A new methodology to determinate linac photon spectra using the EPID signal

Abstract

A precise estimation of the photon spectra emitted by radiation therapy Linacs is especially important to accurately estimate the dose received by patients in order to improve treatment plannings. Moreover the knowledge of the spectra is useful to perform routinely Quality Control procedures of these equipments. In this work we propose the use of the Elekta Linac EPID (Electronic Portal Imaging Device) to obtain the fluence spectrum emitted by this unit. To that, several images using different thicknesses of solid water slabs have been taken with the EPID, setting 100 UM, a 10 cm x 10 cm field size and a 6 MeV energy beam. The relation between the signal response recorded by the EPID with several solid water slabs and the primary photon spectrum emitted is calculated by means of a Response matrix, which has been performed via Monte Carlo simulations. To that, the MCNP5 code has been used to simulate the EPID signal response obtained after irradiation with consecutive monoenergetic beams for different slabs. Each row of the Response Matrix represents the Monte Carlo EPID signal registered using a specific energy bin. Since this matrix is ill conditioned we have applied the Truncated Singular Value Decomposition algorithms to unfold the spectra. Results have been compared with theoretical spectrum, showing that the mixed methodology (experimental-Monte Carlo simulation) can offer an accurate way to determinate the photon spectrum in radiation therapy.