FAPI-negative Papillary Renal Cell Carcinoma With Markedly FAPI-avid Sacral Insufficiency Fracture

Area of Science:

• Nuclear medicine
• Oncologic imaging
• Radiopharmaceuticals

Background:

• Fibroblast activation protein inhibitor (FAPI) PET imaging is increasingly used in oncology.
• FAPI uptake can occur in various tumor types and non-malignant conditions.
• Distinguishing benign FAPI uptake from metastatic disease is crucial for patient management.

Purpose of the Study:

• To report a case where sacral FAPI uptake mimicked bone metastasis.
• To highlight fracture-related FAPI uptake as a potential diagnostic challenge.
• To emphasize the importance of integrating imaging findings with clinical context.

Main Methods:

• A 77-year-old woman with papillary renal cell carcinoma underwent [68Ga]Ga-FAPI PET/CT.
• Magnetic resonance imaging (MRI) was performed for further evaluation of sacral uptake.
• A 2-year clinical follow-up was conducted.

Main Results:

• The primary renal tumor showed no significant [68Ga]Ga-FAPI uptake.
• Intense FAPI uptake was observed in the sacrum, mimicking bone metastasis.
• MRI and follow-up confirmed a sacral insufficiency fracture, not metastatic disease.

Conclusions:

• Sacral insufficiency fractures can exhibit intense FAPI uptake, mimicking bone metastases.
• This finding represents an important imaging pitfall in oncologic FAPI PET/CT.
• Careful correlation with clinical information and other imaging modalities is essential for accurate diagnosis.