Patterns of failure after adjuvant stereotactic body radiation therapy in patients with pancreatic cancer with close or positive margins.

Abstract

755 Background: There is no consensus on treatment volumes for stereotactic body radiation (SBRT) in patients with pancreatic cancer (PCa). Herein, we report patterns of failure following adjuvant SBRT for close/positive margins in patients with pancreatic cancer, which may inform appropriate target volume design for SBRT. Methods: An IRB-approved retrospective review of patients with PCa treated with adjuvant SBRT for close/positive margins from 2009-2018 was conducted. Patterns of failure were assessed by review of imaging and were defined as local (LF), regional (RF), local and regional (LRF), or distant (DF). The Kaplan-Meier method was used to calculate long-term failure rates. In-field failures were defined as LFs completely within the PTV (planning target volume). The location of LFs was compared to the RTOG consensus volumes for adjuvant treatment of PCa to determine if conventional radiation volumes would have included the LF. Results: Seventy-six patients were treated with adjuvant SBRT for close (51.3%) or positive (48.7%) margins, with a median follow-up of 17.0 months (interquartile range [IQR] 7.4-28.3 mos.). Adjuvant SBRT was delivered at a median of 2.2 months after surgery (IQR 1.7-3.0 mos.). Most patients (81.6%) received 36 Gy in 3 fractions. The median PTV volume was 17.8 cc (IQR 12.3-25.2 cc). Upon examination of first failure sites, crude rates of isolated LF, isolated RF, isolated LRF, and DF +/- LF or RF were 9.2%, 6.6%, 2.6%, and 56.6% respectively; 2-year rates were 12.4%, 11.5%, 7.0%, and 66.5%, respectively. Thirty-two patients (42.1%) developed a LF at some point during follow-up. Of 28 LFs with available plans and imaging, 21.4% were in-field failures, while the remainder were completely outside (60.1%) or partially outside (17.9%) the PTV. Most LFs outside the PTV (90.9%) would have been encompassed by the RTOG consensus target volumes for postoperative conventional radiation. Conclusions: In patients with PCa who receive adjuvant SBRT for close/positive margins, the majority of LFs are outside the PTV. Future trials involving SBRT or hypofractionated radiation should consider expansion of treatment volumes if feasible.