The Potential Role of 99mTc-FAPI SPECT/CT in the Staging of Mucinous Appendiceal Adenocarcinoma

We present a 55‑year‑old woman with adnexal masses and elevated CEA. Malignancy workup, adnexal biopsy, and intraoperative assessment revealed the appendix as the primary site, with pathology confirming mucinous adenocarcinoma. While mucinous tumors often show limited avidity on 18F‑FDG PET/CT due to low cellularity, this case demonstrates the utility of fibroblast activation protein inhibitor (FAPI) imaging. 99mTc‑FAPI SPECT/CT successfully identified widespread skeletal metastases, initially detected on bone scan, and extensive soft‑tissue involvement in the peritoneum, appendiceal region, and pleura, highlighting its potential as a superior staging modality for mucinous neoplasms and suggesting the feasibility of FAPI‑targeted theranostics in advanced disease.