Generalized equivalent field size for nonuniform fluence maps in IMRT dose calculation

Purpose: The equivalent field size (EFS) method is widely used to estimate dose of nonstandard fields, such as elongated or arbitrary shaped fields, for both central axis and off axis points. However, its application is limited to fluence maps with uniform intensity. In this work, we propose a generalized EFS (GEFS) for nonuniform fluence maps and present a formula for GEFS-based dose calculation. Methods: A parallel-beam dose table (PDT) consisting of central axis dose of circular fields of various diameters at various depths is used to define scatter contributions, based on which we calculate GEFS of any given fluence map. Such obtained GEFS, together with the radiological depth and PDT, is used to determine the dose at the point of interest. We tested GEFS-based dose calculation on a water phantom for both uniform and nonuniform fluence maps and compared the results with those by the collapsed cone convolution/ superposition (CCCS) method. Results: For all test cases, the gamma index is less than 1 based on the 3%/1 mm criteria for more than 96% of the calculated points. Larger discrepancies mainly occur along the field edges in the buildup region. Conclusions: A generalized equivalent field size for nonuniform fluence maps was proposed and its application in calculating dose at any point was presented and verified through comparison with the CCCS method.