Independent dose verification for brain stereotactic radiotherapy using different add-on micro multi-leaf collimators

An add-on micro multi-leaf collimator (mMLC) is used for stereotactic radiosurgery (SRS) and brain stereotactic radiotherapy (SRT), in which rotational radiotherapy may make more complex and time-consuming. We performed a retrospective study of an independent dose calculation verification for brain SRS and SRT in two institutions to show the accuracy of the verification system and propose a tolerance value for the verification. Several comparisons of static plans and patients' plans were conducted using a phantom measurement, and patients' plans using the patients' own computed tomography image. We evaluated the accuracy of the Clarkson-based dose calculation based on either the equivalent square field formed by the mMLC or by the collimator jaws to determine the collimator scatter factor (S _c). The results for the static plans showed good agreement (<1%), except when we used a 1 cm² field size (<4%). The phantom measurements for the patients' plans showed deviations of 0.1 ± 2.3 and 1.2 ± 1.6% (2 SD) for the treatment planning system and the verification system, respectively. The patients' plans showed a deviation of 2.0 ± 2.1% (2 SD). Depending on the mMLC system, the S _c was calculated using the equivalent field size formed by the mMLC. In this study, we suggest a tolerance level for the brain SRS and SRT of 2-3.5%. However, beam modeling in the treatment planning system would affect the deviation. The S _c should be computed according to the size of the collimator fitted to the MLC.