Interpret the Tc-99m methylene diphosphonate (MDP) bone scan with SPECT/CT (26 mCi injection, delayed whole-body and cervical spine images) in a patient with no known malignancy, which shows multiple intense foci in the left upper and mid ribs consistent with healing fractures, cervical and lumbar facet arthropathy, subchondral sclerosis of the C6 vertebral body, and a 9 mm lytic lesion in the left C6 facet that does not demonstrate tracer uptake.

Interpretation of Tc-99m MDP Bone Scan with SPECT/CT

This bone scan shows no evidence of osteoblastic metastatic disease, but the 9 mm lytic lesion at the left C6 facet that lacks tracer uptake requires urgent further evaluation with MRI and possible biopsy, as purely lytic lesions are poorly detected by bone scintigraphy and may represent metastatic disease, multiple myeloma, or plasmacytoma.

Key Findings Requiring Action

Critical Finding: C6 Lytic Lesion Without Tracer Uptake

• The absence of Tc-99m MDP uptake in a lytic lesion is highly concerning and does not exclude malignancy. Bone scintigraphy detects metastases through osteoblastic activity, making it relatively insensitive for purely osteolytic lesions 1.

• Proceed directly to MRI of the cervical spine without and with IV contrast as the next immediate step. MRI detects very early marrow changes before cortical bone destruction occurs and provides superior assessment of spinal cord compression and epidural extension 1, 2, 3.

• Consider CT-guided biopsy if imaging remains equivocal. In bone-only disease with few lesions or equivocal imaging, histological confirmation of metastatic disease is strongly recommended 1.

• Obtain serum and urine protein electrophoresis with immunofixation immediately. Multiple myeloma and solitary plasmacytoma characteristically present with lytic bone lesions, and the diagnostic pathway differs significantly from other malignancies 1, 3.

Benign Findings Adequately Explained

Left Rib Fractures (5-6 sites):

• The intense focal uptake in the posterior upper and mid left ribs correlating with healing fractures on low-dose CT is consistent with posttraumatic changes 1, 4, 5.
• Tc-99m MDP uptake peaks at day 7 post-injury when injury sites are occupied by mature osteoblasts, and uptake corresponds to osteoblast generation throughout the healing process 4.
• These findings do not require additional workup in the absence of known malignancy 1.

Cervical and Lumbar Degenerative Changes:

• Focal uptake at C3-4 left facet and C6 vertebral body subchondral sclerosis, along with L2-3 and L4-5 degenerative changes, represent typical degenerative arthropathy 1, 6.
• Tc-99m bone scan has a high false-positive rate secondary to benign processes with increased bone turnover, such as degenerative osteoarthrosis 1.
• Facet joint activity is the most common finding on SPECT/CT of the spine, reported in 50% of patients 6.

Limitations of Current Study

Bone Scan Inadequacy for Lytic Lesions:

• Tc-99m MDP bone scan has poor sensitivity (62-100%) and low specificity (48%) for detecting bone metastases, with the lowest sensitivity occurring in predominantly lytic lesions 2.
• Plain radiographs require 50-70% bone destruction before detecting osteolytic changes, so the low-dose CT component may underestimate the extent of disease 2, 3.

Missing Critical Information:

• The bone scan cannot assess epidural disease, spinal cord compression, or neural foramina involvement—critical complications that require urgent intervention 2, 3.
• Without prior imaging for comparison, it is impossible to determine if the C6 lytic lesion is new, stable, or progressive 1.

Recommended Diagnostic Algorithm

1. Obtain MRI cervical spine without and with IV contrast within 48-72 hours to evaluate the C6 lytic lesion for soft-tissue extension, spinal cord compression, marrow involvement, and epidural disease 1, 2, 3.

2. Order protein electrophoresis (serum and urine) with immunofixation immediately to evaluate for multiple myeloma or plasmacytoma, as these conditions present with lytic lesions that may not show tracer uptake 1, 3.

3. Consider FDG-PET/CT if MRI is nondiagnostic or to evaluate for additional lesions. PET/CT is superior to bone scintigraphy for detecting lytic metastases (sensitivity 89.7%, specificity 96.8%) and offers simultaneous assessment of skeletal and extraskeletal disease 1, 2, 3.

4. Perform CT-guided biopsy of the C6 lesion if imaging remains equivocal or shows concerning features. Biopsy provides definitive diagnosis and allows biomarker assessment to direct future therapies 1, 3.

5. Obtain prior imaging studies for comparison to determine if the C6 lesion is new or longstanding, which significantly impacts management 1.

Critical Pitfalls to Avoid

• Never rely on bone scan alone for diagnosis of lytic lesions. The low specificity mandates correlation with MRI or CT to characterize lesions 1, 2.

• Do not assume a "cold" lesion on bone scan is benign. Purely lytic metastases, multiple myeloma, and aggressive tumors may show no tracer uptake 1, 2.

• Do not delay MRI if neurological symptoms develop. MRI is mandatory to evaluate for spinal cord compression and epidural extension, which require urgent intervention 1, 2, 3.

• Do not use bone scan to monitor treatment response. The "flare phenomenon" from healing osteoblastic activity can misleadingly suggest disease progression 1, 2.

Incidental Findings Requiring Follow-up

• Large hypodense thyroid nodules (multinodular goiter): Correlate with thyroid function tests and consider ultrasound evaluation 7.
• Left maxillary sinus retention cyst or chronic sinusitis: Clinical correlation and ENT referral if symptomatic.
• Mild pleural thickening with subpleural opacity at left lung base: Follow-up chest imaging if persistent or symptomatic.