SU‐E‐T‐237: Patient‐Specific Quality Assurance of Dose Delivery for Intracranial Linac‐Based Stereotactic Radiosurgery Using High‐Definition Multileaf Collimators

Abstract

Purpose: Patient‐specific Quality Assurance (QA) of dose delivery for intracranial linac‐based Stereotactic Radiosurgery (SRS) is not a common practice. This study describes an efficient patient‐specific measurement using an ion chamber array system to validate the dose at the isocenter plane when High‐Definition Multileaf Collimators (HDMLC) are used for planning dynamic arcs in small fields.Methods: Measurement of dose in typical SRS treatments is a challenge due to the small sizes of the fields. This study focuses in the measurement of dose at one isocenter plane by using the ion chamber array system MatriXX® embedded into the MultiCube® phantom (IBA Dosimetry Inc.) for patients treated with SRS delivered by HDMLC (2.5‐mm leaf thickness) using the Novalis‐TX linear accelerator. The patient plan is mapped into the phantom placing the isocenter of the lesion in the center of the array, and converting the non‐coplanar arcs into coplanar ones. The total dose distribution in the ion‐chamber array plane is calculated using the same Treatment Planning Algorithm (TPA) used for the patient plan. The phantom‐mapped plan is fully delivered on the phantom such that the combined dose distribution at the isocenter plane of the array is measured directly. A statistical analysis using both the gamma‐factor criteria (3% dose difference and 3 mm distance‐to‐agreement) and the absolute dose difference is then performed between the measured and the calculated dose distributions.Results: This patient‐specific QA was applied to 14 lesions (greater than 1.8 cm in the longest extension) in 7 patients. The percent of pixels with gamma factor greater than one were below 3%, and the percent of pixels with absolute dose difference greater than 5% were less than 5% in all cases Conclusions: This patient‐specific QA for HDMLC is feasible and effective to ensure the accuracy of calculations by the TPA.