A method for an external quality audit programme in radiosurgery with TLDs and radiochromic film

Purpose: Radiosurgery is a high-precision technique that delivers a single highly conformed dose to a stereotactically localized target in most cases. This study proposes a simple and reliable postal quality assurance (QA) phantom for use in an independent evaluation. Methods: Two important parameters were verified, including the dosimetric precision of the treatment planning system, by comparing the absorbed dose measured in the target volume using different dosimeters (i.e., radiochromic film and thermoluminescence dosimeters) calibrated against a small volume ion chamber. The head phantom and the instruction sheets were extensively tested and sent by mail to selected institutions. The institutions were chosen according to the type of linear accelerator they had. The three institutions chosen had different radiosurgery systems for the delivery of treatment and the planning system. The objective was to understand whether our system would be applicable to different radiosurgery delivery systems. Results: A comparison between the TPS plan and the measurement plan using the radiochromic film showed that the gamma pass rate was more than 90% for a gamma criteria of 2 mm/2%. In the worst case scenario, the disagreement with TLD measurements at position 1 arose because the highest dose gradient occurred in that region. The other positions measured with thermoluminescence dosimeters (TLDs) varied by less than 3% from the calculated dose. Conclusion: The overall results were very encouraging and suggest that the proposed phantom may be used as a postal system as part of an independent QA tool for radiosurgery.