OA131] Experimental study of dynamic dose interplay for flattening filter free beams in VMAT SBRT lung treatments

Abstract

Purpose The interplay effects can be occurs due to respiratory motion, while it is likely to average out over a large number of fractions, it may play a role in hypofractionated stereotactic body radiation therapy (SBRT) treatments. We investigated discrepancies between the delivered dose in a static and motion phantom simulating the respiratory movement for a different numbers of control points optimized and dose rates. Methods The majority of patients with lung cancer undergoing VMAT treatment have tumor motion exceeding 5 mm on four-dimentional computed tomography scan. Static and dynamic measurements were performed with ionization chamber in a Quasar motion phantom in a truebeam machine. Dynamic measurements with amplitude range of 0.5–2 cm of clinical treatment plans were compared with static measurements. The clinical plans were optimized with two and one full arcs, and two semi arcs and two semi arcs plus 30°, with 356, 178, 196 and 228 control points respectively. For all plans it was used 6 and 10 flattening filter free (FFF) and 6 MV energies, with dose rates 1400, 2400 and 600 MU/min respectively, and three fractions of 18 Gy. Results The measurements showed that interplay effect is negligible for different dose rate studied, the main reason for this effect is due to increasing the range of motion. For the amplitude of 2.0 cm the differences between dynamic and static measurements was up to 25%, while for 1.0 cm was 3%. The full arc presented a large discrepancies compared to semi arcs (∼3%). However there was no difference between two semi arcs and two semi arcs plus 30°. Conclusions The ionization chamber dosimetry showed that interplay effect between leaves and tumor motion is not significant for reasonable respiratory motion, less than 1.0 cm of amplitude; when delivered with two semi arcs instead of full arcs, and FFF energies do not increase this effect. In cases of amplitude greater than 1.5 cm, the use of an abdominal compressor is recommended.