J. Kaas

Linking log files with dosimetric accuracy--A multi-institutional study on quality assurance of volumetric modulated arc therapy.

PURPOSE To systematically evaluate machine specific quality assurance (QA) for volumetric modulated arc therapy (VMAT) based on log files by applying a dynamic benchmark plan. METHODS AND MATERIALS A VMAT benchmark plan was created and tested on 18 Elekta linacs (13 MLCi or MLCi2, 5 Agility) at 4 different institutions. Linac log files were analyzed and a delivery robustness index was introduced. For dosimetric measurements an ionization chamber array was used. Relative dose deviations were assessed by mean gamma for each control point and compared to the log file evaluation. RESULTS Fourteen linacs delivered the VMAT benchmark plan, while 4 linacs failed by consistently terminating the delivery. The mean leaf error (±1SD) was 0.3±0.2 mm for all linacs. Large MLC maximum errors up to 6.5 mm were observed at reversal positions. Delivery robustness index accounting for MLC position correction (0.8-1.0) correlated with delivery time (80-128 s) and depended on dose rate performance. Dosimetric evaluation indicated in general accurate plan reproducibility with γ(mean)(±1 SD)=0.4±0.2 for 1 mm/1%. However single control point analysis revealed larger deviations and attributed well to log file analysis. CONCLUSION The designed benchmark plan helped identify linac related malfunctions in dynamic mode for VMAT. Log files serve as an important additional QA measure to understand and visualize dynamic linac parameters.

Auditing local methods for quality assurance in radiotherapy using the same set of predefined treatment plans

Background and purpose Local implementation of plan-specific quality assurance (QA) methods for intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment plans may vary because of dissimilarities in procedures, equipment and software. The purpose of this work is detecting possible differences between local QA findings and those of an audit, using the same set of treatment plans. Methods A pre-defined set of clinical plans was devised and imported in the participating institute’s treatment planning system for dose computation. The dose distribution was measured using an ionisation chamber, radiochromic film and an ionisation chamber array. The centres performed their own QA, which was compared to the audit findings. The agreement/disagreement between the audit and the institute QA results were assessed along with the differences between the dose distributions measured by the audit team and computed by the institute. Results For the majority of the cases the results of the audit were in agreement with the institute QA findings: ionisation chamber: 92%, array: 88%, film: 76% of the total measurements. In only a few of these cases the evaluated measurements failed for both: ionisation chamber: 2%, array: 4%, film: 0% of the total measurements. Conclusion Using predefined treatment plans, we found that in approximately 80% of the evaluated measurements the results of local QA of IMRT and VMAT plans were in line with the findings of the audit. However, the percentage of agreement/disagreement depended on the characteristics of the measurement equipment used and on the analysis metric.

The characterization of a large multi-axis ionization chamber array in a 1.5 T MRI linac

By combining magnetic resonance imaging (MRI) scanners and radiotherapy treatment units the need arises for new radiation measurement equipment that can be used in the magnetic field of the MRI. This study describes the investigation of the influence of the 1.5 T magnetic field from an MRI linac on the STARCHECKMAXI MR, a large 2D ionization chamber detector panel. Measurements were performed on an MRI linac and a conventional linac to investigate the behaviour of the detector panel with and without the 1.5 T magnetic field. We measured reproducibility, linearity, warm-up effect, saturation/recombination and chamber orientation. A comparison with gafchromic film was performed and the effect of motion of the panel during measurements inside a magnetic field was investigated. The reproducibility, linearity, warm-up effect, saturation/recombination show no significant deviations with or without magnetic field. An absolute difference in reading of 2.1% was found between off-axis chambers on different axes. The comparison with film shows good agreement. Spurious readings are induced while the panel is undergoing a motion in the magnetic field during measurements. The STARCHECKMAXI MR is suited for use in a 1.5 T MRI linac. Care must be taken when comparing un-normalized profiles from different axes of the detector panel and when the panel is undergoing motion during measurements.

SU-E-T-51: A National QA Audit of QA Systems Used for IMRT and VMAT Patient QA

Purpose: To independently validate patient-specific quality assurance for IMRT and VMAT plans using the same set of treatment plans for all institutes. Methods: In February 2014 we devised a set of treatment plans: simple IMRT/VMAT plans; more complex IMRT/VMAT plans and a stereotactic VMAT plan, all 6MV for both Varian and Elekta linacs. In total we used 5 Varian and 8 Elekta plans. The plans were imported in the institute’s treatment planning system for dose computation on the phantom of the audit team and the institute’s phantom. Additionally, 10x10 cm2 fields were made and computed on both phantoms. Next, the audit team performed measurements using the audit equipment. So far, 18 audits have been performed and we expect to have concluded the audits by June 2015. The measurements were performed using an ionization chamber (PinPoint, PTW), Gafchromic film and a 2D ionization chamber array, all in an octagonal phantom (Octavius, PTW, see Figure). Differences between the measured and computed 2D dose distributions were investigated using a gamma analysis with a 5%/1mm criterion for the stereotactic treatment plan and 3%/3mm for the other plans. Additionally, the participating centres performed QA measurements of the same treatment plans according to their local protocol and equipment. Results: For the 10x10 field on the phantom, the first 18 audits showed differences with respect to the planning of −0.21 (range: −1.8; 2.2)%. See Table 1 for more results. The findings compared well with the QA measurement results reported by the institutions according to their local protocols. Conclusion: These preliminary results demonstrate that such a national QA audit is feasible. Importing and computing the prepared treatment plans in the planning systems in use in the country is achievable. The local QA systems provided similar results as found with the audit.