surface of a typical breast. The choice of the prescription point can have significant effect on the dose prediction and the actual dose delivered. Keeping the prescription point away from the field edges will help improve the agreement between the calculated and measured dose.

MO-D-108-02: A Secondary Monitor Unit Calculation Algorithm Using Superposition of Symmetric, Open Fields for IMRT and VMAT Treatment Plans

Purpose: To perform a secondary dose calculation for an IMRT or VMAT plan to a point on or off axis within 2% using open field data. Methods: The calculation is performed by subdividing dose into the contributions from each opposing leaf pair for a given MLC configuration. Leaf pair dose is determined by drawing four rectangular fields based on leaf positions that are symmetric about the point of calculation. Field dose is determined using isocentric monitor unit calculation formalism with open field data. Superposition of these fields yields the dose from the leaf pair to the point. VMAT plans are approximated by a static MLC configuration at four degree intervals. The algorithm requires standard open field data (e.g., Scps, TPRs), and the MLC control point information. These calculations were done with additional measured small field output factors down to a 1.2 cm2. Algorithm doses were compared with homogeneous and heterogeneous Pinnacle calculations of a series of prostate, head and neck, and chest wall treatment plans. For each treatment plan the calculation point was taken as the isocenter and/or the center of the PTV. Delivery techniques included SMLC‐IMRT and VMAT. Results: Good agreement was obtained between doses calculated by the algorithm and the Pinnacle Treatment Planning System. For homogeneous cases, the total calculated doses were within 2% for all patient plans. Individual beam errors were as high as 7.8% with an average of 2.8%. Larger errors were associated for beam segments with leaf positions very near the calculation point. Larger errors were also found for heterogeneous calculations, where a simple ratio of TPR correction factor was utilized in the algorithm. Conclusion: Results demonstrate that clinically acceptable agreement is obtained using this method. Further improvement could be made with more accurate heterogeneity correction factors and/or incorporation of MLC leaf end effects. This work was supported in part by a research agreement with Oncology Data Systems, Inc.

SU-E-T-13: An Evaluation of RadCal Software in Its Monitor Unit Calculation Accuracy for Small Dynamic Fields Using HDMLC

Purpose: To investigate the accuracy of an independent monitor unit (MU) verification program for small fields used in dynamically modulated highdefinition multileaf-collimator (HDMLC)-based stereotatic radiosurgery (SRS). Methods: HDMLC-SRS patient-treatment plans were calculated using the BrainLab-pencil beam dose-calculation algorithm implemented in the BrainLab-iPlan treatment planning system (TPS) (v.4.1.2) for the Novalis-TX 6X-SRS beam. Percent depth doses, beam profiles and output tables from this clinical beam were utilized to commission the RadCalc software (v.6.2) for independent MU verification. The MU difference between the MU calculation of the treatment planning system and the independent MU verification program for 187 dynamic arcs and 44 intensity modulated beams corresponding to 41 intracranial lesions from 35 patients was calculated. Field sizes of the arcs and beams studied ranged from 15 to 34 mm in their longest extension. Results: The average difference in the MUs with one standard deviation was −0.1% +/− 1.0% for the dynamic arcs and −0.0%+/− 2.7% for the intensity-modulated beams. The intensity modulated beams verifications showed a much higher spread in the difference compared to the non-modulated dynamic arcs; however, the average results can be considered clinically acceptable. The MU difference correlated inversely with the lesion size, indicating the limitation of the TPS algorithm with smaller field sizes. Conclusion: Use of the RadCalc software as an independent monitor unit verification system for both modulated and non-modulated small fields based on high-definition multileaf collimation is adequate to independently validate the point dose calculation accuracy. The system can be utilized as a part of the SRS quality assurance program.

SU-E-T-04: Application of a Semi-Empirical Formula to Determine Spread Out Bragg Peak Factors and Range Shifter Factors From Measured Depth Dose Curves for Passively Scattered Proton Beams

Purpose: To apply two semi-empirical formulas to calculate the spread out Bragg peak ( SOBP ) factors ( SOBPFs ) and range shifter ( RS ) factors (RSFs) used for monitor unit calculation1 of passively scattered proton beams (PSPB). Methods: Following two recently developed2 and refined formulas were used to calculate the SOBPFs and RSFs from measured depth dose curves for various options in our PSPB. For a specific option,SOBPF(W)=PDDWr(0)/PDDW(0).((SSD+dcW)/(SSD+dcWr))2 .NSF ( W).For the same SOBP width and option, but for different RS thickness, t,RSF (W, t) = (( SSD+dcWt) /)( SSD+dcW)) 2.( PDDW( 0) /PDDW(t)). NSF(W,t),W is the width of the SOBP under consideration, Wr is the reference SOBP width relative to which other SOBPF are determined, SSD is the source to surface distance for the percentage depth dose (PDD) measurement, PDDWr(0) and PDDW(0) are PDDs at the surface, and dcW and dcWr are the depths of the centers of SOBP under consideration and reference SOBP respectively, t is the RS thickness, dcWt and dcW are the depths of the centers of SOBP under consideration with RS of thickness t and without any RS respectively, and PDDw(t) and PDDw(0) are the PDD at depth t and surface respectively for the SOBP width W for which the RSFs are being determined. NSF(W) and NSF (W,t) accounts for the contribution of the nozzle elements including the aperture to the back scatter dose at the dose monitor. Results: The SOBPFs and RSFs calculated using above formulas agreed within 2 percent with their respective measured values using an ionization chamber in a water tank for all of our clinically used options for PSPB. Conclusion: The proposed formulas were found to be valid to calculate the required SOBPFs and RSFs for monitor unit calculation of PSPB fields from measured PDD curves. 1N. Sahoo et. al, Med. Phys. 35, 5088, (2008).2N. Sahoo et. al, IJROBP, 84(3S) S849, (2012).

