P Myers

Characterization of a novel 2D array dosimeter for patient‐specific quality assurance with volumetric arc therapy

PURPOSE In this study, the authors are evaluating a new, commercially available 2D array that offers 3D dose reconstruction for patient specific intensity modulated radiation therapy quality assurance (IMRT QA). METHODS The OCTAVIUS 4D system and its accompanying software (VERISOFT) by PTW were evaluated for the accuracy of the dose reconstruction for patient specific pretreatment IMRT QA. OCTAVIUS 4D measures the dose plane at the linac isocenter as the phantom rotates synchronously with the gantry, maintaining perpendicularity with the beam, by means of an inclinometer and a motor. The measurements collected during a volumetric modulated arc therapy delivery (VMAT) are reconstructed into a 3D dose volume. The VERISOFT application is used to perform the analysis, by comparing the reconstructed dose against the 3D dose matrix from the treatment planning system (TPS) that is computed for the same geometry and beam arrangement as that of the measurement. In this study, the authors evaluated the 3D dose reconstruction algorithm of this new system using a series of tests. Using the Octavius 4D phantom as the patient, dose distributions for various field sizes, beam orientations, shapes, and combination of fields were calculated using the Pinnacle3, TPS, and the respective DICOMRT dose was exported to the VERISOFT analysis software. Measurements were obtained by delivering the test treatment plans and comparisons were made based on gamma index, dose profiles, and isodose distribution analysis. In addition, output factors were measured and the dose linearity of the array was assessed. Those measurements were compared against measurements in water using a single, calibrated ionization chamber as well as calculations from Pinnacle for the same delivery geometries. RESULTS The number of voxels that met the 3%/3 mm criteria for the volumetric 3D gamma index analysis ranged from 92.3% to 98.9% for all the patient plans that the authors evaluated. 2D gamma analysis in the axial, sagittal, and coronal planes produced similar results to those in the 3D gamma analysis. The new detector system does not require an angular dependence correction because it rotates in synchrony with the gantry and the detector array maintains a constant SAD while always perpendicular to the beam axis. Output factors were within 2% when compared to ionization chamber measurements and Pinnacle calculations. Similar agreement was observed when testing the MU linearity (for MU values above 2) as well as dose rate effect. CONCLUSIONS The OCTAVIUS 4D system has some unique characteristics that can potentially improve the patient specific pretreatment IMRT QA data collection and analysis. The ability of the software to reconstruct from the measurements the true 3D dose distribution in the phantom, provides a unique perspective for the medical physicist that evaluates a patients QA plan.

Evaluation of PTW Seven29 for tomotherapy patient-specific quality assurance and comparison with ScandiDos Delta(4).

For routine quality assurance of helical tomotherapy plans, an alternative method, as opposed to the TomoTherapy suggested cylindrical solid water phantom with film and ionization chamber, is proposed using the PTW Seven29 2D-ARRAY inserted in a dedicated octagonal phantom, called Octavius. First, the sensitivity of the array to pitch was studied by varying the pitch during planning to 0.287, 0.433, 1.0, and 2.0. For each pitch selected, the dependence on field size was investigated by generating plans with field widths (FWs) of 1.06 cm, 2.49 cm, and 5.02 cm, for a total of 12 plans. Secondly, a total of 15 patient QA plans were delivered using helical tomotherapy with the Delta4 and Seven29/Octavius for comparison. Using the clinical gamma criteria, 3% and 3 mm, all FW and pitch plans had a passing percentage of >90%. For patient QA plans, the average gamma pass percentage was 97.0% (94.4–99.8%) for the Delta4 and 97.6% (92.5-100.0%) for the Seven29/Octavius. Both the Seven29/Octavius and Delta4 performed to a high standard of measurement accuracy and had a 90% or greater gamma percent for all plans and were considered clinically acceptable.

Comparing conformal, arc radiotherapy and helical tomotherapy in craniospinal irradiation planning

