Is the 3 cm lytic lesion in the right posterior‑fossa occipital skull of a 30‑year‑old woman with neck pain, dizziness, nausea, and visual changes most likely a benign cystic bone lesion (e.g., aneurysmal bone cyst) rather than metastatic disease or multiple myeloma, and what laboratory and imaging work‑up is appropriate?

Diagnostic Assessment of Posterior Fossa Lytic Lesion

This lesion is overwhelmingly likely to represent a benign cystic bone lesion, most probably an aneurysmal bone cyst, rather than metastatic disease or multiple myeloma, and the appropriate workup consists of basic laboratory screening followed by neurosurgical consultation for potential biopsy given her symptomatic presentation and critical anatomic location.

Clinical Context Supporting Benign Diagnosis

The clinical profile strongly argues against malignancy:

• Age and demographics: At 30 years old, she falls within the typical age range for aneurysmal bone cysts, with approximately 80% of patients presenting before age 20-30 years 1
• Absence of systemic cancer features: No weight loss, night sweats, fevers, or other constitutional symptoms that would suggest metastatic disease or lymphoproliferative disorder 2
• Stable lesion size: The lesion remained unchanged between CT and MRI over time, which is inconsistent with aggressive malignancy 2
• Anatomic location: While skull involvement by aneurysmal bone cysts is rare (approximately 12-30% occur in spine, with skull being exceptional), it does occur in young adults 3, 1, 4

Imaging Characteristics Consistent with Benign Process

The MRI findings support a benign cystic lesion:

• Expansile lytic appearance: The 3 cm × 1 cm × 3.7 cm lesion with no mass effect or edema is characteristic of slow-growing benign processes 3, 1
• Location in occipital bone: Aneurysmal bone cysts can involve the skull, particularly the occipital region, as documented in multiple case reports 3, 4, 5
• Absence of aggressive features: No cortical destruction, no soft tissue mass, and no vascular compromise argue strongly against malignancy 2

Appropriate Laboratory Workup

The recommended laboratory evaluation to exclude multiple myeloma is appropriate and sufficient 2:

• Complete blood count
• Comprehensive metabolic panel (kidney function, calcium)
• Serum protein electrophoresis (SPEP) with immunofixation
• Quantitative immunoglobulins (IgG, IgA, IgM)
• Serum free light chains and kappa/lambda ratio

If these laboratories are normal, the probability of multiple myeloma is extremely low 2. The absence of other bone pain, anemia, hypercalcemia, or renal dysfunction makes systemic myeloma highly unlikely 2.

Imaging Recommendations

No Additional Imaging Currently Indicated

• PET/CT is not appropriate: The American College of Radiology explicitly states that FDG-PET/CT is not routinely used for evaluation of benign-appearing bone lesions and should be reserved only for development of concerning systemic symptoms such as progressive weight loss, night sweats, fevers, or new bone pain 2

• Current MRI is adequate: MRI without and with contrast is the optimal modality for characterizing symptomatic bone lesions and identifying complications such as secondary aneurysmal bone cyst formation 2

• CT role is limited: While CT can be useful for surgical planning in symptomatic benign lesions, it adds little diagnostic value when MRI has already been performed 2

Management Algorithm

Step 1: Complete Laboratory Screening

Obtain the recommended myeloma workup as outlined above 2

Step 2: Neurosurgical Consultation

Given the symptomatic presentation (persistent neck pain, nausea, dizziness, visual changes for nearly 2 years) and critical anatomic location near the foramen magnum and vertebral artery, neurosurgical consultation is warranted 2:

• The American College of Radiology notes that symptomatic benign lesions may require further evaluation for definitive characterization before intervention, particularly when at risk for complications 2
• Aneurysmal bone cysts in the skull can be highly vascular and require careful surgical planning 3, 5
• Complete surgical resection is curative and prevents recurrence 5, 6

Step 3: Biopsy Consideration

Image-guided biopsy or surgical excision should be discussed with neurosurgery 2:

• While imaging strongly suggests benign pathology, definitive diagnosis requires tissue confirmation 2
• Solitary plasmacytoma cannot be completely excluded without biopsy 2
• The differential includes aneurysmal bone cyst, giant cell tumor, osteoblastoma with secondary ABC, or other benign bone lesions 1, 5, 6

Critical Pitfalls to Avoid

• Do not pursue extensive metastatic workup: Without a known primary malignancy, recent normal colonoscopy, and absence of systemic symptoms, metastatic disease is extremely unlikely 2

• Do not delay neurosurgical evaluation: The proximity to the foramen magnum and vertebral artery, combined with persistent symptoms for 2 years, warrants specialist assessment even if the lesion appears benign 2

• Do not assume observation is sufficient: While observation is reasonable for asymptomatic benign lesions 7, this patient has significant ongoing symptoms that may be related to the lesion and require intervention 2

• Recognize that aneurysmal bone cysts can be highly vascular: Preoperative planning should account for potential significant intraoperative bleeding 5, 6

Definitive Diagnosis Requires Tissue

The only way to definitively exclude solitary plasmacytoma or other rare pathology is through biopsy 2. Given her young age, presence of a young child, symptomatic presentation, and the lesion's critical location, proceeding with tissue diagnosis through either image-guided biopsy or surgical excision is the most prudent approach after laboratory results return 2.