SU-F-T-511: Feasibility Study of Using Flattening-Filter-Free Photon Beams to Deliver Conventional Flat Beam.

PURPOSE: Various dosimetric benefits such as increased dose rate, and reduced leakage and out of field dose has led to the growth of FFF technology in the clinic. In this study, we concentrate on investigating the feasibility of using flattening-filter-free (FFF) beams to deliver conventional flat beams (CFB), since completely getting rid of the flattening-filter module from the gantry head can not only simplify the gantry design but also decrease the workload on machine maintain and quality assurance.

METHODS: The sliding window based IMRT technique was utilized to generate the CFB from the FFF beam for various beam configurations on the Elekta Versa HD. The flat beam reproducibility and MU efficiency were compared for each beam configuration among FFF planning, delivery and CFB planning.

RESULTS: Compared to the CFB plan, the 3%3mm passing rates of the FFF beams from both measurement and plan are 100% and 95%(or better) for 15×15 cm(2) or smaller field size and for any field size greater than 15×15 cm(2) at 10 cm depth, respectively. The largest discrepancy is about 5% and typically appears at the field shoulder area. The MU increase average was 80% for FFF compared to CFB, however has a minimal impact on treatment delivery time.

CONCLUSION: The ability to deliver conventional flat treatments is not absent when operating in FFF mode. With proper TPS manipulation and beam modulation, FFF mode can achieve reasonable flat profiles and comparable dose coverage as CFB does for various conventional treatment techniques, such as four field box, or long spine treatment techniques. The ability to deliver most clinical treatments from the same treatment unit, will allow for less quality assurance as well as maintenance, and completely eliminate the need for the flattening filter on modern linacs.