68Ga-PSMA-11 PET/CT derived quantitative volumetric tumor parameters for classification and evaluation of therapeutic response of bone metastases in prostate cancer patients

To evaluate the role of ⁶⁸Gallium prostate-specific membrane antigen-positron emission tomography/computed tomography (⁶⁸Ga-PSMA-11 PET/CT) derived quantitative volumetric tumor parameters in comparison with fully diagnostic conventional CT and serum-PSA levels for classification and evaluation of therapeutic response of bone metastases in patients with metastasized prostate cancer (PC).

A total of 177 men with biochemical recurrence of prostate cancer suffering from bone metastases underwent PET/CT with [⁶⁸Ga] Ga-PSMA-HBED-CC (⁶⁸Ga-PSMA-11). To calculate ⁶⁸Ga-PSMA-11 PET quantitative volumetric tumor parameters including whole-body total-lesion PSMA (TL-PSMA), whole-body PSMA-tumor volume (PSMA-TV), as well as the established maximum standard uptake values (SUVmax) and mean standard uptake values (SUVmean), all 443 ⁶⁸Ga-PSMA-11-positive bone lesions in the field of view were assessed quantitatively. Quantitative volumetric tumor parameters were correlated with CT-derived volume and bone density measurements of metastatic bone lesions, serum prostate-specific antigen (PSA) levels, and Gleason Scores. In the 20 patients suffering from bone metastases who underwent ⁶⁸Ga-PSMA-11 PET/CT before and after therapy, CT-derived volume and bone density measurements of metastatic lesions were compared to biochemical response determined by serum-PSA levels.

In 177 patients, a total of 443 ⁶⁸Ga-PSMA-11 PET-positive bone lesions were detected. Of these, 50 lesions (11%) were only detectable on PET but not on conventional CT. PET-positive/CT-negative bone metastases demonstrated a significantly lower PSMA uptake compared to PET-positive/CT-positive bone lesions (p < 0.05). SUVmax, SUVmean, PSMA-TV, and TL-PSMA of bone metastases were significantly higher (p < 0.05) in patients with Gleason Scores > 7 compared to those with Gleason Scores ≤ 7. In the linear regression analysis, an association was determined between SUVmean, Gleason Scores, lesion classification, and serum-PSA levels but not for CT-derived bone density measurements. No significant correlation could be found between changes of bone density and CT-derived volume measurements of metastatic bone lesions and changes of serum-PSA levels (p > 0.05) before and after therapy, while a highly significant correlation was observed for changes of PSMA-TV, TL-PSMA, and serum-PSA levels (p < 0.001).

Our results suggest that ⁶⁸Ga-PSMA-11 PET/CT might be a valuable tool for the detection and follow-up of bone metastases in patients with metastasized prostate cancer. ⁶⁸Ga-PSMA-11 PET-derived quantitative volumetric parameters demonstrated a highly significant correlation with changes of serum-PSA levels during the course of therapy. No such correlation could be determined for bone density measurements of metastatic bone lesions. Compared to the fully diagnostic CT scan, a significantly higher proportion of bone metastases was detected on ⁶⁸Ga-PSMA-11 PET.