M Taylor

Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system

PURPOSE To present our method and experience in commissioning dose models in water for spot scanning proton therapy in a commercial treatment planning system (TPS). METHODS The input data required by the TPS included in-air transverse profiles and integral depth doses (IDDs). All input data were obtained from Monte Carlo (MC) simulations that had been validated by measurements. MC-generated IDDs were converted to units of Gy mm(2)/MU using the measured IDDs at a depth of 2 cm employing the largest commercially available parallel-plate ionization chamber. The sensitive area of the chamber was insufficient to fully encompass the entire lateral dose deposited at depth by a pencil beam (spot). To correct for the detector size, correction factors as a function of proton energy were defined and determined using MC. The fluence of individual spots was initially modeled as a single Gaussian (SG) function and later as a double Gaussian (DG) function. The DG fluence model was introduced to account for the spot fluence due to contributions of large angle scattering from the devices within the scanning nozzle, especially from the spot profile monitor. To validate the DG fluence model, we compared calculations and measurements, including doses at the center of spread out Bragg peaks (SOBPs) as a function of nominal field size, range, and SOBP width, lateral dose profiles, and depth doses for different widths of SOBP. Dose models were validated extensively with patient treatment field-specific measurements. RESULTS We demonstrated that the DG fluence model is necessary for predicting the field size dependence of dose distributions. With this model, the calculated doses at the center of SOBPs as a function of nominal field size, range, and SOBP width, lateral dose profiles and depth doses for rectangular target volumes agreed well with respective measured values. With the DG fluence model for our scanning proton beam line, we successfully treated more than 500 patients from March 2010 through June 2012 with acceptable agreement between TPS calculated and measured dose distributions. However, the current dose model still has limitations in predicting field size dependence of doses at some intermediate depths of proton beams with high energies. CONCLUSIONS We have commissioned a DG fluence model for clinical use. It is demonstrated that the DG fluence model is significantly more accurate than the SG fluence model. However, some deficiencies in modeling the low-dose envelope in the current dose algorithm still exist. Further improvements to the current dose algorithm are needed. The method presented here should be useful for commissioning pencil beam dose algorithms in new versions of TPS in the future.

SU‐E‐T‐157: Improving Spot Scanning Proton Therapy Patient‐Specific Quality Assurance Through Early Application of Second‐Check Dose Calculation Software

PURPOSE We discuss the spot scanning proton therapy (SSPT) patient-specific quality assurance (QA) procedure at MD Anderson Proton Therapy Center-Houston, with emphasis on how we verify the dose calculation of the Eclipse treatment planning system. We present our second-check dose calculation software, HPlusQA, and then present a case study to show how a slightly unorthodox application of the second-check software can greatly improve the efficiency of SSPT QA. METHODS In our study, we validated HPlusQA and determined its performance with respect to Eclipse by comparing the calculations of point doses to measured values. To validate the use of HPlusQA as a dose calculation second check, we determined how well gamma index scores comparing HPlusQA to Eclipse predict the gamma index scores comparing dose measurement to Eclipse. To present a more informative distribution of the gamma index score comparison, we introduced the αβ? transformation. RESULTS In our study, both HPlusQA and Eclipse agreed with the measured dose to within 2% on average. The agreement was worse at the end of the proton beam range. HPlusQA had a 79% success rate at predicting when the Eclipse dose calculations would be outside the QA tolerance levels.More importantly, we show how we plan to apply HPlusQA to patient-specific QA. We use a patient case study to demonstrate how HPlusQA can lead to great improvement in the efficiency of SSPT patient-specific QA. CONCLUSION A dose calculation second check such as HPlusQA allows for early detection of many treatment planning system dose calculation errors. Early detection of errors means less time spent on measurements and a more efficient SSPT patient-specific QA process.

