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Deconvolution of detector size effect for output factor measurement for narrow Gamma Knife radiosurgery beams

The difficulties associated with the dosimetry of narrow radiosurgical beams are well recognized. Measurements are complicated by the loss of lateral electronic equilibrium, volume averaging artifacts due to the finite size of the detector and also by the detector response in narrow beams. An example is a recent controversy regarding the output factor for 4 mm collimator of the Gamma Knife (4 mm is the diameter of the geometrical projection of the collimator aperture at the focal spot). The accurate measurement of this output factor has practical consequences for treatment planning and also for the comparison of radiation dosages between Gamma Knife and linac radiosurgery. In this study, the output factor for the 4 mm collimator was determined using a miniature ion chamber. This chamber was still too large to measure the output factor for the 4 mm collimator accurately. We corrected the measured output factor using the deconvolution method to unfold the chamber response from the measured data. The dose profile along one of the principal axes of the 4 mm collimator was measured with this ion chamber and also with radiochromic films. The corrected output factor was obtained by deconvolving the chamber size effect from the measured profile data, based on the dose profile data obtained using radiochromic films.