Background: 99mTc-Methylene diphosphonate (MDP) bone scan of a patient with known breast carcinoma showed a solitary osteoblastic lesion in the femoral head. Solitary bone metastatic lesion and a benign cause-like remodeling phase of avascular necrosis (AVN) were the top most differential diagnoses and therefore can easily be misinterpreted for each other. Magnetic resonance imaging (MRI) is considered as the gold standard for diagnosis of AVN and can help in differentiation between AVN and skeletal metastasis.
Case Presentation: A 40-year-old female patient of right breast carcinoma, treated with mastectomy, chemotherapy, and radiotherapy underwent 99mTc-MDP bone scintigraphy for pain in left hip with restricted movements. The bone scan showed focal uptake in the head of left femur with suspicion of AVN, which turned out to be a metastatic lesion on MRI. Later on, longitudinal follow-up of bone scan after 1 year showed a lesion in the left femoral head extending up to the trochanteric region with additional new lesions confirming skeletal metastasis.
Conclusion: Solitary skeletal metastatic lesion in the head of femur is a rare finding, although in the presence of local symptoms AVN always remains at the top of the list for differential diagnosis of solitary osteoblastic focus in the femoral head. Bone scan is a primary tool for skeletal metastatic survey due to high sensitivity but due to low pathognomonic specificity, additional imaging with MRI must be considered as a safe, non-invasive, and easily available option for further characterization of solitary lesions.
Key words: 99mTc-MDP bone scan, AVN, bone metastasis, MRI, case report
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