Planning study of inhomogeneous doseescalated SBRT in pancreatic cancer.

Purpose: Stereotactic body radiotherapy (SBRT) has emerged as a novel therapeutic option to improve outcome of patients with locally advanced pancreatic cancer (LAPC). More intensive schedules are often challenging due to proximity of organs at risks (OARs). Local control is closely related to a biologically effective dose (BED) of 100 Gy10, corresponding to a dose of 50 Gy in 5 fractions. An in-silico study was performed to evaluate the feasibility of dose escalation in SBRT treatments of LAPC through inhomogeneous dose prescription, and to identify patients suitable for this strategy, based on anatomical proximity between target volumes and OARs. Materials and methods: We collected dosimetric data from 14 patients treated at our center for LAPC. For each patient, a CyberKnife (CK) Synchrony re-planning for fiducial-guided pancreatic SBRT treatment was developed for a planned dose of 50 Gy and 40 Gy in 5 fractions to gross tumor volume (GTV) and target volume planning (PTV), respectively. Priority was given to OAR constraints. Criteria for acceptable coverage of the target were: a) 50 Gy and 47.5 Gy to ≥90% and ≥95% of the GTV, respectively, and b) 40 Gy to ≥95% of the PTV. For each plan, a Expansion-Intersection Volume (EIV), corresponding to the intersection volume between PTV and OARs expanded by 5 mm, was calculated. The planned doses to the target volumes and the OARs were evaluated and statistically analyzed. Results: Median GTV and PTV sizes were 40.8 (range 22.3-205.3) cc and 73.7 (range 36.1-266.7) cc, respectively. Treatment plans have been optimized to keep a V35 in the duodenum, stomach and intestines below 0.5cc in all cases. Median V50 and V47.5 for GTV was 91.0% (range 82.4% -97.8%) and 96.8% (range 92.5% -99.9%), respectively: GTV coverage was acceptable in 10 out of 14 cases. Median V40Gy for PTV was 96.8% (range 90.0% -99.8%): PTV coverage was acceptable in 11 of 14 cases. Median EIV was 12.9 (3.9- 25.1) cc. Spearman correlation showed a significant association between EIV and V47.5Gy for GTV (rho -0.77228, p<0.001) and V40Gyfor PTV (rho -0.68352, p <0.001), respectively (Figure 1). Conclusions: Inhomogenous dose escalated prescription to a BED≥100Gy10 is a feasible treatment strategy in selected patients with LAPC (Figure 2). EIV represents a simple tool to identify suitable patients for this approach [ABSTRACT FROM AUTHOR]