SU-E-T-06: Comparison of Different Commercial MU Verification Software in Terms of Accuracy and Performance

Purpose: All radiation therapy departments have a need for a quick and accurate verification of their treatment plans ranging from conventional, brachytherapy, to IMRT. The aim of this study is to perform an inter‐comparison of different commercially available Monitor Unit (MU) secondary/independent software. Methods: In this study, four independent MU verification software were examined (IMSure, DIAMOND, MuCheck, and Radcalc) as quality assurance tools for RTP systems. An inter‐comparison of the treatment plans of 13 patients was performed using those MU verification software. All the plans were generated using the Pinnacle v9.2 treatment planning system. The treatment techniques include VMAT, MLC‐based step‐and‐shoot IMRT and Conventional Conformal plans for different treatment sites (breast, head and neck, chest, pelvis, abdomen, and brain). The parameters that had to be adjusted after importing the treatment plans into the different software were the average SSD and effective depth. Results: The average percent differences between the MUs provided by the Pinnacle and the RadCalc, ImSure and DIAMOND software were found to be −1.7%, −1.9% and 3.4%, respectively. The variation of the percent differences among the individual patients were 2.9% (−7.2 − 2.5), 3.7% (−7.2 − 3.7) and 7.0% (−9.9 – 16.2) for RadCalc, ImSure and DIAMOND, respectively. Conclusion: Importing the files from the Pinnacle RTP system was equally easy for all the software. It was found that Radcalc was the software that required the minimum changes/interventions when inserting the average SSD and effective depth. However, the Radcalc was the slower among the examined software in computing the MUs of the different beams for the VMAT technique. Overall, the variation of the MU calculations between the examined software was found to be very similar indicating that their ability to be used as quality assurance tools of the calculations provided by the RTP systems is equivalent.

Comprehensive evaluation and clinical implementation of commercially available Monte Carlo dose calculation algorithm

A commercial electron Monte Carlo (eMC) dose calculation algorithm has become available in Eclipse treatment planning system. The purpose of this work was to evaluate the eMC algorithm and investigate the clinical implementation of this system. The beam modeling of the eMC algorithm was performed for beam energies of 6, 9, 12, 16, and 20 MeV for a Varian Trilogy and all available applicator sizes in the Eclipse treatment planning system. The accuracy of the eMC algorithm was evaluated in a homogeneous water phantom, solid water phantoms containing lung and bone materials, and an anthropomorphic phantom. In addition, dose calculation accuracy was compared between pencil beam (PB) and eMC algorithms in the same treatment planning system for heterogeneous phantoms. The overall agreement between eMC calculations and measurements was within 3%/2 mm, while the PB algorithm had large errors (up to 25%) in predicting dose distributions in the presence of inhomogeneities such as bone and lung. The clinical implementation of the eMC algorithm was investigated by performing treatment planning for 15 patients with lesions in the head and neck, breast, chest wall, and sternum. The dose distributions were calculated using PB and eMC algorithms with no smoothing and all three levels of 3D Gaussian smoothing for comparison. Based on a routine electron beam therapy prescription method, the number of eMC calculated monitor units (MUs) was found to increase with increased 3D Gaussian smoothing levels. 3D Gaussian smoothing greatly improved the visual usability of dose distributions and produced better target coverage. Differences of calculated MUs and dose distributions between eMC and PB algorithms could be significant when oblique beam incidence, surface irregularities, and heterogeneous tissues were present in the treatment plans. In our patient cases, monitor unit differences of up to 7% were observed between PB and eMC algorithms. Monitor unit calculations were also preformed based on point‐dose prescription. The eMC algorithm calculation was characterized by deeper penetration in the low‐density regions, such as lung and air cavities. As a result, the mean dose in the low‐density regions was underestimated using PB algorithm. The eMC computation time ranged from 5 min to 66 min on a single 2.66 GHz desktop, which is comparable with PB algorithm calculation time for the same resolution level.PACS number: 87.55.K‐

EP-1168: RapidArc independent monitor unit calculations with Diamond. Practical considerations

EP-1168: RapidArc independent monitor unit calculations with Diamond. Practical considerations

EP-1196: Evaluation of an independent monitor unit calculation software based on AAPM task group 114 report

EP-1196: Evaluation of an independent monitor unit calculation software based on AAPM task group 114 report

Quantitative analysis of in-air output ratio

Output factor (Scp) is one of the important factors required to calculate monitor unit (MU), and is divided into two components: phantom scatter factor (Sp) and in-air output ratio (Sc). Generally, Sc for arbitrary fields are calculated using several methods based on Sc determined by the absorbed dose measurement for several square fields. However, there are calculation errors when the treatment field has a large aspect ratio and the opening of upper and lower collimator are exchanged. To determine Sc accurately, scattered photons from the treatment head and backscattered particles into the monitor chamber must be analyzed individually. In this report, a simulation model that agreed well with measured Sc was constructed and dose variation by scattered photons from the treatment head and by backscattered particles into the monitor chamber was analyzed quantitatively. The results showed that the contribution of scattered photons from the primary collimator was larger than that of the flattening filter, and backscattered particles were affected by not only the upper jaw but also the lower jaw. In future work, a new Sc determination algorism based on the result of this report will be proposed.