Cholangiocarcinoma size on magnetic resonance imaging versus pathologic specimen: Implications for radiation treatment planning

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 ¹⁸F-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 ¹⁸F-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

GTV _M ^PET/MR (median: 94 ml, range 16-655 ml) was significantly greater than GTV _M ^MR (69 ml, 11-635 ml) (p = 0.0001) and GTV _M ^CT (96 ml, 4-564 ml) (p = 0.035). There was no significant difference between GTV _M ^CT and GTV _M ^MR (p = 0.078). Subgroup analysis of intrahepatic and extrahepatic tumors showed that the median GTV _M ^PET/MR was significantly greater than GTV _M ^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 GTV _LN ^PET/MR (8.5 ml, 1-27 ml) was significantly higher than GTV _LN ^CT (5 ml, 4-16 ml) (p = 0.026). GTV^PET/MR had the highest similarity to the GTV^MR, i.e., DSC^PET/MR-MR (0.82, 0.25-1.00), compared to DSC ^PET/MR-CT of 0.58 (0.22-0.87) and DSC^MR-CT of 0.58 (0.03-0.83).

CONCLUSION

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.