Poster Poster Program Therapy Physics

A Universal TPS Beam Model for Varian Truebeam SRS Treatment out-Performs Local Center-Specific Beam Models

Abstract

Purpose

Treatment planning system (TPS) beam modelling is identified as the largest cause of independent dosimetry audit failure. This study aims to develop and validate a universal beam model (UBM) specific to single-iso multi-target (SIMT) stereotactic radiosurgery (SRS) on Varian TrueBeam to mitigate risk of local TPS beam modelling errors.

Methods

Percentage depth-dose at 10cm (PDD10), Eclipse dose calibration factor, high-definition multi-leaf collimator (HDMLC) dosimetric leaf gap (DLG)/transmission, and small-field output factors were measured on Varian TrueBeam by an international consortium of 11 centers to assess dosimetric equivalence. Center-averaged measurements, together with Varian reference beam data, were then used to configure a 6FFF AcurosXB dose-to-medium UBM in the Eclipse v18.1 TPS. To validate UBM accuracy, nine non-coplanar SIMT-SRS VMAT plans (target diameters: 5-20mm) were produced and calculated (1mm grid size) on a virtual 20cm diameter spherical water phantom (TPSUBM). Five of the centers calculated the plans with fixed monitor units (TPSinstitute) and measured the dose within the planned virtual phantom using an electronic portal imaging device (EPID) via EPID-to-dose conversion.

Results

The range for PDD10, Eclipse dose calibration factor, HDMLC DLG and HDMLC transmission measured 0.630-0.639, 0.924-0.936Gy/100MU, 0.29-0.84mm and 0.98-1.04% respectively. The coefficient of variation in output factors measured < 2.0% for fields down to 5x5mm and < 2.5% for PTV D99%, Dmean and Dmax for the dynamic SIMT-SRS plan deliveries. For plan verification measurement, percentage difference in PTV Dmean was on average 2.2% (range: -0.5 to 6.6%) for TPSinstitute, which reduced to 0.4% (range: -1.6 to 3.2%) for TPSUBM. Similarly, global 5%/1mm gamma pass rates improved on average by 0.5% (range: -2.3 to +5.2%) utilising TPSUBM. Significantly, four of the five centers showed benefit using the UBM.

Conclusion

Dosimetric equivalence of Variance TrueBeam makes feasible the application of a UBM for safe and accurate SIMT-SRS dose-calculation.

People

Cameron Stanton, MScPresenting Author · Central Coast Cancer Centre Peter B. Greer, PhDAuthors · University of Newcastle Iliana Peters, MScAuthors · Central Coast Cancer Centre Ching-Ling Teng, PhDAuthors · Departments of Radiation Medicine and Neurosurgery, Zucker School of Medicine at Hofstra/Northwell Ashley Cullen, PhDAuthors · Chris O'Brien Lifehouse Conor McGarry, PhDAuthors · Northern Ireland Cancer Centre Andrew Dipuglia, PhDAuthors · Calvary Mater Hospital Emily Searle, PhDAuthors · Central Coast Cancer Centre D. Michael Lovelock, PhDAuthors · Icahn School of Medicine at Mount Sinai Victoria RobinsonAuthors · Northern Ireland Cancer Centre Baozhou SunAuthors · Baylor College of Medicine Juan-Francisco Calvo-OrtegaAuthors · Hospital Quironsalud Barcelona Michael Barnes, PhDAuthors · Calvary Mater Hospital Benjamin J. Zwan, PhDAuthors · Central Coast Cancer Centre Xiaodong Neo ZhaoAuthors · Washington University in St. Louis School of Medicine Seng Boh Gary Lim, PhD, MSAuthors · Department of Medical Physics, Memorial Sloan Kettering Cancer Center Richard A. Popple, PhDAuthors · University of Alabama at Birmingham Matthew C. Schmidt, PhDAuthors · Washington University in St. Louis Sergei Zavgorodni, PhDAuthors · BC Cancer - Victoria