Session Item

Focal dose escalation to the gross tumor volume (GTV) in definitive radiotherapy (RT) of prostate cancer (PCa) offers the potential to further improve oncologic outcomes and is currently under investigation. Multiparametric MRI (mpMRI) is routinely used for GTV delineation but PSMA-PET/CT is emerging as a diagnostic tool to give complementary information and thus improve detection of intraprostatic tumor lesions. This study aims to intraindividually compare PSMA-PET/CT and mpMRI for delineation of GTV. Additionally, a subgroup is compared to whole mount histopathology as a reference standard.

The data of 94 patients with primary PCa who underwent ⁶⁸Ga-PSMA-11-PET/CT and mpMRI before treatment were retrospectively examined. GTVs were manually delineated in the mpMRI (consensus contour by experts, GTV-MRI) and PET/CT (scaling SUVmin-max: 0-5, GTV-PET). The study endpoints were laterality (left/right/both lobes), cT stage and tumor volume according to the respective GTVs. Additionally, clinical parameters (age, initial PSA and Gleason score in the biopsy) were determined. Binary regression was performed to detect a putative impact of clinical parameters on the concordance between the GTV volumes. Finally, the GTVs of 17 patients were compared to histopathology of the resected specimen. Image validation was conducted on a slice by slice level in quadrants based on the distribution of PCa in histopathology.

Median volume was significantly smaller in mpMRI (2.8 ml, range: 0.1- 67.7 ml) than in PSMA-PET (4.7 ml, range: 0.5 – 139.8 ml) (p < 0.0001). In 83% and 71% of patients GTVs had a difference of >25% and 50%. Laterality was different in 41% of the patients and PSMA-PET showed lesions in both lobes whereas mpMRI did not in 31% of the patients (p<0.05). Accordingly, most common cT-stage in mpMRI was cT2a (42.5%) and cT2c (53.2%) in PSMA-PET. Binary regression revealed that no clinical surrogate parameter had an impact on the concordance of the GTV volumes. The histopathologic subgroup analysis revealed a sensitivity and specificity of 86% and 87% for PSMA-PET, 58% and 94% for mpMRI and 91% and 84% for their GTV-union.

This study demonstrates in a large cohort of primary PCa patients that tumor volumes delineated in PSMA-PET/CT are significantly larger than in mpMRI. Additionally, PSMA-PET revealed lesions in both lobes in almost one third of the patients, whereas mpMIR did not.  Subgroup analysis demonstrates an increased consensus of PSMA-PET with histopathology and a higher sensitivity. These results yield that PSMA-PET/CT may give augmented information for intraprostatic tumor lesion detection and thus may be the favorable diagnostic modality for focal RT.