SU-E-T-559: Evaluation of Flatening Filter Free (FFF) Beams in Radiotherapy of Head and Neck Cancer.

Abstract

To investigate th applicability of the flattening filter free (FFF) to fractionated radiotherapy of head and neck cancer. Twelve patients previously treated with IMRT were selected. A SIB protocol was chosen so that 66/54 Gy were prescribed to the gross tumors/elective nodal volumes in 30 fractions. Photon beams of 6X(6X-FFF) were used for both IMRT (@ 9 gantry angles) and VMAT (single and dual full arcs). Consistent optimization and acceptance criteria were used for all plans, with prescription dose to cover 90% of PTV66 and hot spots limited to 110% and within PTV66. Both plan quality and deliver-ability were compared. All plans met clinical objectives. The IMRT plan quality is similar or slightly better than VMAT, except that PTV66 conformity index is significantly better in 2-arc VMAT (1.18) than IMRT (1.32). The total MUs for IMRT are 1680/2090 for 6X/6X-FFF, a 24% increase. However, the total beam-on time (BOT) under treatment automation is 430/400 seconds for 6X(@300 MU/min)/6X-FFF (@400 MU/min), a 6% decrease. Increasing dose rate to the maximum (600/1400 for 6X and 6X-FFF) can reduce BOT by up to 50%, but at a cost of increased MU (up to 40%) and degraded plan quality. Significant reduction in MU and BOT are realized in VMAT, with ∼600 MU for both 6X and 6X-FFF and either single or dual-arcs. The BOT for VMAT is governed by the gantry speed and approximately 1 min for single arc and 2 min for dual arcs. Plan quality is similar between IMRT and 2-arc VMAT for both 6X and 6X-FFF. Total MU for IMRT and delivery time is significantly higher for IMRT than VMAT, but not much different between flat and non-flat beams. For standard fractionated radiotherapy of HN cancer, there is no advantage for the non-flat beam. The recommended choice is 2-arc VMAT.