Independent Monitor Unit Research Articles

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.

SU‐E‐T‐24: A Simple Correction‐Based Method for Independent Monitor Unit (MU) Verification in Monte Carlo (MC) Lung SBRT Plans

Purpose:Lung‐SBRT uses hypo‐fractionated dose in small non‐IMRT fields with tissue‐heterogeneity corrected plans. An independent MU verification is mandatory for safe and effective delivery of the treatment plan. This report compares planned MU obtained from iPlan‐XVM‐Calgorithm against spreadsheet‐based hand‐calculation using most commonly used simple TMR‐based method.Methods:Treatment plans of 15 patients who underwent for MC‐based lung‐SBRT to 50Gy in 5 fractions for PTV V100%=95% were studied. ITV was delineated on MIP images based on 4D‐CT scans. PTVs(ITV+5mm margins) ranged from 10.1‐ 106.5cc(average=48.6cc). MC‐SBRT plans were generated using a combination of non‐coplanar conformal arcs/beams using iPlan XVM‐Calgorithm (BrainLAB iPlan ver.4.1.2) for Novalis‐TX consisting of micro‐MLCs and 6MV‐SRS (1000MU/min) beam. These plans were re‐computed using heterogeneity‐corrected Pencil‐Beam (PB‐hete) algorithm without changing any beam parameters, such as MLCs/MUs. Dose‐ratio: PB‐hete/MC gave beam‐by‐beam inhomogeneity‐correction‐factors (ICFs):Individual Correction. For independent‐2nd‐check, MC‐MUs were verified using TMR‐based hand‐calculation and obtained an average ICF:Average Correction, whereas TMR‐based hand‐calculation systematically underestimated MC‐MUs by ∼5%. Also, first 10 MC‐plans were verified with an ion‐chamber measurement using homogenous phantom.Results:For both beams/arcs, mean PB‐hete dose was systematically overestimated by 5.5±2.6% and mean hand‐calculated MU systematic underestimated by 5.5±2.5% compared to XVMC. With individual correction, mean hand‐calculated MUs matched with XVMC by – 0.3±1.4%/0.4±1.4 for beams/arcs, respectively. After average 5% correction, hand‐calculated MUs matched with XVMC by 0.5±2.5%/0.6±2.0% for beams/arcs, respectively. Smaller dependence on tumor volume(TV)/field size(FS) was also observed. Ion‐chamber measurement was within ±3.0%.Conclusion:PB‐hete overestimates dose to lung tumor relative to XVMC. XVMC‐algorithm is much more‐complex and accurate with tissues‐heterogeneities. Measurement at machine is time consuming and need extra resources; also direct measurement of dose for heterogeneous treatment plans is not clinically practiced, yet. This simple correction‐based method was very helpful for independent‐2nd‐check of MC‐lung‐SBRT plans and routinely used in our clinic. A look‐up table can be generated to include TV/FS dependence in ICFs.

In-house spread sheet based monitor unit verification program for volumetric modulated arc therapy

Independent monitor unit verification calculation (MUVC) has been recommended by several authors for intensity modulated radiotherapy (IMRT) as a patient specific quality assurance tool. Aim of the present work is to develop an in-house excel spread sheet based MUVC program for volumetric modulated arc therapy (VMAT) using Clarkson's integration technique. Total scatter factor (Sc,p) and tissue maximum ratio (TMR) for circular fields obtained from Treatment planning system (TPS) were used for the calculation. Multileaf collimator (MLC) interleaf leakage, MLC round edge transmission and tongue and groove effect were accounted. MUVC calculation was performed for 58 patients both for patient anatomy and for homogenous cylindrical phantom. Radiological path lengths were used as water equivalent depths (WED) for calculations using patient anatomy. Monitor unit (MU) discrepancies between −2.60% and 0.28% with mean deviation of −0.92% ± 0.75% were obtained for homogenous cylindrical phantom calculations. MUVC for patient anatomy resulted in large variations between −19.02% and 0.67% for 14 plans where isocenter was at a region below −350 HU. But For 44 plans where the isocenter was at a region above −350 HU, variations between −3.44% and 0.48% were obtained with mean deviation of −1.73% ± 1.12%. For VMAT patient specific quality assurance, the independent MUVC algorithm can be used as an easy and quick auxiliary to measurement based verification for plans with isocenter at a region above −350 HU.

放射線治療における独立したMonitor Units検証の現状調査

To prevent the recurrence of irradiation accidents, monitor-unit (MU) verification that is independent of the MU values calculated using the radiation treatment planning system (RTPS) is required. In this study, we investigated whether independent verification of MU values is being conducted in medical settings and examined the types and methods used. Our results indicate that 93.8% of facilities conduct independent MU verification. The methods used vary according to the facility: 30.4% conduct actual measurements, 18.6% use MS-Excel calculation sheets, 16.7% use specialized commercially-available software, 14.7% use spreadsheets 13.7% use calculators, and 5.9% use other methods. In this study, we were able to formulate basic data that can be used for information-sharing between facilities.

