Dosimetric Constraints In Stereotactic Body Radiation Therapy For Locally Advanced Pancreatic Cancer: Capacity For Dose Escalation

Dose escalation in stereotactic body radiation therapy (SBRT) for locally advanced pancreatic cancer (LAPC) is typically limited by dose constraints to nearby gastrointestinal organs, but it is unclear how often and to what degree these constraints are limiting dose to the target. The purpose of this study was to characterize the role that dose constraints may play in limiting dose escalation in SBRT for LAPC. Dose and volume relationships were reviewed for all patients with LAPC who were treated with definitive SBRT at our institution from 2010 to 2016. The main dose constraints used at our institution include V15<9 mL, V20<3 mL, and V33<1 mL as applied to the following structures: proximal duodenum, proximal bowel, and proximal stomach. Patients and plans were grouped and stratified by the degree to which these constraints were met. Any patients with missing or incomplete dosimetric data for the above structures were removed from analysis. Of 151 patients from 2010 to 2016 treated with definitive SBRT for LAPC at our institution, 64 had all requisite dosimetric and volumetric data available for analysis. The distribution of anatomic tumor locations included 56% pancreatic head, 29% body, and 4% tail, with the remainder spanning multiple adjacent locations. The goal prescription dose for all patients was 33 Gy in 5 fractions, but 22 patients (34%) were prescribed a dose <33 Gy (median, 30.0 Gy; range, 25-32.5 Gy) due to difficulties with achieving dose constraints. For patients who received 33 Gy as well as for those who received <33 Gy, the proximal duodenum was the most frequent dose-limiting structure (43.8%) as identified by the constraint most narrowly met or even minimally exceeded for that plan (median, 84.9% of the standard constraint; range, 52.8%-99.2%). We also identified “safe” cases with the potential for further dose escalation in which all volumetric dose constraints were met even when adjusting constraints to 90% of the standard constraint. Of the 44 patients who received full-dose 33 Gy SBRT, 32 (76.2%) had “safe” plans. The range over which constraints could be theoretically minimized in “safe” plans was 16.7% to 89.6% of standard constraint (median 80.8%). The proximal duodenum was the most common dose-limiting structure in our institution’s experience with SBRT for LAPC. Although a minority of patients (about one-third) required dose-reduction due to proximity of normal structures, safe planning and delivery of full-dose SBRT to 33 Gy in 5 fractions was achieved in most patients. Among patients who received full dose, we were able to comfortably meet dose constraints in the vast majority, implying that there may be an opportunity to escalate dose for these patients.