Po‐Thur Eve General‐07: Dosimetry of Small Lung Lesions with EGSnrc Monte Carlo and Treatment Planning Systems

Abstract

Early stage lung cancer, presenting as a small solitary pulmonary mass, is treated with radiation when concurrent disease precludes a surgical option. These small lesions are usually surrounded by less dense normal lung, which affects the ability to deliver a homogenous dose to the target volume. In low‐density tissue, such as lung, there is increased transmission of photons. In addition, lateral scatter of electrons out of the beam can lead to increased penumbral width. The magnitude of these effects is known to be dependent on beam energy. Some of the commonly used commercial treatment planning systems have had limited success in predicting accurately dose distributions under these highly inhomogeneous conditions.We present a quantitative comparison between Monte Carlo simulation and commercial planning systems for a select range of clinically relevant target geometries and beam parameters. Small water equivalent cylindrical lung tumors of diameter 3 and 5 cm were incorporated within a CT dataset at different locations. A Parallel Opposed Pair (POP) field arrangement with 6MV or 15MV photons and variable field‐edge margins were considered. These plans were calculated using BEAMnrc Monte Carlo code and on two planning systems; ADAC Pinnacle III Version 7.4 and MDS Nordion Theraplan Plus v3.8.The analysis of dose profiles and DVH's show considerable and unique differences between Monte Carlo and the results from each TPS within the tumor and at the junction between tumor and lung. For both planning systems, the severity of these errors, increases with photon energy, and decreases with field size.