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

Area of Science:

• Oncology
• Radiation Oncology
• Medical Physics

Background:

• Low dose-averaged linear energy transfer (LETd) in carbon-ion radiotherapy (CIRT) may correlate with unfavorable prognosis for local recurrence in pancreatic cancer.
• Inter-fractional variations in LETd during daily CIRT sessions are not well understood.
• This study investigates inter-fractional LETd profile variations in pancreatic cancer CIRT.

Purpose of the Study:

• To evaluate inter-fractional variations in dose-averaged linear energy transfer (LETd) profiles during carbon-ion radiotherapy (CIRT) for pancreatic cancer.
• To assess the impact of daily anatomical changes on LETd coverage within target volumes.

Main Methods:

• Retrospective analysis of 7 locally advanced pancreatic cancer patients treated with CIRT (55.2 Gy RBE in 12 fractions).
• Comparison of LETd parameters (L98%, L95%, L50%, L2%) between treatment planning CT (Plan-CT) and daily in-room CT (Daily-CT) images.
• Deformable image registration was used to align Daily-CT images to Plan-CT for LETd analysis.

Main Results:

• Significantly lower L50% and L2% values were observed on Daily-CT compared to Plan-CT, indicating modest reductions in LETd coverage.
• No significant differences were found for L98% and L95%, suggesting minimal impact on the lowest dose-covered regions.
• One patient showed consistent LETd reduction due to increased bowel gas, causing a distal shift in LETd distribution and reduced target coverage.

Conclusions:

• Inter-fractional LETd variations in pancreatic cancer CIRT can be more pronounced for larger target volumes and lower L-X% values.
• While generally modest, variations highlight the potential impact of inter-fractional changes, such as bowel gas, on LETd coverage.
• Bowel gas variations represent a potential risk factor for compromised LETd delivery in CIRT for pancreatic cancer.