Pages: 154-159

Authors: Dario Faj, Hrvoje Brkić, Vjekoslav Kopačin, Marija Majer, Željka Knežević, Svjetlana Marić, and Mladen Kasabašić

Abstract

This paper aims to describe the photon beam characteristics in terms of energy and angular distribution during breast megavoltage photon radiotherapy of pregnant patients. Photon beam characteristics are investigated at treatment volume (breast) and the position of the fetus in the Tena phantom using Monte Carlo simulation. Photon beam energy spectra are compared across various materials used as substitutes for constructing physical and computational phantoms. Mean energies calculated in substitute materials developed by our group and used to build the Tena phantom, differ up to 10 % from the calculated ones in ICRU reference tissue materials. It was found acceptable since this is less than the differences between ICRP and ICRU materials. Then, the photon beam characteristics are investigated in the anthropomorphic phantom, Tena. Photon beam mean energy in the fetal region of the phantom (out-of-field) is significantly lower (more than 1 MeV) than at the breast position (in-field). The angular distribution of the photon beam at the breast position predominantly shows a forward direction, whereas, at the fetus position, the distribution is more scattered. When selecting a detector, it is crucial to consider the differences in photon energy and angular distributions between in-field and out-of-field measurement points to reduce measurement uncertainties and ensure reliable data.

Key words: megavolt photon beam radiotherapy, Monte Carlo, beam characteristic, out-of-field pregnant patient

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