A 62-year-old man with a history of prostate cancer presents with localized right hip pain and elevated alkaline phosphatase. A technetium-99m methylene diphosphonate (MDP) bone scan is performed. The scan shows focal increased uptake in the right femoral head with normal surrounding bone. Which of the following is the most likely explanation for this finding?

Explanation

## Analysis of Technetium-99m MDP Bone Scan Finding ### Understanding Tc-99m MDP Mechanism **Key Point:** Technetium-99m methylene diphosphonate (MDP) is a bone-seeking radiopharmaceutical that binds to hydroxyapatite in bone. Increased uptake reflects osteoblastic activity and increased blood flow to bone, not necessarily bone destruction. ### Clinical Context The patient has: - History of prostate cancer (high propensity for bone metastases) - Localized hip pain - Elevated alkaline phosphatase (marker of osteoblastic activity) - Focal increased uptake in femoral head on bone scan **High-Yield:** Prostate cancer commonly metastasizes to bone, particularly the axial skeleton and proximal femur. These metastases are typically **osteoblastic** (sclerotic), not osteolytic, and produce increased alkaline phosphatase. ### Why This is Metastatic Bone Disease The combination of: 1. **Risk factor:** Known prostate cancer 2. **Biochemical marker:** Elevated alkaline phosphatase indicates new bone formation 3. **Imaging finding:** Focal increased uptake = increased osteoblastic activity 4. **Clinical presentation:** Localized pain at site of uptake ...all point to **osteoblastic metastatic disease**. The bone scan is highly sensitive for detecting areas of increased bone metabolism, making it excellent for identifying skeletal metastases. ### Differential Consideration | Diagnosis | Bone Scan Pattern | Alkaline Phosphatase | Clinical Clue | |-----------|------------------|---------------------|---------------| | **Metastatic (osteoblastic)** | Focal increased uptake | Elevated | Known malignancy, pain at site | | Avascular necrosis | Photopenia (cold defect) early; increased uptake late | Normal | History of trauma, steroids, or sickle cell | | Osteoarthritis | Mild diffuse uptake in joint | Normal | Degenerative pattern, not focal | | Paget's disease | Intense, diffuse uptake in affected bone | Markedly elevated | Pathognomonic radiograph, polyostotic | **Clinical Pearl:** In a cancer patient with a focal hot spot on bone scan and elevated alkaline phosphatase, metastatic osteoblastic disease is the leading diagnosis until proven otherwise. ### Citation [cite:Harrison 21e Ch 397]