ESTRO 38 Abstract book

S499 ESTRO 38

PO-0929 Focal boost dose escalated prostate SBRT on the Halcyon fast-rotating O-ring linac R. De Roover 1 , W. Crijns 1 , S. Michiels 1 , C. Draulans 1 , K. Poels 1 , K. Haustermans 1 , T. Depuydt 1 1 Catholic University of Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective A commercially available fast-rotating O-ring linac, Varian Halcyon (HA), has been shown to improve the time- efficiency of conventionally fractionated volumetric modulated arc therapy (VMAT). The dual-layer multi-leaf collimator’s (MLC) 10 mm leaf-width, however, may limit its use for stereotactic body radiotherapy (SBRT). In this study, HA is compared with a SBRT-dedicated C-arm linac for the planning and delivery of focal boost dose escalated prostate SBRT. Material and Methods Sixteen patients with intermediate or high risk prostate cancer were planned for whole-gland prostate SBRT with a focal boost to the macroscopic tumor using VMAT. Prescription was 35 Gy to the prostate (PTV prostate ), 30 Gy to the seminal vesicles (PTV SV ), and 40 Gy to the focal boost (GTV boost ) in 5 fractions. The GTV boost should be aimed to receive up to 50 Gy as long as the organ at risk (OAR) sparing is not at risk. Plans were generated for HA using 6MV flattening filter free (FFF) dual arc (HA2) and triple arc (HA3) VMAT. Field shaping was performed by both MLC layers independently (10 mm leaf-width, with a 5 mm offset between the staggered layers). The collimator angles were 10°/80° and 10°/45°/80° for dual arc and triple arc respectively. For comparison, plans were generated on a Varian TrueBeam STx (2.5 mm leaf-width) using 6MV dual arc (TB2) VMAT, with collimator angles 10°/80°. The Photon Optimizer and Anisotropic Analytical Algorithm were used in Varian Eclipse (v15.6) for each plan. Identical optimization objectives were used for each plan within a patient. Plans were normalized to 35 Gy covering 95% of PTV prostate . PTV coverage and OAR sparing were compared between HA2, HA3 and TB2 using a two-sided Wilcoxon matched- pairs signed-rank test. Pre-treatment quality assurance (QA) was performed using portal dosimetry using a 2%(local)/2mm gamma evaluation with a low dose exclusion threshold set to 20% of the prescribed dose. The time needed to deliver the plans in auto-sequencing mode was recorded. Results Median target coverage, OAR doses and plan properties are compiled in Table 1. Figure 1 shows the median dose- volume histogram for a selection of OARs. Both HA2 and HA3 achieved similar target coverage and focal boost dose escalation compared to TB2. All OAR doses were well within our clinical criteria. Similar urethral sparing was obtained for HA2 and HA3 compared to TB2. Increased D max to rectum and bladder was observed for HA2 and HA3 (median increase of 0.6 Gy and 0.4 Gy respectively). The median plan delivery time was reduced with of 34 sec and 16 sec for HA2 and HA3, respectively. All plans passed pre- treatment QA with gamma agreement scores above 94.6%.

Conclusion Compared to a SBRT-dedicated C-arm linac, HA achieved clinically comparable quality for focal boost dose escalated prostate SBRT planning. This study demonstrates the potential of a mainstream fast-rotating O-ring linac to deliver increasingly used prostate SBRT treatments. PO-0930 Knowledge-based planning of head and neck cancer; comparisons of target and normal tissue parameters A.I.S. Holm 1 , L. Kaplan 1 , U.V. Elstrøm 1 , S.S. Korreman 2 1 Aarhus University Hospital, Department of oncology, Aarhus C, Denmark ; 2 Aarhus University Hospital, Department of oncology and Danish Center for Particle Therapy, Aarhus C, Denmark