Currently, radiotherapy treatment plan acceptance is based primarily on dosimetric performance measures. However, use of radiobiological analysis to assess benefit in terms of tumor control and harm in terms of injury to normal tissues can be advantageous. For pediatric craniospinal axis irradiation (CSI) patients, in particular, knowing the technique that will optimize the probabilities of benefit versus injury can lead to better long‐term outcomes. Twenty‐four CSI pediatric patients (median age 10) were retrospectively planned with three techniques: three‐dimensional conformal radiation therapy (3D CRT), volumetric‐modulated arc therapy (VMAT), and helical tomotherapy (HT). VMAT plans consisted of one superior and one inferior full arc, and tomotherapy plans were created using a 5.02 cm field width and helical pitch of 0.287. Each plan was normalized to 95% of target volume (whole brain and spinal cord) receiving prescription dose 23.4 Gy in 13 fractions. Using an in‐house MATLAB code and DVH data from each plan, the three techniques were evaluated based on biologically effective uniform dose (D¯¯), the complication‐free tumor control probability (P+), and the width of the therapeutically beneficial range. Overall, 3D CRT and VMAT plans had similar values of D¯¯ (24.1 and 24.2 Gy), while HT had a D¯¯ slightly lower (23.6 Gy). The average values of the P+ index were 64.6, 67.4, and 56.6% for 3D CRT, VMAT, and HT plans, respectively, with the VMAT plans having a statistically significant increase in P+. Optimal values of D¯¯ were 28.4, 33.0, and 31.9 Gy for 3D CRT, VMAT, and HT plans, respectively. Although P+ values that correspond to the initial dose prescription were lower for HT, after optimizing the D¯¯ prescription level, the optimal P+ became 94.1, 99.5, and 99.6% for 3D CRT, VMAT, and HT, respectively, with the VMAT and HT plans having statistically significant increases in P+. If the optimal dose level is prescribed using a radiobiological evaluation method, as opposed to a purely dosimetric one, the two IMRT techniques, VMAT and HT, will yield largest overall benefit to CSI patients by maximizing tumor control and limiting normal tissue injury. Using VMAT or HT may provide these pediatric patients with better long‐term outcomes after radiotherapy. PACS number: 87.55.dk

Pediatric Cranio-spinal Axis Irradiation Comparison of Radiation-induced Secondary Malignancy Estimations Based on Three Methods of Analysis for Three Different Treatment Modalities

Pediatric cranio-spinal axis irradiation (CSI) is a valuable treatment for many central nervous system (CNS) diseases, but due to the life expectancies and quality of life expectations for children, the minimization of the risk for radiation-induced secondary malignancies must be a high priority. This study compared the estimated CSI-induced secondary malignancy risks of three radiation therapy modalities using three different models. Twenty-four (n = 24) pediatric patients previously treated with CSI for tumors of the CNS were planned using three different treatment modalities: three-dimensional conformal radiation therapy (3D-CRT), volume modulated arc therapy (VMAT), and Tomotherapy. Each plan was designed to deliver 23.4 Gy (1.8 Gy/fraction) to the target which was defined as the entire brain and spinal column with a 0.7 cm expansion. The mean doses as well as the dose volume histograms (DVH) of specific organs were analyzed for secondary malignancy risk according to three different methods: the effective dose equivalent (EDE), the excess relative risk (ERR), and the linear quadratic (LQ) models. Using the EDE model, the average secondary risk was highest for the 3D-CRT plans (37.60%), compared to VMAT (28.05%) and Tomotherapy (27.90%). The ERR model showed similarly that the 3D-CRT plans had considerably higher risk (10.84%) than VMAT and Tomotherapy, which showed almost equal risks (7.05 and 7.07%, respectively). The LQ model requires organ-specific cell survival parameters, which for the lungs, heart, and breast relevant values were found and applied. The lung risk for secondary malignancy was found to be 1.00, 1.96, and 2.07% for 3D-CRT, VMAT, and Tomotherapy, respectively. The secondary cancer risk for breast was estimated to be 0.09, 0.21, and 0.27% and for heart it was 9.75, 6.02 and 6.29% for 3D-CRT, VMAT, and Tomotherapy, respectively. Based on three methods of secondary malignancy estimation, the 3D-CRT plans produced highest radiation-induced secondary malignancy risk, and the VMAT and Tomotherapy plans had nearly equal risk. Pediatric patients must be treated with reducing long term sequelae as a priority.

SU‐E‐T‐350: Three Year Analysis Using Ionization Chamber Array for Patient Specific IMRT QA

Purpose: To analyze the patient specific IMRT QA results at our institution over the past 3 years and evaluate the procedures and methods of our IMRT QA program. Methods: 1466 patient specific IMRT QAs were performed at our institution from March 2009 to December 2011. The passing criteria for each IMRT QA are that 90% of all evaluated points must have a gamma =1.0. The gamma is calculated using the TPS calculated planar dose as reference and the reference value is 90% of the maximum dose in the plane. A dose threshold of 10% is used in order to remove very low dose points from the calculation. The planar dose and the measurements are computed and obtained using the actual beam angles. The measurements were performed on Varian linacs equipped with Millennium 80, Millennium 120 and High‐Definition 120 MLC.Results: The IMRT QA results were analyzed with respect to the linac, treatment site, number of beams, IMRT vs. TOMO vs. VMAT, and number of control points. The overall average gamma index value was 96.85% (±1.5%). Head and Neck had the lowest gamma index (95.97%) while brain had the highest (97.85%). Conclusions: After evaluation of 700 patients, it can be determined that there are significant variations in the average measured gamma value based on treatment site, number of beams, machine type, and number of control points. This work provides a foundation for future analysis of possible underlying issues in the gamma value deviations for each comparison.

