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

Abstract

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 18F-FDG PET/MR to MR and CT for target volume delineation for RT planning. We retrospectively included 22 patients with newly diagnosed unresectable primary CC who underwent 18F-FDG PET/MR for initial staging. Gross tumor volume (GTV) of the primary mass (GTVM) and lymph nodes (GTVLN) 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. GTV MPET/MR (median: 94ml, range 16-655ml) was significantly greater than GTV MMR (69ml, 11-635ml) (p = 0.0001) and GTV MCT (96ml, 4-564ml) (p = 0.035). There was no significant difference between GTV MCT and GTV MMR (p = 0.078). Subgroup analysis of intrahepatic and extrahepatic tumors showed that the median GTV MPET/MR was significantly greater than GTV MMR in both groups (117.5ml, 22-655ml vs. 102.5ml, 22-635ml, p = 0.004 and 37ml, 16-303ml vs. 34ml, 11-207ml, p = 0.042, respectively). The GTV LNPET/MR (8.5ml, 1-27ml) was significantly higher than GTV LNCT (5ml, 4-16ml) (p = 0.026). GTVPET/MR had the highest similarity to the GTVMR, 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). 18F-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.