A 41 Year‐Old Woman with a Mass in the Posterior Cranial Fossa

Clinical History and Imaging

A 41‐year‐old previously healthy Japanese female presented with a 2‐month history of left occipital headache and dizziness. She had instability in standing on her left foot. A contrast‐enhanced T1‐weighted MR scan showed a high signal, 4.6 × 3.2 × 3.5 cm extra‐axial mass containing a cyst located in the left posterior cranial fossa (Figure 1A). It attached to the dura mater along the occipital bone and the petrous bone, a part of which was extending into the sigmoid sinus. Catheter angiography illustrated a hypervascular tumor fed by the middle meningeal artery and the ascending pharyngeal artery. The sigmoid sinus was occluded by the tumor invasion.

Figure 1.

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The patient underwent preoperative embolization of the tumor in order to decrease blood loss. Tumor resection was performed via left lateral suboccipital approach. The tumor was whitish‐yellow, elastic hard, rich in fibrous tissue and was strongly adherent to the dura mater. The cystic part was surrounded by soft membrane and filled with clear fluid. Brain and the tumor were clearly bounded by the arachnoid membrane. Gross total resection including the dura mater and the sigmoid sinus invaded by the tumor was achieved. After surgical treatment, her symptom gradually improved. She was discharged from our hospital with no neurological deficits.

Microscopic Pathology

Histopathological examination revealed the tumor had biphasic features. H&E staining (Figure 1B,C) showed a proliferation of short spindle cells in a vague arrangement with branching vascular structures and many broad bands of collagenous tissue. There were no whorls or psammoma bodies. Necrosis and mitoses were not present. Mixed with the spindle cell tumor, glandular duct structures of various sizes were found. These tissues consisted of serous and mucous acini and diversely dilated ducts. There were no atypical cells in this structure. Immunohistochemically, the spindle cells were diffusely positive for CD34 (Figure 1D), vimentin (Figure 1E) and nuclear STAT6 expression (Figure 1F); EMA and progesterone receptor were negative. On the other hand, the glandular duct structures were positive for EMA (Figure 1G), AE1/AE3, CK7 (Figure 1H); CD34 and STAT6 were negative. And smooth muscle actin stain illustrated myoepithelial cell layer surrounding the secretory acini (Figure 1I). The Ki67 index was 7%. What is your diagnosis?

Diagnosis

Meningeal solitary fibrous tumor/hemangiopericytoma with ectopic salivary grand tissue

Discussion

The 2016 WHO classification of tumors of the 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 is a WHO grade I tumor with patternless, low cellularity, spindle cells with abundant collagen. In contrast, the tumors with the HPC phenotype characterized by hypercellularity, necrosis and high mitotic count are classified as WHO 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 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 an 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 grand 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 grand 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 grand tissue in posterior cranial fossa is extremely rare, to our knowledge only a few cases were reported. Some literature proposes it is associated with the elongation and differentiation of epibranchial placodes during embryogenesis 3, 4.

Coexistence of SFT and ectopic salivary grand 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. The author wondered about the association with SFT and the ectopic salivary grand tissues 3. In the past literature, several authors have reported various types of neoplasms possibly from ectopic salivary grand 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 some 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.