Contributed by Bunsho Asayama MD, Yoshinobu Seo MD, Yoshimaru Ozaki MD, PhD, Satoshi Tanikawa MD, Takanori Hirose MD, PhD, Shinya Tanaka MD, PhD, Hirohiko Nakamura MD, PhDFINAL DIAGNOSIS
Meningeal solitary fibrous tumor/hemangiopericytoma with ectopic salivary gland tissue
DISCUSSION
The 2016 WHO classification of tumors of the central nervous system (CNS) combines meningeal solitary fibrous tumor (SFT) and hemangiopericytoma (HPC) under a single mesenchymal tumor entity. It is defined by a genomic inversion at the 12q13 locus, NAB2-STAT6 gene fusion, which results in strong positivity for STAT6 nuclear expression in immunohistochemistry. Meningeal SFT / HPC is rare, constitutes less than 1% of all CNS tumor. As a feature not present in other CNS tumors, SFT / HPC contains three-tiered grading system within oneself. The classic SFT phenotype characterized by a patternless architecture, relatively low cellularity, spindle cell lesion with rich collagenous tissue correspond to grade I, which is considered to behave as a benign tumor. In contrast, the tumors with the HPC phenotype characterized by hypercellularity, necrosis, and high mitotic count are classified as grade II or grade III, which have a high rate of recurrence and occasional extracranial metastasis.
In the present case, the tumor consisted of two different types of components. Strong immunoreactivity to CD34 and nuclear STAT6 expression, together with negative results in EMA and progesterone receptor suggested that the short spindle cell neoplasm was not fibrous meningioma but meningeal SFT / HPC. Because of low mitotic activity and the absence of necrosis, it was considered to be SFT phenotype. Admixed with spindle cell tissue, the presence of mucoserous acini, normal ducts structure, and myoepithelial cell layer surrounding the acini confirmed normal, while ectopic, salivary grand tissue. Because of lack of atypical cells in these glandular duct structures, metastatic tumor and carcinosarcoma were not suggestive.
Salivary gland tissue heterotopia is defined as the presence of normal salivary gland tissue distant from the normal location of the tissues. It is often documented in the head and neck, including the middle ear, external auditory canal, thyroglossal duct and other extracranial regions (1). Intracranial salivary gland heterotopia is an unusual phenomenon and mainly limited in the sellar region, and it is thought largely to result from differentiation of the remnants of Rathke's cleft pouch which is derived from the ectoderm of stomodeum. Ectopic salivary gland tissue in posterior cranial fossa is extremely rare, to our knowledge only a few cases were reported. Some literatures propose it is associated with the elongation and differentiation of epibranchial placodes during embryogenesis (3, 4).
Coexistence of SFT and ectopic salivary gland tissue in our patient appears to be a unique condition. In 2004, Rodriguez et al. described SFT of the cerebellopontine angle with salivary gland tissues in a 53-year-old female, whose pathological condition was similar to our case's one. The author wondered about the association with SFT and the ectopic salivary gland tissues (3). In the past literatures, several authors have reported various types of neoplasms could be originated from ectopic salivary gland tissues (1). In addition, it has been often documented that SFT arises from both major and minor salivary glands (2). Therefore, we hypothesize about the developmental mechanism of our present case that apart of salivary gland primordial cells had been misplaced into dura mater of the posterior cranial fossa during embryonic migration and then neoplastic transformation into SFT was brought about by the result of differentiation.
The existence of ectopic tissue makes surgeons and pathologists to take an interest in its possible origin.
REFERENCES
Contributed by Bunsho Asayama MD, Yoshinobu Seo MD, Yoshimaru Ozaki MD, PhD, Satoshi Tanikawa MD, Takanori Hirose MD, PhD, Shinya Tanaka MD, PhD, Hirohiko Nakamura MD, PhD
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