Patterns of Regional Failure in Patients with Pancreatic Cancer treated with Stereotactic Body Radiation Therapy

Abstract

Stereotactic body radiation therapy (SBRT) is a treatment option for locally advanced pancreatic cancer. Current practice is to target the gross tumor volume (GTV) only and omit elective nodal irradiation, which may lead to increased regional failures. In this study, we review the patterns of regional failure of our patients treated with SBRT. We retrospectively reviewed all patients diagnosed with pancreatic cancer treated with SBRT to the GTV only from 2003 to 2019 in our institution with regional nodal failure on at least one follow-up scan. Major blood vessels (celiac artery (CA), superior mesenteric artery (SMA), portal vein (PV), and aorta) were contoured, and the distance of the nodal failures measured. Regional nodal failure was defined as new lymphadenopathy within 1.5 cm of CA, SMA, and PV, and within 2cm of aorta in accordance with RTOG consensus contouring guidelines. Follow-up scans were fused with the treatment planning scan, and doses to the regional failures were computed. They were then drawn to one template CT scan for mapping. Patients were excluded for regional mapping if they had incomplete SBRT plan data or follow-up scan images. Out of 355 patients diagnosed with pancreas cancer treated with SBRT in our institution, 23 patients (6.5%) had identified regional nodal failure in their follow-up. Thirteen (57%) patients had locally advanced disease treated definitively, 4 (17%) were neoadjuvant prior to Whipple, 3 (13%) were treated palliatively and were metastatic at presentation, and 3 (13%) were recurrence after Whipple or conventional radiotherapy. The median prescribed dose to the planning treatment volume (PTV) was 33 Gy (range, 24 – 40 Gy) in 5 (range, 1-5) fractions. The mean PTV dose delivered BED 10 is 66.63 Gy (range, 27.01-121.19Gy) There were 0 isolated regional failures; 17 (74%) regional failures occurred with simultaneous distant failure, 2 (9%) had distant recurrence before they had locoregional failure, 3 (13%) had simultaneous local progression, and 1 (4%) had local progression before regional failure. We identified 30 nodal failures from 23 patients. Nineteen (63%) nodal failures occurred within the para-aortic nodes, followed by 4 SMA nodal failures (17%), 5 CA failures (22%), and 2 PV failures (9%). The mean distance from CA, SMA, aorta, and PV were 0.72, 1.12, 0.80, and 0.58 cm respectively. The median mean, min, and max doses received by the regional nodes were 2.12 Gy (range, 0.07-20.77Gy), 0.87 Gy (range, 0-17.51Gy), and 4.02 Gy (range, 0.07-24.2 Gy) respectively. Regional failures with pancreatic SBRT were overall rare with a crude rate of 6.5%, none of which occurred in isolation as the first site of failure. The most common sites of regional failure were the para-aortic nodes followed by the CA nodes and then SMA nodes. Although this data supports the continued exclusion of elective nodal regions with pancreatic SBRT, the risk of nodal failures may increase with improvements in systemic therapy.