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.

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

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

SU-E-T-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.

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-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.

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

Determination of monitor unit check tolerances based on a comparison with measurement and treatment planning system data

This work describes the experimental validation of treatment planning system monitor unit (MU) calculations against measurement for a range of scenarios. This, together with a comparison of treatment planning system MUs and an independent MU check method, allows the derivation of confidence intervals for the check process. Data were collected for open and 60° motorized wedge fields using an Elekta Synergy linac at 6 and 8MV using homogeneous and heterogeneous phantoms. Masterplan (Version 4.0) pencil-beam and collapsed cone algorithms were used for the primary MU calculations with full inhomogeneity correction. Results show that both algorithms agree with measurement to acceptable tolerance levels in the majority of the cases studied. The confidence interval for the pencil-beam algorithm MU against an independent check was determined as + 1.6% to -3.4%. This is modified to + 2.3% to -2.5% when data collected with low-density heterogeneities are removed as this algorithm is not used clinically for these cases. The corresponding interval for the collapsed cone algorithm was + 1.2% to -4.3%, indicating that an offset tolerance for the independent check is appropriate. Analysis of clinical conformal treatment plan data generated using the pencil-beam algorithm (1393 beams) returned 93% of beams within the independent check tolerance. Similarly, using the collapsed cone algorithm as the primary MU calculation, 77% (of 1434 beams) were within the confidence interval.

SU-E-T-203: Development of a QA Software Tool for Automatic Verification of Plan Data Transfer and Delivery.

Consistency verification between the data from treatment planning system (TPS), record and verification system (R&V), and delivered recorder with visual inspection is time consuming and subject to human error. The purpose of this work is to develop a software tool to automatically perform such verifications. Using Microsoft visual C++, a quality assurance (QA) tool was developed to (1) read plan data including gantry/collimator/couch parameters, multi-leaf-collimator leaf positions, and monitor unit (MU) numbers from a TPS (Xio, CMS/Elekta, or RealART, Prowess) via RTP link or DICOM transfer, (2) retrieve imported (prior to delivery) and recorded (after delivery) data from a R&V system (Mosaiq, Elekta) with open database connectivity, calculate MU independently based on the DICOM plan data using a modified Clarkson integration algorithm, and (4) compare all the extracted data to identify possible discrepancy between TPS and R&V, and R&V and delivery. The tool was tested for 20 patients with 3DCRT and IMRT plans from regular and the online adaptive radiotherapy treatments. It was capable of automatically detecting any inconsistency between the beam data from the TPS and the data stored in the R&V system with an independent MU check and any significant treatment delivery deviation from the plan within a few seconds. With this tool being used prior to and after the delivery as an essential QA step, our clinical online adaptive re-planning process can be speeded up to save a few minutes by eliminating the tedious visual inspection. A QA software tool has been developed to automatically verify the treatment data consistency from delivery back to plan and to identify discrepancy in MU calculations between the TPS and the secondary MU check. This tool speeds up clinical QA process and eliminating human errors from visual inspection, thus improves safety.

Comparison of 3D conformal breast radiation treatment plans using the anisotropic analytical algorithm and pencil beam convolution algorithm

PurposeTo investigate (1) dosimetric differences between plans calculated using the anisotropic analytical algorithm (AAA) and pencil beam convolution (PBC) algorithm, (2) the plan quality achieved using AAA compared to PBC based on dosimetric parameters and (3) discrepancies with an independent MU verification calculation for breast treatment planning. Materials and methodsThis study included 10 lumpectomy (Group I) and 10 mastectomy (Group II) cases. Target volumes were defined as breast for Group I and chest-wall for Group II based on the isodose distribution of PBC plans in order to evaluate plans. All plans were initially calculated with PBC. For study aim (1), plans were re-calculated using AAA with the same monitor units (MUs). For study aim (2), plans were calculated using AAA with modifications of wedges, subfields and beam weightings from the original plans to achieve optimal coverage. For study aim (3), independent MU verification was performed. A 3% difference between primary MUs and verification MUs was considered an action level. Results(1) Plans using PBC overestimate the dose to the target volume compared to plans using AAA (Group I V95%=90.4%:84.4%; Group II V95%=83.0%:74.5%; PBC:AAA). (2) The new plans using AAA achieved similar target coverage to the original PBC plans based on dose–volume histograms (DVHs). Yet, the high-dose volume (V105%) was significantly larger in AAA plans than PBC plans for Group II (V105%=19.5%:24.0%). For both groups, there was a significant increase in the ipsilateral lung volume receiving low dose with AAA plans (Group I V5Gy=23.6%:39.9%; Group II V5Gy=21.2%:33.6%). Isodose distributions of AAA plans displayed insufficient coverage in the superior area. (3) In Group I, all PBC cases passed MU verification versus 30% of AAA cases. In Group II, 80% of PBC plans versus 65% of AAA plans were within the action level. ConclusionsPlans using AAA calculation can achieve a similar level of target coverage based on DVH as PBC calculation. Nevertheless, the dose distribution shows insufficient coverage in the superior area with AAA plans compared to PBC plans. The lung volume receiving the low-dose (i.e. 5Gy) is larger and the dose to the skin is greater in AAA plans than PBC plans. Compared with PBC calculation, a larger tolerance in discrepancy between AAA and independent MU verification should be allowed to account for the inadequate heterogeneity corrections in the latter.

