Application of intrahepatic fiducial markers in carbon ion radiotherapy for pancreatic cancer-Achieving higher precision treatment.

[BACKGROUND] Carbon-ion radiotherapy (CIRT) delivers highly conformal tumor doses while sparing surrounding organs-at-risk (OAR). However, density variations along the beam path and interfractional target motion can reduce target coverage and expose OAR to high doses. These motions are typically corrected using target matching (TM) or marker matching (MM). Because the pancreas is close to the gastrointestinal tract and major vessels, fiducial marker implantation carries a risk of organ or vascular injury, and metal artifacts from markers can affect beam range calculations. TM with computed tomography (CT) also reduces treatment efficiency and prevents real-time motion monitoring.

[PURPOSE] To improve pancreatic cancer CIRT by correcting interfractional target motion using fiducial markers implanted in hepatic segments 5 or 6.

[METHODS] Twenty patients with implanted fiducial markers who underwent liver cancer CIRT at our hospital were selected. The effect of interfractional target motion correction by MM in pancreatic cancer CIRT was evaluated as simulation study. The initial planning CT and in-room CT images were used for bone matching (BM), liver-dome matching (LM), TM, and MM. Correlations between target motion and fiducial marker or liver-dome motion were analyzed, and fractional dose distributions were calculated. Variations in clinical target volume coverage (ΔCTV V95%) and OAR doses were examined across strategies per fraction.

[RESULTS] A strong correlation was observed between the motion of the target and the fiducial marker. The median ΔCTV V95% values were -4.68%, -3.97%, -1.07%, and -1.69% for BM, LM, TM, and MM, respectively. TM and MM significantly improved target coverage compared to BM and LM. OAR doses stayed within dose constraints for all methods.

[CONCLUSION] The proposed MM approach corrected interfractional target motion, achieving target coverage comparable to TM. In pancreatic cancer CIRT, managing gastrointestinal gas and body contour changes along with tumor motion may further optimize target coverage and improve clinical outcomes.