Evaluation of IMRT QA Point Measurement Rocesses Using a Commercial Heterogeneous Phantom

Lung cancer is the leading cause of cancer mortality in the United States. Significant effort has been carried out in developing highly conformal dose delivery techniques using multiple coplanar beams for these patients. IMRT is being used for lung cancer treatment and has shown the potential to deliver a highly conformal dose to the target while minimizing dose to the healthy tissues and is thus an effective tool for dose escalation in lung cancer treatment (Arriagada et al., 2004). Due to the complexity of IMRT treatment plans, patient specific quality assurance (QA) is the standard of practice (Palta et al., 2008). Different measuring devices and techniques such as TLD, EPID and gamma analysis were adopted to investigate the dose distributions for the QA purpose. An independent dose and monitor unit verification (MUV) software was investigated and benchmarked for clinical applications by Goerge et al. (Georg et al., 2007a; Georg et al., 2007b). A common procedure is based on point dose measurements using ion chamber or planar dose measurements using films in a homogeneous solid water phantom. While these measurements provide an evaluation of the linac and MLC performance for a specific IMRT plan, they do not directly verify the dose calculation accuracy of a treatment planning system (TPS) for the IMRT plan in heterogeneous materials. This is particularly important for the case of lung cancer IMRT, where the lateral electron transport at the boundaries of lung and tissue (or tumor) can not be easily handled by a conventional pencil beam algorithm (Butson et al., 2000; Laub et al., 2001; Ma et al., 2003; McDermott et al., 2003; Jang et al., 2006; Davidson et al., 2007).