Region of interest selection and volume averaging for Gamma Knife radiochromic film dosimetry

<i>Objective.</i>Accurate determination of relative output factors (ROFs) is paramount in stereotactic radiosurgery dosimetry, particularly for small fields employed in Leksell Gamma Knife (LGK) systems. Existing guidelines for LGK film-based dosimetry lack specificity regarding the selection of a region of interest (ROI) for ROF measurements.<i>Approach.</i>This investigation assessed the dosimetric performance of Gafchromic^TMEBT4 film, focusing on establishing a quantitative metric for the unambiguous selection of a small ROI, defined relative to the beam's full width at half maximum (FWHM), for LGK ROF measurements. Film irradiations were performed on an LGK ICON unit using 16-, 8-, and 4 mm collimators. Calibration curves were generated by irradiating 15 films with 16 mm beams in 15 s increments, with films subsequently scanned in both portrait and landscape orientations at various resolutions. Dosimetric accuracy was assessed using the net optical density transfer function along with corresponding uncertainty propagation. The influence of detector size on ROF determination was calculated using ROIs with diameters systematically varied to include 1/10, 1/5, 1/4, 1/3, 2/5, 1/2, 2/3, and 3/4 of the beam's FWHM.<i>Main results.</i>ROF measurements demonstrated high sensitivity to ROI size. A metric employing a ¼ FWHM ROI yielded ROFs values within 1% of the established Leksell GammaPlan (LGP) reference data, effectively minimizing film volume-averaging effects. Systematic dose errors, up to ±20% on average, were observed due to orientation mismatches between calibration and measurement scans. Furthermore, across all evaluated scan resolutions both FWHM and penumbra measurements consistently conformed to LGP specifications, specifically within ±0.5 mm and ±1 mm, respectively.<i>Significance.</i>The ¼ FWHM ROI provides a clinically practical method for LGK ROF determination, addressing the absence of a clear ROI definition in TG-178 report. This metric achieves a suitable balance for film-based measurements, ensuring a sufficient number of ROI pixels for statistical significance while minimizing volume-averaging effects.