Dose distribution of stereotactic irradiation for thorax

Abstract

Stereotactic irradiation has been adopted in many hospitals recently. For this treatment, accurate dosimetry is important for determining the absorbed dose to target volume. But there are some difficulties caused by small field. Especially in the case of lung cancer, it is difficult to determine the absorbed dose for narrow beam field. Because it is composed variety materials, it makes determining absorbed dose to be difficult. In the lung, which has a low density, the number of recoil electron per unit volume is smaller than the soft tissue and travels longer distance. Consequently, the recoil electron arrives the outside of the field and electron equilibrium does not exist. In this study, the absorbed dose distribution of thorax was calculated using the EGS4 Monte Carlo simulation. The thorax model was composed of 3 layers; the thickness of front chest wall is 3 cm, the thickness of back chest wall is 5 cm, and between them, there is lung in which including small target volume. The variation of absorbed dose distribution as a function of field size, energy of incident X-ray and depth of target volume was investigated.