Incidental Intense Fibroblast Activation Protein Inhibitor (FAPI) Uptake in Bilateral Gluteal Myositis Ossificans: A Case Report

Abstract

Positron emission tomography/computed tomography (PET/CT) using 18F-fluorodeoxyglucose ([18F]-FDG) is a widely adopted imaging modality for detecting hypermetabolic lesions. However, emerging positron-emitting tracers, such as radiopharmaceuticals featuring fibroblast activation protein (FAP) inhibitors (FAPI) labeled with [18F] or [68Ga], have opened new avenues in nuclear medicine. This case report focuses on the unique behavior of [68Ga]-FAPI in bilateral gluteal myositis ossificans, an infrequent condition characterized by soft tissue ossification. A 45-year-old woman with gastric adenocarcinoma underwent subtotal gastrectomy and received neoadjuvant and adjuvant chemotherapy; [68Ga]-FAPI PET revealed metastatic processes and unexpected [68Ga]-FAPI avid intramuscular ossifications in the pelvic and bilateral thigh muscles. Even though there was no history of trauma, the patient was diagnosed with myositis ossificans, a condition marked by non-cancerous ectopic ossifications. Diagnosis relies on history, radiology, and/or histology. FAPI imaging, increasingly used for inflammatory and infectious diseases, can exhibit uptake in benign conditions, including those involving bones and joints. This case report is the first to document incidental bilateral [68Ga]-FAPI uptake in bilateral gluteal myositis ossificans. The robust [68Ga]-FAPI activity in myositis ossificans highlights the importance of considering myositis ossificans in the context of soft tissue calcifications with intense [68Ga]-FAPI uptake.

Keywords: fapi; incidental; myositis ossificans; non-neoplastic; pet/ct.

Figure 1. MIP images: (A) baseline [18F]-FDG PET/CT scan and (B) follow-up [68Ga]-FAPI PET/CT image

MIP: maximum intensity projection

Figure 2. [68Ga]-FAPI PET/CT images

Follow-up [68Ga]-FAPI PET/CT images. (A) CT soft tissue window axial and left sagittal views. (B) Fused PET/CT axial and left sagittal views. (C) CT bone window axial and left sagittal views reveal extensive soft tissue ossification surrounding the bilateral hip joints, mainly involving the gluteus minimus and medius muscles. These ossifications exhibit a distinctive pattern, characterized by peripheral mature lamellar bone and a central region consisting of immature bone, demonstrating intense [68Ga]-FAPI activity with a maximum standardized uptake value (SUVmax=18.3) within the left gluteal area

Figure 3. Radiographs of both hip joints

Anteroposterior (A), right lateral (B), and left lateral (C) radiographs of both hip joints illustrate the presence of multiple mature heterotopic ossifications encircling both hip joints. These ossifications extend superiorly, in close proximity to the inferior aspect of the ilium, and inferiorly, just below the level of the lesser trochanter