Is an indeterminate expansile lytic lesion in the inferior right occipital bone likely to be benign in a patient with impaired renal function and elevated total bilirubin?

Indeterminate Expansile Lytic Lesion in Occipital Bone: Likely Benign

An indeterminate expansile lytic lesion in the inferior right occipital bone is most likely benign, particularly given the patient's impaired renal function and elevated bilirubin, which are unrelated to the bone lesion and do not suggest systemic malignancy such as multiple myeloma.

Primary Diagnostic Considerations

The key to determining whether this lesion is benign or malignant lies in understanding what conditions cause expansile lytic bone lesions in the skull:

Benign Entities Are More Common in This Location

• Xanthoma of bone presents as an expansile lytic lesion with cortical expansion or disruption, particularly in the skull (including occipital bone), and despite its aggressive radiographic appearance, it is benign 1
• Aneurysmal bone cysts (ABC) appear as lytic and expansile lesions that can spontaneously heal, especially in older adolescents and young adults 2
• These benign lesions characteristically show expansion without the systemic features of malignancy 1

Why Multiple Myeloma Is Unlikely

The patient's laboratory abnormalities argue against multiple myeloma:

• Impaired renal function with elevated bilirubin is not a typical presentation of myeloma. Multiple myeloma causes renal insufficiency through light chain deposition, hypercalcemia, or direct tubular damage—not through mechanisms that would elevate bilirubin 3
• Myeloma presents with lytic lesions (not expansile lesions) plus systemic features: anemia, hypercalcemia, renal failure from myeloma kidney, and elevated serum M-protein ≥3 g/dL with ≥10% bone marrow plasma cells 3
• A solitary lytic bone lesion without these systemic features would be classified as solitary plasmacytoma, not myeloma, and even then would require bone marrow biopsy showing <10% plasma cells 3
• The guidelines specifically note that "in patients with a single asymptomatic lytic bone lesion, the possibility of an associated benign bone cyst or a bone angioma should be considered" 3

The Elevated Bilirubin and Renal Dysfunction Context

• Elevated bilirubin with renal impairment suggests a separate hepatorenal process unrelated to the bone lesion 3, 4
• This combination does not fit the CRAB criteria (hypercalcemia, renal failure, anemia, bone lesions) required for myeloma diagnosis, as the renal and hepatic dysfunction appear to have alternative etiologies 3
• Elevated bilirubin can actually be protective in chronic kidney disease and does not suggest malignancy 5

Recommended Diagnostic Approach

Imaging Characterization

• CT scan with contrast (if renal function permits) or MRI should be obtained to better characterize the lesion's internal architecture, cortical integrity, and soft tissue extension 3
• MRI is particularly useful for differentiating benign from malignant bone lesions and can detect soft tissue masses or bone marrow involvement 3

Laboratory Workup to Exclude Myeloma

Despite the low probability, complete exclusion requires:

• Serum protein electrophoresis (SPEP) and immunofixation to detect M-protein 3
• Serum free light chain assay 3
• Complete blood count to assess for anemia 3
• Serum calcium to exclude hypercalcemia 3
• If M-protein is absent and calcium is normal, myeloma is effectively ruled out 3

Biopsy Consideration

• Biopsy should be performed if imaging remains indeterminate after advanced imaging, as histologic diagnosis is definitive for benign entities like xanthoma or ABC 1, 2
• Curettage with histologic examination is both diagnostic and potentially curative for benign expansile lesions 1

Clinical Pitfalls to Avoid

• Do not assume all lytic bone lesions with renal dysfunction are myeloma. The pattern must fit: multiple lytic (not expansile) lesions, M-protein elevation, bone marrow plasmacytosis, and myeloma-related renal failure 3
• Do not overlook infectious causes. Primary tuberculosis can present as a lytic bone lesion mimicking tumor, though this is rare in the skull 6
• Do not delay biopsy if imaging is equivocal. Benign lesions like xanthoma have aggressive radiographic appearances that can only be distinguished from malignancy histologically 1