Gross tumor volume delineation in primary prostate cancer on ¹⁸F-PSMA-1007 PET/MRI and ⁶⁸Ga-PSMA-11 PET/MRI

We aimed to assess the clinical value of ¹⁸F-PSMA-1007 and ⁶⁸Ga-PSMA-11 PET/MRI in the gross tumor volume (GTV) delineation of radiotherapy for prostate cancer (PCa).

Sixty-nine patients were retrospectively enrolled (57 in the ¹⁸F subgroup and 12 in the ⁶⁸Ga subgroup). Three physicians delineated the GTV and tumor length by the visual method and threshold method with thresholds of 30%, 40%, 50%, and 60% SUVmax. The volume correlation and differences in GTVs were assessed. The dice similarity coefficient (DSC) was applied to estimate the spatial overlap between GTVs. For 51 patients undergoing radical prostatectomy, the tumor length (Lpath) of the maximum area was measured, and compared with the longest tumor length obtained based on the images (L_MRI, L_PET/MRI, L_PET, L_PET30%, L_PET40%, L_PET50%, L_PET60%) to determine the best delineation method.

In the ¹⁸F subgroup, (1) GTV-PET/MRI (p < 0.001) was significantly different from the reference GTV-MRI. DSC between them was > 0.7. (2) GTV-MRI (R² = 0.462, p < 0.05) was the influencing factor of DSC. In the ⁶⁸Ga subgroup, (1) GTV-PET/MRI (p < 0.05) was significantly different from the reference GTV-MRI. DSC between them was > 0.7. (2) There was a significant correlation between GTV-MRI (r = 0.580, p < 0.05) and DSC. The longest tumor length measured by PET/MRI was in good agreement with that measured by histopathological analysis in both subgroups.

It is feasible to visually delineate GTV on PSMA PET/MRI in PCa radiotherapy, and we emphasize the utility of PET/MRI fusion images in GTV delineation. In addition, the overlap degree was the highest between GTV-MRI and GTV-PET/MRI, and it increased with increasing volume.