Control chart analysis of data from a multicenter monitor unit verification study

Background and purposeThis study aims to investigate the process of monitor unit verification using control charts. Control charts is a key tool within statistical process control (SPC), through which process characteristics can be visualized, usually chronologically with statistically determined limits. Material and methodsOur group has developed a monitor unit verification software that has been adopted at several Swedish institutions for pre-treatment verification of radiotherapy treatments. Deviations between point dose calculations using the treatment planning systems and using the independent monitor unit verification software from 9219 treatment plans and five different institutions were included in this multicenter study. The process of monitor unit verification was divided into subprocesses. Each subprocess was analyzed using probability plots and control charts. ResultsDifferences in control chart parameters for the investigated subprocesses were found between different treatment sites and different institutions, as well as between different treatment techniques. 19 of 37 subprocesses met the clinical specification (±5%), i.e. process capability index was equal to or above one. ConclusionsControl charts were found to be a useful tool for continuous analysis of data from the monitor unit verification software for patient specific quality control, as well as for comparisons between different institutions and treatment sites. The derived control chart limits were in agreement with AAPM TG114 guidelines on action levels.

SU-E-T-360: A Novel Procedure to Determine the Dose Output Change with Spread Out Bragg Peak Width and Range Shift in Passively Scattered Proton Beams

Purpose: Two of the major factors affecting the change in output of a passively scattered proton beam are the spread‐out Bragg peak width factor (SOBPF) and range shifter factor (RSF), both of which are usually measured during clinical commissioning. The purpose of this study is to determine these two factors by numerically constructing the SOBP from experimentally measured pristine Bragg peak (PBP), which can serve as an independent monitor unit (MU) calculation procedure. Methods: SOBPs were constructed by superposition of PBPs that are shifted by the water‐equivalent thicknesses (WET) of the corresponding steps of the range modulation wheel. A least‐squares fit was used to determine the weightings of the shifted PBPs to form the required SOBPs. With a simplification in modeling that MU monitor in the nozzle receives dose proportional to the surface dose of the SOBP (higher by an inverse‐square factor), we can relate the MU to the dose at the center of SOBP and compute the SOBP factors for different SOBP widths. When the range shifter was added, the SOBP was shifted toward the surface by the WET of the range shifter and the RSF can also be calculated from the change in the surface dose.Results: The SOBP and range shifter factors were calculated for SOBPs with modulation widths between 4 and 16 cm of 250 and 200 MeV proton beams. The calculated SOBPF and RSF agreed within 2%, and 1.5% with the respective measured values. Conclusions: Starting with measured pristine Bragg peaks, SOBPs can be numerically constructed by simulating the range modulation process, and important dosimetry factors dependent on the modulation width and range shifters can be calculated within 2% accuracy.

SU‐E‐T‐348: Commissioning of An Independent Monitor Unit Verification Program (RadCalc Version 6.1) in Photon Point Dose Calculation

Purpose: To commissioningRadCalc program for secondary point dose check in conventional 2D and 3DCRT calculation techniques. Methods: Test plans for 6 MV and 10 MV photon beams with conventional and 3DCRT planwere created inRadCalc program.Field was varied with a range of field sizes and modifier techniques referred to IAEA‐TECDOC‐1583 with central axis and off central axis calculation points. The measured dose for each field was then compared to the calculated dose (RadCalc program and Eclipse treatment planning system). Results: In conventional field validation,RadCalc shows good agreement with measured doses.For open and almost all wedge fields, RadCalccalculationagree with dose measurement within +/−1%. Differences of over 1% were found in maximum wedge fields and some half fields. However, the dose difference between calculation and measurementwas within +/−2% in all tests. In 3DCRT plan validation, the central axis point dose between calculation and measurement in all tests are agree within the IAEA‐TECDOC‐1583 criteria. When measured/ calculated dose points were in non‐tissue equivalent such as lung and bone which were off central axis situation, dose difference is more than +/−10%. Unfortunately if measured/ calculated dose points areoutside the field (scatter dose), the result cannot be validated. Part of this large difference between two calculated and measured can be the volume effect of the chamber.In comparison between dose calculated by RadCalc and Eclipse, overall results show dose difference within +/−5%. Conclusions: RadCalc program can be used as a point dose calculation check in conventional 2D and 3DCRT techniques.

