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.