Clinical relevance of post-therapy SPECT/CT in prostate cancer patients treated with Lutetium Lu 177 vipivotide tetraxetan

Introduction: Lu-177 PSMA-617 (Lutetium Lu 177 vipivotide tetraxetan) is approved for the treatment of metastasized castrate-resistant prostate cancer patients progressing after at least one line of novel axis androgen deprivation therapy and one of the taxane-based therapies. PCWG3 criteria is currently used for the follow-up of these patients, which may be misleading in some patients. This retrospective study looked at the clinical relevance of SPECT/CT performed following Lutetium Lu 177 vipivotide tetraxetan which are performed as part of our standard of care.

Methods: After IRB approval, charts of patients treated with Lutetium Lu 177 vipivotide tetraxetan and referred to our Theranostic Tumor Board (TTB) were retrospectively evaluated. SPECT/CT images (noncontrast-enhanced low dose CT) were acquired from vertex to mid-thigh at an early time point (same day) or delayed (next day or later) based on clinical logistical considerations. SPECT/CT results were categorized by a single experienced nuclear medicine physician without use of laboratory data in the following groups: group 1: favorable response - decrease in intensity of expression and/ or number of the lesions, group 2: progressive disease - patients with new and/or progressive lesions on SPECT and/ or CT component of the SPECT/CT, group 3: neither PD nor favorable response including mixed response, and group 4: other clinically relevant non tumor related findings

Results: Out of the 68 patients recommended for Lutetium Lu 177 vipivotide tetraxetan therapy, 22 patients did not get any SPECT/CT at the time of analyses of the data mainly due to insurance or logistical issues (5 were treated in a clinical trial). Forty-six patients underwent SPECT/CT either on the same day (n=8; 17.4%) or next day or later (n=38; 82.6%). The mean age of these patients was 69 ± 9 years. Patients received a median of 3 cycles (range 1-6). A total of 131 SPECT/CTs were performed. Results of the SPECT/CT showed favorable response in 14 (30%) of patients whereas 21 (45.6%) belonged to group 3. Progressive disease was observed in 8 (17.4%) of patients. We observed hydronephrosis, new fracture, lesion suitable for biopsy and pleural effusion in one patient each (group 4).

Conclusions: Post-Lutetium Lu 177 vipivotide tetraxetan SPECT/CT scans appear to provide clinically meaningful information some of which cannot be determined by serum PSA alone and should strongly be considered for performance in all patients after every cycle.