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Background
Glioblastoma multiforme (GBM) is both the most common and most aggressive primary brain malignancy in adults, accounting for 16% of primary CNS tumors with a median survival time of 14 months despite current maximal treatment with surgical resection followed by radiation and chemotherapy. In part due to its aggressive course and short survival period, GBM metastasis beyond the primary neuraxial tumor site is rare, occurring in less than 2% of cases. Although the exact mechanism of extracranial GBM spread is uncertain, the majority of cases have been reported to occur following surgical resection or stereotactic biopsy of the primary tumor site. Such procedures are postulated to provide avenues of escape that circumvent traditional barriers of GBM spread, including the absence of true CNS lymphatics as well as the presence of both an intact blood-brain barrier and dense connective tissue which limit access to hematogenous spread. However, a small minority of cases have also demonstrated extracranial GBM spread before any invasive procedure has taken place. This exhibit will highlight the most common sites for extracranial GBM spread, as well as potential routes for extracranial GBM metastasis through illustrative cases of patients before and after resection. Recognition of these patterns of GBM metastasis is integral to clinicians and radiologists alike, as the incidence of extracranial GBM has steadily increased over time, paralleling improvements in management and survival time for patients with GBM.

Educational Goals / Teaching Points
The goals of this exhibit are to familiarize the audience on patient demographics and factors associated with extracranial GBM spread, innate CNS barriers to extracranial GBM metastasis, as well as the most common sites for extracranial GBM spread. Finally, we will highlight potential mechanisms and imaging characteristics of systemic GBM spread through illustrative biopsy-proven cases.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The imaging characteristics and symptomology of extracranial GBM are heterogenous and varied, requiring correlation with both imaging and biopsied tissue features. However several common sites of systemic GBM metastasis have been identified, including to regional lymph nodes, to lungs/pleura, and to bone, with vertebral bodies most commonly involved. Examples of resultant presentations include progressive dyspnea secondary to malignant pleural effusion with pleural and lung parenchymal involvement, as well as progressive neck swelling and pain secondary to metastatic cervical lymphadenopathy. Our cases will highlight imaging findings demonstrating perineural and hematogenous dissemination of GBM to these sites.

Conclusion
Extracranial GBM spread is a rare but increasingly common phenomenon associated with poor patient outcomes. Most cases occur following surgical resection or biopsy of the primary tumor, and several common sites of metastasis have been demonstrated. Recognition of potential routes and sites of extracranial GBM spread are integral to future systemic management and detection of this aggressive malignancy.