Room: AAPM ePoster Library
The high complexity of IMRT planning, dose calculation and treatment delivery requires careful quality assurance which is most commonly performed with patient specific end-to-end tests using 2D and 3D detector arrays. The vendors SNC and PTW are some of the largest providers for dosimetry equipment including various types of PSQA devices. The authors compare detection sensitivities for dosimetric errors of four competing IMRT and VMAT PSQA devices.
Verification plans for 22IMRT and 21VMAT treatments were projected on solid water phantoms. The X-ray energy was mostly 6MV, with a few plans based on 10MV, 6MVFFF and 10MVFFF. The treatment plans covered a wide range of sites including highly modulated H&N and homogeneous breast-tangents plans. Additionally, two sets of treatment plans for IMRT and VMAT with MLC position-errors were created.
The IMRT plans were measured with the SNC-MapCheck and the PTW-Octavius. VMAT plans were recorded with the SNC-ArcCheck and the PTW-Octavius4D. The gamma value based on absolute dose with 10% threshold was used to quantify the passing-rate for passing-criteria of 3%/2mm down to 1%/1mm as suggested by TG218. The gamma value was calculated in 2D for IMRT in SNC-Patient and PTW-Verisoft and in 3D for VMAT in SNC-3DVH and PTW-Verisoft.
The average passing rate was significant lower for Octavius1500 than MapCheck and slightly lower for Octavius4D than ArcCheck and the discrepancy increased with tighter passing-criteria. For the 3%/2mm criterion, passing rates for VMAT plans are unchanged for single MLC shifts and decrease by 3% for 5mm shifts in IMRT plans; a shift of the full MLC-bank by 2mm reduces the passing rate in average by 15% for VMAT and 53% for IMRT.
The passing rate for the PTW devices is generally lower. PSQA on IMRT plans is more sensitive to shifts in MLC positions than for VMAT plans.
Funding Support, Disclosures, and Conflict of Interest: Muthukumaran Manavalan is employee of the PTW North America Corporation and provided the PTW Octavius QA devices.