SU‐E‐T‐38: Are the Calculation Methods for Determining Tissue‐Maximum Ratios from Percent Depth Dose Valid for Flattening Filter‐Free Photon Beams?

Purpose: The calculation methods used to determine tissue‐maximum ratio (TMR) from percent depth dose (PDD) were derived by considering differences between TMR and PDD such as the geometry and field size. Phantom scatter factors or peak scatter factors are used to correct dosimetric variation due to field size differences. This relation matches well when the photon beam is flat. However, validity of this equation for flattening filter‐free (FFF) photon beams has never been evaluated. This study aims to evaluate the validity of the equation for FFF beams. Methods: Measured TMR's of 6FFF and 10FFF beams (TrueBeam, Varian) were compared to calculated TMR's. The TMR's were calculated from PDD's using scatter factors both from BJR Supplement 25 and from our own measurements. The same methods were repeated for standard flat 6MV and 10MV beams to confirm the soundness of the measurements and calculations. Results: for the flat beams, the calculated and measured TMR's agree within 1.2% over all field sizes and depths. For the FFF beams, differences are minimal up toa depth of 10 cm for all field sizes. For field sizes less than 20 cm, no noticeable differences are found down to 30 cm depth. The calculated and measured TMR's start to deviate around 15 cm depth for field sizes larger than 20 cm. Differences increase with depth and field size, resulting indeviations up to 3.2%. Conclusions: For field sizes smaller than 20 cm and depths shallower than 15 cm, traditional calculation models for TMR are acceptable for FFF beams; however, beyond those limits, the models start to deviate from measurement. We believe this is due to decreased lateralscatter of the FFF beam. For clinical FFF beams, TMR should either be measured directly, or a new calculation model should be developed.