Inter-fractional Variation of Linear Energy Transfer Profiles During Carbon-ion Radiotherapy for Pancreatic Cancer

In carbon-ion radiotherapy (CIRT), a low dose-averaged linear energy transfer (LET_d) within the tumor has been suggested as a potential unfavorable prognostic factor for local recurrence. However, inter-fractional variations in LET_d during daily CIRT sessions remain unelucidated. This study aimed to investigate inter-fractional variations in LET_d profiles during CIRT for pancreatic cancer.

Seven patients with locally advanced pancreatic cancer treated with CIRT were retrospectively analyzed. Treatment plans delivering 55.2 Gy (relative biological effectiveness) in 12 fractions were generated based on computed tomography (CT) images (Plan-CT). For each CIRT session, in-room CT images were acquired immediately after irradiation and referred to as Daily-CT; all 12 series of images were deformably registered to the Plan-CT. LET_d parameters (L_98%, L_95%, L_50%, and L_2%) for target volumes were compared between Plan-CT and Daily-CT (L_X% was defined as the minimum LET_d delivered to X% of the target volume).

The L_50% and L_2% values for the planning target volumes were significantly lower on Daily-CT than on Plan-CT, although the differences were modest. Meanwhile, no significant differences were observed for L_98% and L_95%. One patient exhibited a consistent decrease in LET_d on Daily-CT across all LET_d parameters and target volumes examined. For this patient, increased bowel gas within the beam path was observed during daily sessions, which caused a distal shift of the LET_d distribution, which then reduced LET_d coverage of the target volumes.

During CIRT for pancreatic cancer, inter-fractional variations in LET_d profiles may be more pronounced at larger target volumes and at lower L_X% values. Although the variations were modest in most cases, inter-fractional changes in bowel gas within the beam path are a potential risk factor for reduced LET_d coverage.