SU‐E‐T‐403: Intensity Modulated Proton Therapy Plans with Multiple Fields for Prostate Cancer

PURPOSE To investigate the possible improvements in the dose conformity in intensity modulated proton therapy (IMPT) plans with multiple fields as compared to widely used single field optimized (SFO) plans with two lateral fields for spot scanned proton therapy (SSPT) and to verify the accuracy of the delivered dose using the Radiological Physics Center Proton Prostate Phantom (RPCPPP). METHODS The CT images of RPCPPP were used to design multiple IMPT plans using both SFO and multi field optimization (MFO) methods in the Varian Eclipse treatment planning system (TPS). The standard SFO clinical plan used two lateral fields going through the femoral heads. Non-standard plans were generated using three or more oblique fields based on the SFO and MFO options of the TPS. The angles were carefully chosen to avoid bladder and rectum as much as possible. The RPC phantom containing films and TLDs was irradiated using the standard clinical SFO plan and the four field MFO plan to compare the planned and delivered dose. RESULTS Based on the DVHs, MFO plan with multiple oblique fields is found to have superior target conformity with reduced dose to organs at risk (OAR) and normal tissue, especially in the high dose regions, as compared to SFO plans. The point doses measured by TLDs and 2-D dose distribution measured by GafChromic films in the RPCPPP irradiation passed the commonly used RPC acceptance criteria, namely +/- 7% agreement in point does and more than 85% of the voxels meeting the gamma index criteria of 7% dose or 4 mm distance agreement. CONCLUSION The MFO SSPT plans for prostatic targets with multiple fields lead to better target conformity with lower doses to OARs as compared to SFO plans. The results of irradiations of the RPCPPP show that these plans can be delivered with acceptable accuracy.

MO‐G‐137‐05: Improving Passive Scattering Proton Therapy Plan Quality by Optimizing Compensator Parameters

PURPOSE To demonstrate the importance of optimizing compensator parameters in passive scattering proton therapy (PSPT). METHOD We examined the plans of four patients treated for lung tumors at MD Anderson Cancer Center. Comparable IMRT and PSPT plans had been created for each of these patients. The clinical PSPT plans were calculated using 5 mm drill bit compensators with smearing values of 0.5 to 0.9 cm. Two new plans were created for each patient: one with a smearing value of 0 cm and a second with 0 cm smearing and smooth compensators. The prescribed isodose line of each plan was changed so that PTV D95 was equal in a given patients plans and robustness analysis was performed to evaluate the impact of the compensator changes. A plan was considered robust if ITV D95 remained greater than 95% of the prescribed dose in the worst case scenario for target coverage. The robust plan with altered compensator parameters and most improved normal tissue sparing was considered to be the compensator optimized plan. RESULTS Mean lung dose for the 4 patients is 19.8, 12.7, 11.9 and 12.4 Gy in the compensator optimized plans compared to 21.8, 14.2, 12.9 and 13.6 Gy in the clinical plans; esophagus V60 is 39.2%, 13.4%, 7.7% and 17.7% compared to 48.6%, 18.1%, 11.6% and 20.7%; mean heart dose is 23.3, 1.2, 8.2 and 4.6 Gy compared to 24.7, 2.4, 9.5 and 8.3 Gy. CONCLUSION Robustness analysis allows reduction of the smearing parameter while keeping confidence that the plan is robust against range and setup uncertainties. Compensator parameters should be individually optimized if PSPT plans are to display maximum advantage over IMRT plans in terms of dose volume data.

SU‐E‐T‐48: Relative Proton Stopping Power Ratio Database for Common Dosimetry Phantom Materials

PURPOSE To create a database of relative proton stopping power ratios (RPSPRs)of various phantom materials, especially those that simulate different kinds of human tissue. More importantly, we intend to use these stopping powers to gain a better understanding of and more accurately estimate range uncertainties in heterogeneous phantoms and, ultimately, in patient treatment plans. METHODS RPSPRs were determined using two independent techniques to measure the range shift produced by the various materials. The first technique used a PTW water tank scanning system with a PTW TN23343 Markus chamber to measure and compare the percent depth dose profiles of uniform proton beams with and without the various materials of interest blocking the beam. The second technique was similar to the first, except that an IBA Zebra, a water equivalent multi-layer ionization chamber system, was used to measure the percent depth dose profiles. Each material was evaluated using unmodulated proton beams of two different energies to examine possible stopping power energy dependencies. RESULTS Currently, we have obtained RPSPR data for three materials that compose the CIRS Model 002LFC IMRT Thorax Phantom. We have also measured range shifts for Lucite and homogeneous rods from a Gammex RMI 467 Electron Density CT Phantom. The range shifts, based on the distal 90% ranges, measured with the water tank and Zebra agree within 0.45 mm, with a mean difference of 0.2 mm, and with no dependence on the energy of the incident proton beam. The difference in RPSPRs between the two measurement techniques was no more than 0.045, with a mean difference of 0.011. CONCLUSION The PTW water tank scans and Zebra measurements have allowed us to accurately determine the RPSPRs of different materials used to simulate human tissue. For the materials that we measured, the stopping powers had no apparent energy dependence.

