Point dose verification of Cranial Stereotactic Radiosurgery using micro Ionization Chamber and EBT3 film for 6MV FF and FFF beams in Varian TrueBeam® LINAC

Abstract

Abstract Introduction: Achieving high positional and dosimetric accuracy in small fields is very challenging due to the imbalance of charged particle equilibrium (CPE), occlusion of the primary radiation source, and overlapping penumbra regions. These factors make the choice of the detector for Stereotactic Radiosurgery (SRS) patient-specific quality assurance (PSQA) difficult. The aim of the study is to compare the suitability of EBT3 Gafchromic film against CC01 pinpoint chamber for the purpose of SRS and stereotactic Radiotherapy (SRT) dose verification. Material and Method: EBT3 Gafchromic film was calibrated against Treatment Planning System (TPS) doses (1 Gy – 35 Gy). CC01 pinpoint chamber and EBT3 film was used to verify Patient-Specific point doses of 21 intracranial lesions each planned with Static, Dynamic Conformal Arc (DCA), and Volumetric Arc Therapy (VMAT) using Varian TrueBeam Accelerator 6MV Flattening Filter (FF) and 6MV Flattening Filter Free (FFF) beams. The lesion sizes varied from 0.4 cc to 2.9 cc. The lesions were categorized into <1cc, 1cc-2cc and 2cc-3cc. Results: High variations in measured doses from TPS calculated dose were observed with small lesion volumes irrespective of the dosimeter. As the sizes decreased high uncertainty was observed in ion chamber results. CC01 was observed under-responding to film in small lesion sizes (<1cc), where nearly 50% of results under-responded in comparison with Film results. Film results were more or less consistent for static and DCA plans. Static and DCA plans were consistent passing more than 73% of the plans of the smallest lesion size category. VMAT showed very poor PSQA agreement for all three volumes (32.1% for <1cc, 14.3% for 2cc-3cc and 39.3% for 2cc-3cc). No significant difference was observed between 6MVFF and 6MVFFF beams from the chi-squared test. Conclusion: EBT3 Film was observed to be a more suitable detector for small lesion sizes less than 1cc, compared to CC01. As the volume increases, the response of CC01 and EBT3 film have no significant difference in performing PSQA for intracranial SRS/SRT. VMAT techniques for intra cranial SRS shows deviation from TPS planned dose for both EBT3 film and CC01 and should not be preferred choice of verification tools.