Purpose: To experimentally determine the sensitivity of gamma analysis for detecting geometric errors in multi-target SRS treatment delivery using 2D and 3D dosimetry.
Methods: A singleâ€?isocenter SRS VMAT plan consisting of three 1cm targets located at 0, 4.1, and 7.4cm from isocenter was generated for a cylindrical ClearView gel phantom in Eclipse v.13.6. Each target received D99%>22Gy prescription dose, using a total of three coplanar and non-coplanar arcs. The plan was delivered to gels dosimeters with and without an intentional 1-degree table offset introduced after CBCT registration, representing the maximum allowable couch offset per standard QA guidelines. The clinical relevance of the error was determined by comparing target volume coverage, conformity index, homogeneity index, and unintended normal tissue dose relative to standard SRS cases. Each target in the gel plan was individually verified using Gafchromic EBT-XD film in a cylindrical solid water phantom. Gel and film were calibrated according to established protocols. Gamma sensitivity analysis was performed in Matlab and FilmQA Pro for gels and film, respectively, using various tolerance criteria.
Results: At 7.4cm from isocenter, D99% was reduced by 15% when the intentional error was introduced. Gel-measured comparisons using gamma analysis with 4%/1mm criteria had passing rates of 45%-85% and 88%-95% for targets with and without intentional errors, respectively. Film dose was calculated using 4%/1mm and 3%/3mm gamma criteria. The results with intentional errors were 71â€“82% and 98-100% for 4%/1mm and 3%/3mm, respectively. These results demonstrate that both film and gel are suitable detectors for single-isocenter, multi-target SRS QA, but that careful selection of gamma tolerance criteria is required.
Conclusion: This study demonstrates the sensitivity of both gel and film to detect subtle, but clinically relevant, geometric errors during SRS pre-treatment QA. However, the gamma criteria used should be optimized by treatment modality.