In a patient with hyperostosis along the sagittal suture (suggesting bone invasion typical of parasagittal or falx meningioma) and a prior MRI that showed no intracranial lesion, what are the differential diagnoses?

Differential Diagnoses for Hyperostosis with Suspected Parasagittal/Falx Meningioma

When hyperostosis is present along the sagittal suture without an intracranial lesion on prior MRI, the primary differential diagnoses include neurosarcoidosis, primary calvarial osteosarcoma, fibrous dysplasia, and osteoma, though meningioma en plaque remains most likely and may have been missed on initial imaging.

Key Diagnostic Considerations

Primary Differential: Meningioma En Plaque

While meningioma is the most common cause of hyperostosis in this location, the absence of an intracranial mass on prior MRI creates diagnostic uncertainty. However:

• Meningioma en plaque can present with extensive hyperostosis disproportionate to the tumor mass 1
• Prior imaging may have missed a subtle dural-based lesion, as SSTR PET detects meningiomas with superior sensitivity compared to contrast-enhanced MRI alone 2
• Hyperostosis in meningiomas is caused by actual tumor invasion histologically in 96% of cases (25/26 patients with radiographic hyperostosis) 3

Critical Alternative Diagnoses

Neurosarcoidosis represents the most important mimicker:

• Can present as dural-based lesions with hyperostosis of the sphenoid wing, radiologically indistinguishable from meningioma 4
• Hyperostosis is typically absent in neurosarcoidosis, making this presentation "unusual" but documented 4
• This diagnosis is crucial because management differs entirely from meningioma

Primary calvarial osteosarcoma:

• Rare (0.33-0.41 per million incidence) but can mimic parasagittal meningioma both clinically and radiologically 5
• Key distinguishing feature: osseous erosions are uncommon in meningiomas and should raise suspicion for alternative diagnoses 6
• Particularly relevant when meningioma has extensive intraosseous component 5

Fibrous dysplasia and osteoma:

• Classic hyperostosing conditions that can be confused with meningioma en plaque 7
• CT features help differentiate: meningiomas show periosteal pattern of hyperostosis, inward bulging of vault lesion, and surface irregularity of hyperostotic bone 7

Diagnostic Algorithm

Immediate Next Steps

1. Obtain high-resolution CT without contrast - this is the neuroimaging method of choice for evaluating meningioma en plaque 7

   • Look for: periosteal pattern, inward bulging, surface irregularity
   • Assess for osteolytic changes (suggests osteosarcoma over meningioma) 6

2. SSTR PET imaging (68Ga-DOTATATE/DOTATOC preferred):

   • Provides superior detection sensitivity compared to MRI alone 2
   • Can detect small meningeal lesions previously undetected on MRI 2
   • Detects osseous involvement with higher accuracy than MRI 2
   • If SSTR-avid: strongly supports meningioma diagnosis
   • If SSTR-negative: consider neurosarcoidosis, osteosarcoma, or other pathology

3. Repeat contrast-enhanced MRI with attention to:

   • Subtle dural enhancement
   • Brain-tumor interface characteristics
   • Perilesional edema patterns

Location-Specific Considerations

Skull base versus convexity location matters:

• Hyperostosis is significantly more common in skull base meningiomas than convexity meningiomas (p = 0.001) 8
• For parasagittal/falx location (middle third), hyperostosis is common but carries specific surgical implications 9

Common Pitfalls to Avoid

1. Assuming hyperostosis always equals meningioma - while meningioma is most common, the documented cases of neurosarcoidosis with hyperostosis 4 and osteosarcoma mimicking meningioma 5 mandate consideration of alternatives

2. Relying solely on MRI - CT is superior for characterizing hyperostotic bone changes 7, and SSTR PET provides critical functional information 2

3. Overlooking the temporal sequence - in the reported case 1, CT 4 years prior showed only hyperostosis without intracranial lesion, then later developed an intracranial mass, suggesting hyperostosis can precede obvious tumor

4. Misinterpreting bone invasion patterns - presence of osseous erosions (rather than hyperostosis) should prompt consideration of osteosarcoma or other aggressive pathology 6

Histopathological Correlation

Biopsy remains the definitive diagnostic tool when imaging is equivocal, as:

• Hyperostosis correlates with bone invasion in meningiomas 3, 8
• Neurosarcoidosis, osteosarcoma, and meningioma can be radiologically identical 4, 5
• Ki-67 index is higher in meningiomas without hyperostosis (p = 0.03), suggesting different biological behavior 8