**1. Introduction**

This chapter will cover, in brief, clinical and pathological characteristics of what are known as primary ectopic meningiomas (PEM) and the presence of tissue histomorphologically and immunophenotypically consistent with meningeal tissue (meningothelial) occurring in other organs or as part of teratomas or hamartomas/choristomas. PEMs, that is, those that occur outside of the central nervous system (CNS) can occur as a result of direct extension of a primary CNS meningioma (through calvarial bone into adjacent soft tissue), as a metastatic lesion, or as a primary ectopic meningioma [1]. Cutaneous meningiomas or primary cutaneous meningiomas describe a subset of PEMs mostly found in the scalp and have a classification system delineated by whether lesions are congenital or acquired and whether they have connection to a primary intracranial meningioma. Type I are congenital and may present as midline scalp cystic lesions (rudimentary meningoceles, acoelic meningeal hamartomas). Rarely sinus tracts have been found connecting these to the CNS. Type II are soft tissue meningiomas that have predilection for the nose, mouth, eyes, and ears and have no connection to an intracranial meningioma. Type III are soft tissue extensions of a primary intracranial meningioma [2]. Meningothelial tissue (not meningioma) can be seen most

notably described in the lungs and rarely in hamartomas/choristomas (lesions composed of tissue types arranged haphazardly but indigenous to the location; hamartoma or not indigenous to location; choristoma) particularly in the head and neck location. PEM have been described in teratomas and meningothelial tissue is not an infrequent component of mature neuroglial tissues in teratomas (tumors composed of tissues derived from all three primordial germ layers). This review will focus on those some aspects of meningiomas that occur outside the CNS; the primary ectopic meningiomas and lesions where meningothelial tissue has been found with particular focus on pulmonary and gonadal (in the context of gonadal mature teratomas) meningiomas and meningothelial proliferations and separately hamartomas/choristomas particularly of the head and neck. It is these latter lesions that the author has the most familiarity from practicing in the discipline of pediatric pathology.
