Solitary rib lesions showing PSMA uptake in pre‐treatment staging 68Ga‐PSMA‐11 PET scans for men with prostate cancer: benign or malignant?

OBJECTIVES To determine the proportion of solitary rib lesions on pre-treatment 68 Gallium-labelled prostate-specific membrane antigen (PSMA)/CT scans in men with prostate cancer that are malignant and examine any predictive factors. PATIENTS AND METHODS This retrospective single tertiary referral institution cohort study of men reviewed the results of 68 Ga-PSMA-11 PET/CT scans performed for primary staging prior to treatment of prostate cancer from July 2014 to September 2019. Men with PSMA uptake outside the prostate in only the rib lesion were included. A solitary rib lesion was considered to be malignant if it increased in size on follow-up imaging. A lesion was considered benign if the PSA remained <0.1 mcg/litre following a radical prostatectomy, < 2 mcg/L above nadir following radiation therapy as per the Phoenix criteria, histology was benign on rib biopsy or follow-up imaging showed no growth of the rib lesion. If a lesion did not meet these criteria it was considered indeterminate. RESULTS Sixty-two men had PSMA uptake in a solitary rib lesion. Fifty-four men went on to have radical prostatectomies and eight underwent radiotherapy. Sixty-one (98.4%) met the criteria for a benign rib lesion. Only one man had a false negative malignant lesion. This man had a rib lesion with a low SUVmax of 2.21 reported as benign, but post-operative PSA was 0.67 mcg/L and the rib lesion progressed on follow-up imaging with development of widespread metastases. Of the benign rib lesions, there were four false positives reported as possible metastases. Three had percutaneous rib biopsies, two of which came back with benign histology and one was indeterminate. The indeterminate biopsy patient had a radical prostatectomy and his post-op PSA was <0.1 mcg/L. Forty-three (69.4%) men with benign rib lesions had a SUVmax greater than the SUVmax of the malignant lesion. CONCLUSION To our knowledge, this is the first cohort study of men with PSMA avid solitary rib lesions on pre-treatment 68 Ga-PSMA PET/CT staging scans for prostate cancer. Our results indicate that the vast majority of these lesions have low intensity uptake and are benign. Intervention to confirm this is not usually required.