Differential Diagnosis for Diploic Space Masses
The differential diagnosis for a mass in the diploic space includes benign lesions (hemangioma, epidermoid cyst, eosinophilic granuloma, fibrous dysplasia), primary malignancies (multiple myeloma, osteosarcoma), and metastatic disease, with metastases being the most common malignant etiology in adults. 1
Primary Diagnostic Approach
When evaluating a diploic mass, age is the single most important demographic factor that narrows your differential, as certain lesions predominate in specific age groups. 2
Benign Lytic Lesions
The most common benign entities include:
- Hemangioma: Presents as a lytic lesion with characteristic "sunburst" or trabeculated appearance on imaging 3
- Epidermoid cyst: Classic presentation is a deep, nonmobile scalp mass in the frontal, parietal, or occipital regions; skull radiographs show a well-defined lytic lesion with sclerotic margins 4
- Eosinophilic granuloma: Appears as an osteolytic lesion with bevelled edges (the "hole within a hole" sign) 1
- Fibrous dysplasia: Demonstrates ground-glass matrix on CT with expansion of the diploic space 3
- Dermoid cyst: Similar to epidermoid but may contain fat density 3
- Aneurysmal bone cyst: Expansile lytic lesion with fluid-fluid levels 3
- Giant cell tumor: Rare in the skull but presents as an aggressive-appearing lytic lesion 3
Malignant Lesions
Metastases are the most frequent cause of skull lesions overall, with breast cancer being the most common primary in adults and neuroblastoma in children. 1
- Multiple myeloma: Presents with classic "punched out" lytic lesions on radiographs, typically multiple 1
- Metastatic disease: Can be lytic, sclerotic, or mixed; common primaries include breast, lung, prostate, kidney, and thyroid 1, 3
- Osteosarcoma: Presents as a sclerotic or mixed lesion with aggressive features 3
- Lymphoma: Can present as a transdiploic lesion crossing the inner and outer tables 3
Transdiploic Lesions (Crossing Both Tables)
These lesions warrant special consideration as they may involve intracranial extension:
- Meningioma: Can involve the skull vault with characteristic hyperostosis and dural tail on MRI 5, 6, 3
- Hemangiopericytoma: Aggressive vascular tumor that crosses the diploic space 3
- Metastasis and lymphoma: Can also present in this pattern 3
Rare Considerations
Leiomyomas and angioleiomyomas should be included in the differential of skull base lesions, though they are extremely rare in the cranium. 7 These benign smooth muscle tumors can be locally destructive and large (up to 7.7 cm). 7
Imaging Strategy
MRI with and without contrast is the initial imaging study of choice for characterizing soft tissue components and intracranial extension. 6 CT with contrast serves as complementary imaging to evaluate bone involvement, hyperostosis, or erosion. 6
Diffusion-weighted imaging (DWI) can help differentiate benign from malignant lesions: ADC values ≥1.08 × 10⁻³ mm²/s effectively rule out lymphoma and nasopharyngeal carcinoma with 96% accuracy. 8 Skull base osteomyelitis (mean ADC 1.26) has significantly higher ADC values than lymphoma (0.59) or nasopharyngeal carcinoma (0.74). 8
Critical Pitfalls to Avoid
Always assess whether the mass involves only the diploic space or crosses into intracranial structures, as deep involvement requires neurosurgical evaluation and changes surgical planning. 2
Consider vascular lesions (hemangioma, hemangiopericytoma) that may require preoperative embolization to reduce bleeding risk if excision is planned. 2
Assess for multiplicity of lesions, which suggests systemic conditions like multiple myeloma, metastatic disease, or eosinophilic granuloma (part of Langerhans cell histiocytosis). 2, 1
Plain skull radiographs, while often overlooked, provide invaluable initial information and can direct further management, particularly for epidermoid cysts and multiple myeloma. 4