Predicting Outcomes of Indeterminate Bone Lesions on 18 F-DCFPyL PSMA PET/CT Scans in the Setting of High-Risk Primary or Recurrent Prostate Cancer.

Purpose : Indeterminate bone lesions (IBLs) on prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography (PET/CT) are common. This study aims to define variables that predict whether such lesions are likely malignant or benign using features on PSMA PET/CT. Methods : 18 F-DCFPyL PET/CT imaging was performed in 243 consecutive patients with high-risk primary or biochemically recurrent prostate cancer. IBLs identified on PSMA PET/CT could not definitively be interpreted as benign or malignant. Medical records of patients with IBLs were reviewed to determine the ultimate status of each lesion. IBLs were deemed malignant or benign based on evidence of progression or stability at follow-up, respectively, or by biopsy result; IBLs were assigned equivocal when insufficient or unclear evidence existed. Post-hoc patient, lesion, and scan variables accounting for clustered data were evaluated using Wilcoxon rank-sum and Chi-square tests to determine features that favored benign or malignant interpretation. Results : Overall, 98 IBLs within 267 bone lesions (36.7%) were identified in 48/243 patients (19.8%). Based on review, 37/98 IBLs were deemed benign and 42/98 IBLs were malignant of which 8/98 had histological verification; 19/98 IBLs remained equivocal. Location and maximum standard uptake value (SUV max ) categorical variables were predictive of IBL interpretation ( P =0.0201 and P =0.0230, respectively). For IBLs with new interpretations, 34/37 (91.9%) considered benign showed SUV max <5 or exhibited focal uptake without coexisting bone metastases; 37/42 (88.1%) deemed malignant demonstrated SUV max ≥5 or were present with coexisting bone metastases. Univariable and multivariable logistic regression predicted IBLs with high SUV max (OR=9.29, P =0.0016; OR=13.87, P =0.0089) or present with other bone metastases (OR=9.87, P =0.0112; OR=11.35, P =0.003) to be malignant. Conclusions : IBLs on PSMA PET/CT are concerning; however, characterizing their location, SUV and additional scan findings can aid interpretation. IBLs displaying SUV max ≥5 or present with other bone metastases favor malignancy. IBLs without accompanying bone metastases that exhibit SUV max <5 and are observed only in atypical locations favor benign processes. These guidelines may assist in the interpretation of IBLs on PSMA PET/CT.