Abstract
1343
Background: 68Ga PSMA PET/CT is known to impact management in patients with prostate cancer. 18F PSMA 1007 is a relatively novel tracer which seems to have advantages over 68Ga PSMA in terms of reduced renal excretion, better tumour to back ground ratio, and longer half-life. As with any imaging modality PSMA targeted imaging has equivocal findings, posing as a diagnostic dilemma. The purpose of the study was to assess the frequency of equivocal lesions on 18F PSMA PET/CT and their significance, which could potentially impact patient management.
Methods: Retrospective analysis of 203 patients who had 18F PSMA PET/CT for either restaging (n=189) or initial staging (n=14) purposes over a 12-month period was undertaken. The scans were evaluated to assess for local disease, lymph node involvement and distant metastasis. The PSMA findings were classified as suspicious for metastasis, equivocal and benign lesion. Experience with PSMA ligand imaging, literature review of physiological pattern, pitfalls and PSMA uptake in non-prostatic cancers were considered while classifying the lesions. The scan results of suspicious and equivocal lesions were assessed through follow up/complementary imaging or clinical multidisciplinary discussion as found suitable for planning patient management.
Results: Median patient age was 64 (range 49-82), Gleason grade (range 6-10) and serum PSA range 0.04- 299 ng /ml. Total of 91 of 203 patients had equivocal lesions, of which 81/91 patients had a solitary lesion (skeletal/lymph nodal/ others) while 10/91 patients had more than one lesion. Skeletal lesions accounted for most frequent site of equivocal findings (61/91patients). The thoracic region (ribs, thoracic vertebrae, sternum and scapula; n=44/60) was the most frequent site for equivocal skeletal lesions, the remaining lesions were mostly found in the pelvic bones and lumbar vertebrae (11/60) and small minority in more than one region (6/60). Equivocal findings in the pelvic lymph nodes were seen in 27 patients (pelvic, retroperitoneal, inguinal), lung nodules in 11 patients and at other sites in 7 patients [prostate bed (2), hilar and mediastinal lymph nodes (2), supraclavicular lymph node (1) and spleen (1)]. 36/91 patients with equivocal lesions were followed up with further imaging (18 F PSMA PET/CT in 10, CT in 14, MRI in 11 and bone scan in 1). In 11/91 a management decision was made following multidisciplinary discussion. On follow up imaging of 36 patients, 18/28 equivocal skeletal lesion were classified as benign, 6/28 lesions still remained equivocal, while 4/28 lesions were confirmed as metastasis. Of the 10 patients with equivocal lymph nodes who had follow-up imaging, 7/10 were proven metastasis and 3/10 were confirmed as benign. 1 lung nodule was proven metastasis on surveillance imaging.
Conclusions: 18F PSMA PET-CT may show significant number of equivocal lesions; many of them in the skeleton, and minorities of them are ultimately proven metastatic. A cautious approach with structured reporting, good understanding of the lesion distribution pattern at typical and atypical sites may help in standardization of reports could assist in proper classification of the lesions. Addition of PSMA RADS grading may help better categorization of these lesions. A progressive assessment through interval imaging or multidisciplinary discussions is immensely helpful in making patient management and should be considered with equivocal lesions.