We present a case of a 71-year-old male patient diagnosed with myoepithelial carcinoma of the pelvis and perineum, who underwent fibroblast activation protein (FAP)-directed radioligand therapy and presented long lasting tumor retention as detected by late whole-body scans.

The patient had been previously submitted to neoadjuvant radiation therapy and surgery, with tumoral relapse at the surgical margins in the gluteal region. Lesions displayed a slow and progressive sarcomatoid growth pattern, with sacral osteolytic involvement, accompanied with obturatory and left common iliac lymph node metastases. Immunotherapy was initiated but discontinued due to grade IV diarrhea. With progressive disease evident on FDG PET/CT and no other viable chemotherapy indicated, FAPI radioligand therapy was proposed, given significant tracer uptake observed in FAP-directed PET/CT. The patient received an intravenous injection of 200 mCi of [177Lu]Lu-FAP-RTX, which was well tolerated, with no acute side effects reported. Blood tests remained within normal range. Beyond partial hair loss,no other adverse events were observed. Imaging studies including whole-body planar and SPECT/CT scans revealed intense tracer retention in the sacral mass and mild to moderate retention in the lymph node metastases up to 15 days post-treatment. Notably, indicative of a response to FAP-directed radioligand therapy, there was a resolution of drainage from a fistula in the intergluteal region. Follow-up imaging is still pending at the moment.

This case is the first report on favorably sustained tumor retention of the radiopharmaceutical in a carcinoma patient undergoing FAP-directed radioligand therapy. With tumor response assessment still pending, longer follow-up and detailed observation is still necessary for a better understanding of potential benefits and side effects of FAP-directed radioligand therapy, especially in patients undergoing subsequent treatment cycles.