Impact of 18F-FDG PET/MR based tumor delineation in radiotherapy planning for cholangiocarcinoma

Purpose Radiation therapy (RT) is an effective treatment for unresectable cholangiocarcinoma (CC). Accurate tumor volume delineation is critical in achieving high rates of local control while minimizing treatment-related toxicity. This study compares F-18-FDG PET/MR to MR and CT for target volume delineation for RT planning. Methods We retrospectively included 22 patients with newly diagnosed unresectable primary CC who underwent F-18-FDG PET/MR for initial staging. Gross tumor volume (GTV) of the primary mass (GTV(M)) and lymph nodes (GTV(LN)) were contoured on CT images, MR images, and PET/MR fused images and compared among modalities. The dice similarity coefficient (DSC) was calculated to assess spatial coverage between different modalities. Results GTVM(PET/MR) (median: 94 ml, range 16-655 ml) was significantly greater than GTVM(MR) (69 ml, 11-635 ml) (p = 0.0001) and GTVM(CT) (96 ml, 4-564 ml) (p = 0.035). There was no significant difference between GTVM(CT) and GTVM(MR) (p = 0.078). Subgroup analysis of intrahepatic and extrahepatic tumors showed that the median GTVM(PET/MR) was significantly greater than GTVM(MR) in both groups (117.5 ml, 22-655 ml vs. 102.5 ml, 22-635 ml, p = 0.004 and 37 ml, 16-303 ml vs. 34 ml, 11-207 ml, p = 0.042, respectively). The GTVLN(PET/MR) (8.5 ml, 1-27 ml) was significantly higher than GTVLN(CT) (5 ml, 4-16 ml) (p = 0.026). GTV(PET/MR) had the highest similarity to the GTV(MR), i.e., DSCPET/MR-MR (0.82, 0.25-1.00), compared to DSC (PET/MR-CT) of 0.58 (0.22-0.87) and DSCMR-CT of 0.58 (0.03-0.83). Conclusion F-18-FDG PET/MR-based CC delineation yields greater GTVs and detected a higher number of positive lymph nodes compared to CT or MR, potentially improving RT planning by reducing the risk of geographic misses.

Automated summary

This study compared the application of F-18-FDG PET/MR to MR and CT for target volume delineation in radiation therapy planning for cholangiocarcinoma. PET/MR showed greater accuracy in tumor volume delineation and detected more positive lymph nodes, potentially reducing the risk of geographic misses in RT planning.