SU‐GG‐T‐235: Consistency and Reproducibility of the VMAT Plan Delivery Using Three Independent Validation Methods

Purpose: The aim of he study is to evaluate the consistency and reproducibility of a prostate and a lung VMAT plans case for 31 consecutive days using three different approaches. Materials and methods: For each plan, a pre‐treatment delivery was performed and the measurements obtained (dynalog files, planar dose with seven29, and the fluence from DAVID) were used as reference measurements. The remaining thirty (n=30) consecutive daily measurements were compared against the reference. The analysis of these DynaLog files have been carried out by in‐house programming in MATLAB, by converting these ASCII files in to a 552 × 552 matrix that can be visualized as a gray level fluence image of the corresponding QA plan. In this work, we used the DAVID system, which is able to perform such quality assurance measurement while the patient is treated. The evaluation software compares the dose measured during radiotherapy to a reference dose, which was taken for each leaf pair during a reference measurement. In the third method, OCTAVIUS phantom with the Seven29 ion chamber array were used for comparing fluence verification. Results: The Dynalog files analysis showed that the variations between the reference fluence and the daily fluence were very low and the mean gamma index was as low as 0.01+/− 0.08. In our 30 days period of measurements using DAVID, the maximum variations were within 3% for both plans. Similar results observed in the analysis of the delivered dose as recorded by the seven29 ionization chamber array. Conclusions: All methods showed minimal daily deviations that contributed to clinically insignificant dose variations from day to day. Based on our results, we conclude that the VMAT delivery using a Varian 2100CD linear accelerator equipped with 120MLC is highly reproducible. “Research sponsored by PTW‐Freiburg Company.”

SU‐E‐T‐226: SRT/SBRT Patient Specific QA with a New High Resolution 2D Detector Array

Purpose: To evaluate the performance of a new commercially available device for patient specific SBRT pretreatment QA.Materials and Methods: The PTW OCTAVIUS1000SRS was used in this study for patient specific QA measurements for patient undergoing SBRT treatments. Forty five (n=45) individual field and five (n=5) composite plans were measured. All plans were optimized and calculated using Eclipse v8.9. All measurements were performed with the gantry static at the upright orientation. The detector array was placed inside the PTW Octavius II phantom and the source to detector plane distance was set to 100cm. The gamma index was the metric of choice in order to quantify the agreement between measurements and calculations. The tolerance criteria used for the evaluation were 3% dose difference and 3mm distance to agreement and also 2%/2mm. Results: The results of the comparison using 3%/3mm were inconclusive. The number of detectors included in the comparison and the rather loose criteria used caused most comparisons to yield a gamma index of 100%. Comparing the measured and calculated planar dose distributions using 2%/2mm as gamma index tolerances, the average gamma index was 95.9% (min:90.9, max:99.2%). The agreement was also reflected while comparing profiles and isodose distributions. Conclusion: The new PTW Octavius 1000SRS device is able to provide a reliable solution for the patient specific QA of SBRT and SRS plans where small fields are commonly used. Project was partially funded by PTW, NY

SU‐E‐T‐281: Pediatric Cranio‐Spinal Axis Irradiation: Comparison of Radiation‐Induced Secondary Malignancy Estimations Based On Three Methods of Analysis for Three Different Treatment Techniques

PURPOSE Pediatric cranio-spinal axis irradiation (CSI) is a valuable treatment for many central nervous system diseases, but due to the life expectancies and quality of life expectations for children, minimizing the risk of radiation-induced secondary malignancies must be a high priority. This study compares CSI risk estimates based on three methods for three delivery techniques. METHODS Twenty-four (n=24) pediatric patients previously treated with CSI for tumors of the CNS were retrospectively planned using three different treatment techniques: 3D-CRT, SmartArc, and Tomotherapy. Each plan was designed to deliver 23.4Gy (1.8 Gy/fraction) to the target which was defined as the entire brain and spinal column with a 3mm expansion. The mean doses as well as the dose volume histogram (DVH) data for specific organs were analyzed for secondary malignancy risk according to three different Methods: effective dose equivalence (EDE), excess relative risk (ERR), and linear quadratic (LQ) model. RESULTS Using the EDE method, the average secondary-risk was highest for the 3D plans, 37.6%, compared to SmartArc, 28.1%, and Tomotherapy, 27.9%. The ERR method showed similarly that the 3D plans had significantly higher risk (10.8%) than SmartArc and Tomotherapy, which almost equally showed lower risks (7.1% and 7.1%). The LQ model requires organ-specific cell survival parameters found in literature of which the lungs, heart, and breast values were found and studied. The lung risk for secondary malignancy was found to be 1.0, 2.0, and 2.1% for 3D, SmartArc, and Tomotherapy respectively. The secondary cancer risk for the breasts was estimated to be 0.1, 0.2, and 0.3% and for heart it was 9.8, 6.0, 6.3% for the 3D, SmartArc, and Tomotherapy, respectively. CONCLUSION Based on three methods of secondary malignancy calculations, 3D plans produced highest radiation-induced secondary malignancy risk, and SmartArc and Tomotherapy plans had nearly equally lowered risk.

