Determination of dosimetric parameters using equivalent square methods for a MLC

The concept of equivalent square has been looked at by many authors. The approaches range from simple approximations to summation methods and integration methods. These studies looked at output factors, collimator scatter, TMR, or percent depth dose. In this study, output factor, collimator factor, and TMR data were measured for rectangular and irregular fields for a Siemens MLC. The measurements were made for 6 and 18 MV photon beams. To provide comparison with previous studies, the field shapes described by Xiao et al., Phys. Med. Bio. 44, 1053-1065 (1999) and Yuen et al., Med. Phys., 26, 2385-2389 (1999) were used. It was found that the average field length approximation is accurate to within 3.4% for all parameters and fields studied. The average deviation for output factor was 0.1% for rectangular fields with 28 /30 fields within 1%. The average deviation was -1.1% for the irregular fields with 17/31 fields within 1%. The average deviation for TMR for rectangular fields was 0.3% with ah fields within 1%. For irregular fields the average was -0.6% with 24/31 within 1%. The average deviation was -1.1% for the irregular fields with 17/31 fields within 1%. The average deviation for collimator scatter factor for rectangular fields was 0.1% with 26/30 fields within 1%. For irregular fields the average was -0.8% with 21/31 within 1%. Thus a simple approximation can be used to determine dosimetric parameters for clinical fields with an accuracy better than 2%.