Dosimetric characterization of a 5 mm diameter BrainLab cone for radiosurgery

Abstract

Purpose. A dosimetric characterization of a 5 mm diameter cone is presented. PDD curves, dose profiles (OAR) and output factors (Scp) were measured utilizing different detectors. Scp values were obtained using distinct methodologies and they were compared to the new gold standard established by TRS-483 IAEA (2017). Methods. A procedure to align the detector axis of motion and the central axis for PDD determination is described. Four ionization chambers (PTW Farmer 30013, PTW Pin Point 31014, IBA CC01 and IBA CC04), a stereotactic diode (IBA SFD), EBT3 radiochromic film and TLD-600 Micro Cubes were used. For Scp determination five methodologies were considered: direct measurement method (DMM), intermediate field size method (IFSM), Monte Carlo correction factor method (CFM), BrainLab recommended method (BLM) and scanning chamber method (SCM), described by Fan et al (2009 Med. Phys. 36 5292–5300). SFD results were compared to Monte Carlo simulations. Results. PDD curves obtained satisfactorily agree (p < 6.4 · 10−3). The best results for OARs were obtained using high spatial resolution devices, namely EBT3 and SFD diode. A 0.666 value was obtained for Scp as of an average of SFD and CC01 detector results utilizing TRS-483 new methodology. Differences between both results were less than 0.5%, showing how robust the new methodology is. SCM, BLM and EBT3 measurement achieved the most similar results with respect to TRS-483 obtained result (discrepancies of 1.7%, 0.1% and 3.7%, respectively). Besides, Monte Carlo result obtained (0.673) perfectly agrees with these methods results. Conclusions. A multidetector study was performed to obtain the main dosimetric functions to characterize a 5 mm radiosurgery cone. The use of ion chambers is not encouraged for OAR determination because of the important volume averaging effect, higher penumbras and steeper profile fall off nearby the maximum. Utilizing TRS-483 methodology we obtained a value of 0.666. SCM, BLM and EBT3 measurement are reliable procedures to obtain Scp values for very narrow beams due to their slight difference when compared to the new gold standard result.