Detector selection impact on small-field dosimetry of collecting beam data measurements among Versa HD 6 MV FFF beams: a multi-institutional variability analysis

Abstract

OBJECTIVE The objective of the present study was to assess the impact of detector type on measurements of small fields for the 6MV-FFF beam within 13 institutes of Elekta Versa HD linear accelerator users. METHODS Using various ion chambers, measurements of the percent depth dose (PDD), off-center ratio (OCR), and output factor (OF) datasets in square areas between 2×2 and 50×50 mm2 for beam modelling in Monaco TPS were collected, and dosimetric differences were compared. The types of used detectors were PTW PinPoint 31014, IBA CC04, IBA CC01, IBA CC13 and Exradin A16, respectively. The effect of the detector type on the OCR was evaluated by including penumbra width of fields, full width half maximum value (FWHM) and FFF beam specific unflatness value. The PDDs were compared using depth of dmax, d5 and d10. Also, the differences between OFs of each field and institution were analyzed with and without correction factor. RESULTS The largest observed variance in dmax was 3 mm for a few institutions, whereas the majority of institutions agreed with GBD dmax values within 2 mm. The agreement of the median PDD values with GBD was all within ±2%. The maximum deviation was under 2% for the shoulder part and 1% for the center part of the OCR profiles for all field sizes. The maximum deviation of the penumbra and FWHM value observed for all field sizes at certain institute OCR data was considered a user-dependent effect instead of a detector. The relative percent difference both the OFuncorr and the OFcorr compared with GBD for all field sizes was within ±3%. CONCLUSION The findings of this study obtained via a large multicentre study can be considered as an external cross-verification for Versa HD users doing beam data collection of Monaco and should help to offer accurate TPS modelling of small fields and minimize the uncertainty of SRS and SBRT. Our results emphasized that the use of several dosimetric systems, comparison of golden beam data, and multi-institutional review are required. © 2024, Turkish Society for Radiation Oncology.