SU‐E‐T‐52: Independent Dose Verification System for Spot Scanning Proton Therapy

Purpose: Similar to IMRT, patient specific quality assurance (QA) is an essential part of intensity modulated proton therapy (IMPT). Patient‐specific QA based on measurements only is a time‐consuming process. In this work, we developed and validated an independent dose verification system which will be used as a part of the patient specific QA process to improve the QA efficiency for IMPT patients. Methods: A recently developed pencil beam algorithm base on measured in‐water single‐spot profiles was implemented. The algorithm added a modified Cauchy‐Lorentz function to accurately model individual scanning spot. For the purpose of independent dose calculation, it is sufficiently different from the in‐air fluence‐based algorithm of our clinical treatment planning system (TPS). QA tasks can be seamlessly communicated between TPS and the verification system through DICOM network streams. Verification results were generated without human intervention and analyzed using data extracting and analyzing tools. Patient specific measured data of ten patients were selected to validate the system, among which five were treated with single‐field optimization (SFO), and five with multiple‐field optimization (MFO). The measured and calculated dose distributions were compared at points, and 2D planes at various depths. Results: AT least 98% of dose pixels passed 2% relative dose/2 mm distance to agreement (DTA) gamma criteria for all 2D dose planes of SFO cases. At least 95% of dose pixels passed the same gamma criteria for all MFO cases. Calculated values are within 3% or 2 mm DTA of measurements for 97% of validated point doses. Conclusions: We havedeveloped and validated an independent dose verification system for IMPT. The proposed system may reduce the amount of measurement and beam time required for patient specific QA processes, while maintaining a reasonable confidence to deliver a treatment safely and effectively.

SU‐C‐BRCD‐04: Development of An Efficient and Effective Patient Specific QA Program for IMPT

Purpose: Intensity modulated proton therapy (IMPT) patient specific quality assurance (QA) program based on measurement alone could be very time consuming due to highly modulated dose distributions of IMPT fields. In this work, we present our efforts for developing an efficient and effective patient specific QA program for IMPT. Methods: The improved QA program, based on a previously reported one for single field uniform dose (SFUD), consists of three components: measurements, independent dose calculation and analysis of patient specific treatment delivery log files. Measurements included two‐dimensional (2D) measurements for each field delivered at the planned gantry angles with electronic medical record (EMR) system in the QA mode and the accelerator control system (ACS) in the treatment mode and additional depths for each field with ACS in physics mode without using the EMR. Dose distributions in a water phantom for each field were calculated independently using a recently developed in‐ house pencil beam algorithm and compared with the results of TPS. The treatment log file for each field was analyzed in terms of deviations in delivered spot positions from their planned positions using various statistic analysis methods. Results: With the improved patient specific QA program, we are able to verify (1) the dose calculation of TPS, (2) integrity of the data transfer from TPS to EMR to ACS and (3) treatment delivery including dose delivered and spot positions. We estimated that the in‐room measurement time required for each complex IMPT patient like head and neck patients would be less than 2 h. Conclusions: We have developed an efficient and effective QA program for IMPT patients with the equipment and resources available in our clinic.

SU‐E‐T‐291: Dosimetry of Double Scattered Proton Beam Fields Used for Cranio‐Spinal Irradiation

PURPOSE To investigate the effect of source to surface distance on treatment field lateral penumbra width and the consequence of setup error on the dose distribution in the junction between two spinal fields of double scattered proton beams. METHODS The CT images of the Spine Phantom from Radiological Physics Center was used to design a double scattered proton beam treatment plan using Varian Eclipse treatment planning system. PTV included spinal cord, vertebral body and part of spinous process. The PTV was divided into superior and inferior parts and two posterior fields were used to cover the targets with the prescribed dose. 2D dose was measured using IBA MatriXX and EBT2 film at a depth close to the center of SOBPs of both the fields located both at the nominal source to axis distance (SAD) of 270 cm and at an extended SAD. The field separation was changed by ±1 mm to study the effect of setup error. The measured and TPS calculated dose distributions in verification plans in a water phantom were compared. RESULTS The measured 2-D doses agreed very well with planned ones for individual fields. 99% of pixels pass 3%/3 mm dose/distance agreement criteria. The CAX dose differences are within 2%. The 80% to 20% penumbra widths at nominal SAD are 7.4/7.7/7.8 mm for planned/MatriXX/EBT2 film respectively, and about 1 mm wider for the extended SAD. The measured maximum dose was about 10% higher than that from the plan, and decreased/increased about 7% when the fields were separated by ±1 mm. CONCLUSIONS The penumbra width is modestly affected by the extended SSD often used for patient treatment. Dose in the junction of two fields is very sensitive to the setup error and the accuracy of the TPS dose calculation in this region may be limited.

