IMRT and VMAT in the ACDS level III audit

Abstract

Introduction Due to clinical demand, IMRT and VMAT have been added as additional optional modalities which may now be included in the level III audit. Facilities may choose the option to audit multiple beam models and algorithms as part of the audit. Method The plans used for the audit are the “C-shape” target volume from AAPM TG119 [1] (with and without lung inhomogeneities) and the “Complex case” with two adjacent target structures (see Fig. 1). The dose is measured in a custom anthropomorphic phantom (CIRS) using 7 CC13 chambers and an eight channel tomotherapy electrometer. Field trials have been conducted at 13 radiotherapy facilities, with 16 beam models tested for each of IMRT and VMAT. Results The planning constraints have been found to be difficult to meet for some planning systems and are intended to produce a suitable degree of modulation only and not to test the ability of planning staff. The average dose difference was + 0.5% with standard deviation 1.6% for IMRT and - 0.5% with standard deviation 1.7% for VMAT from 270 to 282 in volume point dose measurements respectively. Of 552 point doses measured in 92 plans delivered, 3 points had dose differences greater than 5% and 44 were between 3.3 and 5% (see Fig. 2). Conclusion The ACDS level III IMRT and VMAT audits will score the absolute dose difference in all measured points and use ± 5% as the threshold for an out of tolerance audit result and ± 3.3% for pass (action level).L6215694552018-04-11 <br />