EP-1539 VOLUMETRIC MODULATED ARC THERAPY FOR STEREOTACTIC SPINE BODY RADIOTHERAPY: A PLANNING STUDY

plans for Elekta, the best conformity was observed for the dMLC. No significant change of the conformity was seen, after changing the MLC leaf width from 1 cm (MLCi2) to 0.5 cm (Agility). Technique-dependent, we found no significant differences for the amount of segments for the different MLC's. Supposedly, due to the capability of interdigitation (MLCi2, Agility), the amount of segments (87±6 Oncor s+s, 68±2 s+s MLCi2, 69±3 s+s Agility) and MU (734±22 Oncor s+s, 551±29 s+s MLCi2, 549±36 s+s Agility) decrease for s+s-IMRT in case of more complex prescriptions (group 2). Comparing MLC's with 1 cm (MLCi2) vs. 0.5 cm leaf width (Agility), MU's decrease for dMLC and VMAT.shape optimization, VMAT plans show improved homogeneity and steeper DVH fall offs in high dose regions, creating a further potential for OAR sparing or dose escalation. Our results show decreased MU's for dynamic techniques, when using the new Agility MLC, leading to a reduction of secondary radiation and potentially shorter treatment times. However, these results do not lead to improved plan quality for prostate (including or excluding the pelvic lymph nodes) when having a MLC with finer resolution or higher leaf speed (Agility).