Room: Exhibit Hall
Purpose: To develop practical QA solutions for radiotherapy of Multiple-Brain-Metastases (MBM) with single-isocenter VMAT requiring sub-millimeter accuracy of patient setup and delivery.
Methods: Up to ~16 PTVs can be treated with 3-6 VMAT-arcs and couch angles within 90deg-0deg-270deg range. Commissioning of TPS, linac and patient-specific QA are challenging for MBM mostly due to: (a) the presence of many small PTVs treated with small fields with high dose gradients and located up to about 7cm away from the isocenter, (b) different performance of TPS dose calculation algorithms for simple vs complex plans, (c) limitations of QA equipment and software. TPS limitations relate to MLC parameters (transmission, dosimetric leaf-gap offset) and modeling of small beams (0.5cm-3cm). Equipment/software limitations include: (d1) poor in-plane spatial resolution of 2D detector arrays, (d2) lack of direct 3D measurement (except for gel dosimeters), (d3) detector damage due to irradiation of electronics for non-zero couch angles, (d4) lack of QA software tools and metrics suitable for analysis of each individual PTV, (d5) inaccuracy of 3D dose reconstruction (or interpolation) for dosimetry systems based on 2D measurements (detector arrays, EPID dosimetry) or linac log-files, (d6) impracticality or time inefficiency of using existing dosimetry systems for high-throughput QA of MBMs
Results: We investigate the performance of 3 selected candidate dosimetry systems: (i) high-definition 3D CrystalBall polymer gel dosimetry system (MGSResearch), (ii) Delta4 system (ScandiDos) based on two orthogonal diode arrays, (iii) PerFraction (SunNuclear) 3D dose computation based on measured EPID images and linac logfiles.
Conclusion: Practical methods of overcoming or mitigating limitations (d1)-(d6) in TPS commissioning and in routine patient-specific QA are discussed. A new MLC-gantry VMAT test developed for QA of 15 PTVs with custom locations is developed. Approaches to TPS beam-modeling optimization (MLC transmission / dosimetric gap offset) leading to a satisfactory agreement with measurements are presented.
Funding Support, Disclosures, and Conflict of Interest: One of the co-authors (MM) is supported by MGS Research.