Case of the Week

•Background:

• IMT is a rare tumor—most arise in the lung and orbit while less commonly they can have intracranial or intraspinal origin.
• In the WHO 2021 Classification of Tumors of the Central Nervous System, IMT is assigned to the soft-tissue tumor subcategory of mesenchymal, nonmeningothelial tumors.
• The etiology of IMT is uncertain but is hypothesized to result from an abnormal myofibroblastic response to a local inflammatory insult (trauma, surgery, inflammation, etc). ALK gene (2p23) rearrangements and human herpesvirus 8 infection have also been hypothesized to contribute to the development of IMT.
• IMT is a true neoplasm; it has previously been grouped with inflammatory pseudotumor, which is a nonneoplastic process.

•Clinical Presentation:

• Patients may be asymptomatic, have nonspecific symptoms such as headache, pain, seizure, or visual changes, or can have location-dependent symptoms that worsen as the tumor enlarges.

•Key Diagnostic Features:

• Dural-based extra-axial mass with either focal or diffuse presentation
• Displacement of brain cortex and displacement of superficial blood vessels toward the calvarium on imaging as well as direct visualization during surgical resection will help distinguish extra-axial location of IMT
• On MRI, key distinguishing factors include marked low T2 signal intensity and diffuse contrast enhancement. Reduced perfusion on ASL for a low T2 signal extra-axial mass can raise suspicion.
• Diffuse type are more often associated with dural venous sinus thrombosis and tend to have worse outcomes and higher recurrence rates related to incomplete surgical resection and poor response to medical treatment
• Histopathology demonstrates a proliferation of myofibroblastic/spindle cells and inflammatory infiltrate

•Differential Diagnosis:

• Neoplastic: Meningioma and solitary fibrous tumor (both are rare in children and most demonstrate ASL hyperperfusion and increased CBV on bolus perfusion-weighted imaging with dynamic susceptibility contrast), metastases (have a known primary site and brain lesions are typically located at the gray-white matter junction and watershed areas), lymphoma and leukemia (tend to have strong restricted diffusion and either brain or bone involvement)
• Inflammatory/autoimmune: Inflammatory pseudotumor (more commonly occurs in the lung, orbit, sinonasal cavity, nasopharynx, larynx, trachea, and, rarely, solid organs of body), IgG4-related disease (systemic disease with elevated serum IgG4 and typically multifocal organ involvement)
• Granulomatous/infectious/histiocytic: Langerhans cell histiocytosis (circumscribed lytic bone lesion with beveled edge), histiocytic sarcoma (exceptionally rare in CNS), neurosarcoidosis (typically diagnosed in middle-aged or older adults, minority of cases have isolated CNS involvement, and often presents in a variety of ways with parenchymal, pachymeningeal, leptomeningeal, or, less likely, osseous involvement), tuberculosis (most commonly presents as basal leptomeningitis; focal tuberculoma and abscess are less common and typically are intra-axial in location)

•Treatment:

• Surgical resection is the best and most effective treatment; however, the diffuse type may be difficult to remove completely, and recurrence is also possible.
• High-dose corticosteroids have shown some success in controlling symptoms for unresectable lesions; however, they do not represent an ideal long-term treatment option.
• Radiation and chemotherapy may be alternative treatment options in some cases.