SU‐E‐T‐308: Radiobiological Evaluation of the Three Dimensional Conformal Radiation Therapy, SmartArc, and Helical Tomotherapy Treatment Techniques for Pediatric Cranio‐Spinal Axis Irradiation

PURPOSE Currently, radiotherapy treatment planning acceptance is based primarily on dosimetric plan performance and concerns. However, using radiobiological analysis to assess the benefit of tumor control while limiting injury to normal tissues can be greatly advantageous. For pediatric CSI patients in particular, knowing the technique that will optimize the benefit versus injury probability can lead to greater long-term outcomes. METHODS Twenty-four CSI pediatric patients (median age 10) were retrospectively planned with three techniques: three-dimensional conformal radiation therapy (3D-CRT), SmartArc (SA), and helical tomotherapy (HT). Each plan was normalized to 95% of target volume (whole brain and spinal cord) receiving prescription dose 23.4 Gy in 13 fractions. Using an in-house Matlab code and the DVH data from each plan, the three techniques were evaluated based on biologically equivalent uniform dose (BEUD), probability of controlling the tumor without causing severe injury to normal tissues (P+ ), and optimal BEUD and P+ . RESULTS Overall, 3D-CRT and SA plans had similar values of BEUD (24.1 and 24.2 Gy) while HT had a BEUD slightly lower (23.6 Gy). P+ percentages were 64.7, 67.3, and 56.7% on average for 3D-CRT, SA, and HT plans respectively. The optimal values for BEUD were 28.5, 33.0, and 32.2 Gy for the 3D-CRT, SA, and HT plans, respectively. Although the actual P+ values were lower for HT, choosing a more optimal BEUD prescription level yields an optimal P+ of 94.3, 99.6, and 99.4% for 3D-CRT, SA, and HT, respectively. CONCLUSION If the optimal dose level is prescribed using a radiobiological calculation method as opposed to purely dosimetric, the two IMRT techniques, SA and HT, will yield the greatest overall benefit to the CSI patient by maximizing tumor control and limiting normal tissue injury. Using SA or HT, may provide these pediatric patients with better long-term outcomes after radiotherapy.

SU‐E‐T‐151: Application of a 2D Ionization Chamber Array for Monthly and Annual Linear Accelerator QA

Purpose: An evaluation of a commercially available 2D array for monthly and annual linear accelerator QA. Methods: The 2D‐ARRAY seven29 (PTW, Freiburg, Germany) is a two‐dimensional ion chamber array, consisting of 729 (27 × 27) vented ion chambers, with separately housed evaluation electronics. The parallel‐plate chambers are 5 × 5 × 5 mm in size, and have a center to center spacing of 10 mm. To increase resolution, the array can be shifted three times in increments of 5 mm, and the resulting 2916 measurement points can be summed into one image. The array was placed under 10 cm of water‐equivalent material, and irradiated to evaluate: flatness and symmetry, static MLC positioning, dynamic MLC positioning, and output. It was also irradiated without any buildup for the light field and radiation coincidence test. All deliveries were completed using a Varian 2100 C/D linear accelerator with a Millennium‐80 MLC. For improved accuracy, the static and dynamic MLC fields were delivered 4 times each, and were then summed using the “merge” function in the PTW VeriSoft software. Results: Over one month of data acquisition, output fluctuated less than 2%. For light field and radiation coincidence, reproducibility is excellent, with a maximum of 1% variation in asymmetric jaw settings. When using the “merge” function on dynamic MLCmeasurements, results were less than 4% for transmission and 0.5 mm for positioning errors. Measurements for flatness and symmetry and static MLC positioning also had good reproducibility. Conclusions: The PTW 2D‐Array seven29 can be used to evaluate many areas of monthly linear accelerator QA. Particularly in light of the trend for departments to go filmless, it is a viable alternative for obtaining flatness and symmetry, static and dynamic MLC positioning, light field and radiation coincidence, and for measuring output. This work partially supported by PTW (Freiburg, Germany).