SU‐D‐BRCD‐01: Evaluation of Zebra Multi‐Layer Ionization Chamber System for Patient Treatment Field and Machine QA for Spot Scanning and Passive Scattering Proton Beams

PURPOSE To evaluate Zebra multi-layer ionization chamber system for patient treatment field and machine QA for spot scanning proton beams (SSPB) and passive scattering proton beams (PSPB). METHODS Zebra dose measurement system (IBA Dosimetry), consisting of 180 parallel platechambers with 2 mm detector spacing, was used for measuring proton beamdepth dose curves (DDC) for spread out Bragg peaks (SOBP) and single spot pristine Bragg peaks (PBP). The measurements were performed for 100 to 250 MeV PSPB and 89.2 to 221.8 MeV SSPB using the Hitachi ProBeat synchrotron based delivery system. An in-house Matlab based analysis software was used to compare the Zebra measured DDC with those measured by the Markus chamber in a PTW water tank (MC-WT). Several verification plans in the water phantom were created for patient treatment fields using the Eclipse treatment planning system (TPS). The DDC for individual verification fields were measured using the Zebra andcomparisons were made with the TPS calculations. RESULTS The dosedifferences between the Zebra and MC-WT measurements in the plateau regions of the DDC are within 2% for various energies of PSPB, but are larger than 2% at the sharp dose distal gradient regions. The values for distal penumbra widths, range and SOBP widths from Zebra and MC-WT measurements agree within 0.5 mm, 1.5 mm, and 2 mm, respectively. The Zebra measured values of the range of the single spots also agreed within 1 mm with their established values from other measurements. The Zebra measured DDC of verification plan of patient treatment fields showed goodagreement with those from the TPS. CONCLUSIONS Our investigation shows that Zebra can be useful for fast and reasonably accurate measurements of the DDC of pristine and spread-out Bragg peaks of both spot scanning and passive scattering proton beams.

SU‐E‐T‐523: Comparison of Fiducial and Bony‐Anatomy Based Alignment for Prostate Localization in Proton Therapy

Purpose: To quantify and compare differences in localization when aligning proton therapy patients using carbon fiducials versus bony‐anatomy and to evaluate the dosimetric consequences of these differences. Methods: 250 pairs of AP and lateral daily kV images were obtained for 16 prostate cancer patients treated at our institution. Prior to treatment 2–3 carbon fiducials were implanted in each patient. Patients were treated to 76 Co‐Gy‐ Equivalent in 38 fractions, and immobilized with a knee‐foot cradle. A water‐filled endorectal balloon was used to suppress prostate motion. Before each fraction, therapists aligned patients on fiducials and acquired a set of post‐shift images to confirm alignment. Residual errors for fiducial alignment were collected from post‐shift images using a point alignment tool; for bony‐anatomy alignment, a 2D‐3D method was used. The dosimetric implications were analyzed using verification plans offset by the maximum difference between couch shifts determined using the two methods. Results: The average systematic component of residual shifts was less than 0.1 cm for fiducials and bony‐anatomy in all directions. The standard deviation of systematic components was less than 0.1 cm for fiducials and was 0.23, 0.26, and 0.08 cm for bony‐anatomy in the AP, SI, and RL directions respectively. The random component of residual shifts was less than 0.1 cm for fiducials and was 0.16, 0.17, and 0.05 cm for bonyanatomy. Incorporating the maximum difference in shifts into verification plans, CTV coverage was found to be minimally compromised with no less than 96% of the CTV receiving full dose in any plan and V70 of the rectum/bladder varied by a maximum of 13 percentage points in either direction Conclusions: Systematic error for each method was small, while random error was larger in the AP and SI directions for bony‐anatomy than fiducials. Verification plans revealed minimal CTV coverage degradation for the maximum shift differences.