Safe Proton Beam Re-irradiation With a Bioabsorbable Spacer in Pancreatic Cancer: A Case Report

Proton beam therapy (PBT) re-irradiation for pancreatic cancer is challenging because of accumulated dose limits in surrounding gastrointestinal (GI) organs. Bioabsorbable polyglycolic acid (PGA) spacers can reduce radiation exposure to adjacent organs. This report aims to detail the safe execution of PBT re-irradiation for recurrent pancreatic body cancer adjacent to the stomach using a PGA spacer, and physically verify the spacer's proton beam blocking effectiveness via auto-activation positron emission tomography (AAPET). A 77-year-old female with pancreatic body cancer experienced local residual disease 6 months after an initial PBT course (67.5 Gy (RBE)/25 fractions). To enable a second PBT, a 1-cm-thick PGA spacer was successfully placed between the stomach and the pancreas. The second PBT delivered 67.5 Gy (RBE) in 25 fractions using two posterior beams to avoid gastric passage. AAPET-CT was performed three times during the second PBT. Quantitative analysis compared the average Becquerel (Bq)/ml values in the planning target volume (PTV), the stomach (ventral to the spacer), and abdominal regions outside the field. Maximum doses to the stomach and duodenum were limited to 16.1 Gy (RBE) and 16.7 Gy (RBE), respectively. Quantitative AAPET analysis showed the average Bq/ml value in the PTV was significantly higher than in other regions. This visually and quantitatively confirmed the spacer's effectiveness in stopping the proton beams. Three months after the second PBT, the irradiated site was controlled and no adverse events related to PBT or PGA spacers were observed. In a case of recurrent pancreatic body cancer adjacent to the stomach, safe proton beam re-irradiation was successfully performed by placing a PGA spacer. PGA spacer placement may represent an effective strategy for safely achieving PBT re-irradiation for recurrent pancreatic cancer near the GI tract.