A 68-year-old man with biopsy-proven prostate cancer (Gleason 8, PSA 45 ng/mL) is being staged for treatment planning. Which investigation is most appropriate to assess for distant skeletal metastases?

Explanation

## Staging of High-Risk Prostate Cancer: Detection of Skeletal Metastases ### Investigation of Choice for Bone Metastases **Key Point:** Whole-body bone scan with technetium-99m (Tc-99m MDP or HDP) is the standard investigation for detecting skeletal metastases in high-risk prostate cancer. ### Indications for Bone Imaging in Prostate Cancer **High-Yield:** Bone scan is indicated when: - PSA > 20 ng/mL - Gleason score ≥ 8 - Clinical T3–T4 disease - Symptomatic bone pain In this case: PSA 45 ng/mL + Gleason 8 = **definite indication** ### Why Tc-99m Bone Scan? 1. **Mechanism:** Osteoblastic activity (bone remodeling) uptake — detects areas of increased bone turnover 2. **Sensitivity:** 70–80% for detecting metastases 3. **Whole-body coverage:** Screens entire skeleton in one study 4. **Cost-effective:** Relatively inexpensive and widely available 5. **Established standard:** Decades of clinical validation in prostate cancer staging ### Comparison of Bone Imaging Modalities | Modality | Sensitivity | Specificity | Advantages | Disadvantages | |---|---|---|---|---| | **Tc-99m bone scan** | 70–80% | 60–70% | Whole-body, standard, cost-effective | Lower specificity; cannot differentiate benign from malignant lesions | | **FDG-PET-CT** | 85–90% | 85–90% | Higher sensitivity/specificity; anatomic correlation | Expensive; limited availability; not standard for initial staging | | **Plain radiographs** | 30–40% | High | Cheap, available | Poor sensitivity; only detects sclerotic lesions; not screening tool | | **MRI spine** | High | High | Excellent for spinal metastases; no radiation | Limited field; expensive; time-consuming; not whole-body | ### Clinical Pearl **Flare phenomenon:** Patients with metastatic disease treated with androgen-deprivation therapy may show increased bone scan uptake 2–3 months post-treatment due to healing response — this does NOT indicate progression. ### Role of PET-CT FDG-PET-CT is increasingly used for: - Detecting soft-tissue and lymph node metastases - Equivocal bone scan findings - High-risk patients - But NOT yet standard for routine initial staging (cost, availability) **Mnemonic: BONE SCAN INDICATIONS = PSA > 20 or Gleason ≥ 8 or T3/T4 or Symptoms**