Validation of an online replanning technique for prostate adaptive radiotherapy

We have previously developed an online adaptive replanning technique to rapidly adapt the original plan according to daily CT. This paper reports the quality assurance (QA) developments in its clinical implementation for prostate cancer patients. A series of pre-clinical validation tests were carried out to verify the overall accuracy and consistency of the online replanning procedure. These tests include (a) phantom measurements of 22 individual patient adaptive plans to verify their accuracy and deliverability and (b) efficiency and applicability of the online replanning process. A four-step QA procedure was established to ensure the safe and accurate delivery of an adaptive plan, including (1) offline phantom measurement of the original plan, (2) online independent monitor unit (MU) calculation for a redundancy check, (3) online verification of plan-data transfer using an in-house software and (4) offline validation of actually delivered beam parameters. The pre-clinical validations demonstrate that the newly implemented online replanning technique is dosimetrically accurate and practically efficient. The four-step QA procedure is capable of identifying possible errors in the process of online adaptive radiotherapy and to ensure the safe and accurate delivery of the adaptive plans. Based on the success of this work, the online replanning technique has been used in the clinic to correct for interfractional changes during the prostate radiation therapy.

Monte Carlo simulations to replace film dosimetry in IMRT verification

Patient-specific verification of intensity-modulated radiation therapy (IMRT) plans can be done by dosimetric measurements or by independent dose or monitor unit calculations. The aim of this study was the clinical evaluation of IMRT verification based on a fast Monte Carlo (MC) program with regard to possible benefits compared to commonly used film dosimetry. 25 head-and-neck IMRT plans were recalculated by a pencil beam based treatment planning system (TPS) using an appropriate quality assurance (QA) phantom. All plans were verified both by film and diode dosimetry and compared to MC simulations. The irradiated films, the results of diode measurements and the computed dose distributions were evaluated, and the data were compared on the basis of gamma maps and dose-difference histograms. Average deviations in the high-dose region between diode measurements and point dose calculations performed with the TPS and MC program were 0.7 ± 2.7% and 1.2 ± 3.1%, respectively. For film measurements, the mean gamma values with 3% dose difference and 3 mm distance-to-agreement were 0.74 ± 0.28 (TPS as reference) with dose deviations up to 10%. Corresponding values were significantly reduced to 0.34 ± 0.09 for MC dose calculation. The total time needed for both verification procedures is comparable, however, by far less labor intensive in the case of MC simulations. The presented study showed that independent dose calculation verification of IMRT plans with a fast MC program has the potential to eclipse film dosimetry more and more in the near future. Thus, the linac-specific QA part will necessarily become more important. In combination with MC simulations and due to the simple set-up, point-dose measurements for dosimetric plausibility checks are recommended at least in the IMRT introduction phase.

SU‐GG‐T‐211: An Independent MU Calculation Program for Dynamic IMRT

Purpose: The performance of an independent monitor unit calculation program is evaluated to verify the monitor unit (MU) for dynamic IMRT fields. The MU calculation program runs on a PC and is based on an empirical pencil‐beam model to determine D/MU. A correction methodology is used to correct for the output (head‐scatter) change for the dynamic IMRT fields. Methods and Materials: The D/MU of IMRT fields at the same point (mostly the hot spot point) was compared between treatment planning system and the independent MU calculation program. Here MU for an IMRT field refers to the total MU delivered. The fractional MU of each segment of an IMRT field, defined as the MU for the segment per total MU, was assumed to be the same. The commercial treatment planning system is based on a pencil‐beam convolution algorithm with headscatter modeling. D/MU of a subset of IMRT fields, which is delivered to a Mapcheck, were compared among the treatment planning system calculation, the independent MU calculations, and measurements. Results: Among the 10 patients examined, the difference of D/MU for all IMRT fields delivered to a point agreed to within 7% for 90% of patients. This error is slightly worse then the measurement, where 100% of patients are within 7% limit. However, when each individual IMRT field is examined, 15.6% (25/160) of the fields have a discrepancy larger than 7%. All those fields (&gt;7%) lie in the valley region of an IMRT field. Conclusion: The independent MU calculation program can effectively verify the dose calculation accuracy for most IMRT fields. Only in very rare instances, which invariably involves the calculation point to be in very low dose region, actual measurements are needed to further verify the accuracy of dose calculation for IMRT field.