Sci—Fri PM: Delivery — 08: Characterizing the spatially varying fluence and spectra of a kV imaging source for dose calculations

Kilovoltage (kV) daily image‐guided radiotherapy (IGRT) procedures accumulate radiation dose within the patient that is currently not routinely incorporated in the treatment plan. As part of the process of developing a patient‐specific kV dose computation tool, the kV x‐ray source must be characterized. We propose a simple, clinically feasible experimental characterization method using in‐air dose measurements along the transverse axis. We determine half‐value layer (HVL) along the transverse axis, from which we derive the HVL‐specific mass‐absorption coefficient, which is used to determine beam fluence. These values are interpolated over the entire field. The spectrum at each interpolation point in the field is found from HVL and accelerating potential (kVp) using third‐party software Spektr. We use this method to characterize the spatially varying fluence and spectra of a Varian® On‐Board Imaging® source for energies 80, 100 and 125 kVp. This characterization is used to compute dose within a heterogeneous phantom, using our previously validated in‐house dose computation software, which we compare with relative dose measurements. We show that for a 10×10 cm2 field size using no added filtration, the agreement for all three energies is within 2% for the central depth‐dose profile and within 2.6% for the transverse profiles. This clinically feasible experimental characterization method for kV imaging sources represents a crucial step in the development of a patient‐specific dose computation tool.