Modified dose difference method for comparing dose distributions.

The quantitative comparison of two-dimensional dose distributions (e.g., calculated versus measured) has become a key issue in intensity-modulated radiotherapy (IMRT) QA. We proposed a new evaluation method referred to as modified dose difference (MDdiff) evaluation. Hereinafter, features and effectiveness of the MDdiff evaluation method will be described. In this work, the formalism of MDdiff was defined by introducing a dimensionless parameter β(r)(DG)(r(-->)(r)). A new formalism is compared to a gamma method, and the MDdiff and the gamma method are respectively applied to patient-specific IMRT QA. The calculation of the evaluation of dose distributions was performed using a C++ program. Evaluations were performed by counting the number of data points satisfying MDdiff ≥ (1/2)δD⁰, γ ≥ 1. The evaluation result of dose distributions using the MDdiff method had the same tendency as the evaluation result using the gamma evaluation method. The modified dose difference tool also provides a quantitative method for comparing two dose distributions like the gamma evaluation. Furthermore, many problems of gamma evaluation are resolved.