Case 624 -- A 6-year-old girl with an extra-axial mass in the middle cranial fossa

Contributed by 1Kenta Masui, MD, 1Satoshi O. Suzuki, MD, PhD, 1Akira Kondo, MD, PhD, and 1Toru Iwaki, MD, PhD
Departments of Neuropathology1, Neurological Institute, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan


A 6-year-old Japanese girl presented with psychomotor seizures including oral automatism with cloudy consciousness, lasting for 2 months. On admission, no neurological deficit was evident except for seizures, and no abnormal manifestations of the skin were observed. Magnetic resonance (MR) images indicated a tumor with peritumoral edema in the left middle cranial fossa, displaying heterogeneous enhancement with intravenous administration of gadolinium. The medial part of the tumor consisted of an irregular-shaped region, which was hyper-intense on T1-weighted images and hypo-intense on T2-weighted images (Fig. 1), indicating the presence of certain substances that shorten both T1 and T2 relaxation times, such as melanin, calcium and intracellular methemoglobin (4). This region was highlighted as a high-density area on computed tomography (Fig. 2). The patient underwent tumor resection through a left occipital craniotomy. The tumor was an extra-axial mass with focal blackish coloration in its medial portion, and intraoperative frozen sectioning showed a proliferation of pigmented cells among thick collagen fibers.


On gross inspection, the specimen was composed of white-to-grayish hard tissue, focally showing blackish pigmentation with the highest density at the surface of the tumor. The white-to-grayish lesion histologically consisted of a storiform arrangement of spindle-shaped cells with abundant intercellular collagen fibers and occasional concentric structures (arrow, Fig. 3). There were also clusters of cells with large nuclei and prominent nucleoli (Fig. 4). Invasion of these cells into the surrounding gliotic cerebral tissue was focally noted, which was confirmed by glial fibrillary acidic protein (GFAP)-immunostaining (asterisks, Fig. 5). While mitotic figures were rarely seen, the MIB-1 (Ki-67) staining index was increased up to 5.7% (Fig. 6). On the other hand, the blackish pigmentation at the surface of the tumor was composed of a dense accumulation of pigmented cells in the arachnoid membrane of the existing cerebral tissue, to which the tumor was firmly adhered (upper left: brain tissue, lower right: tumor, Fig. 7). The tumor tissue was highly fibrotic at the site of adhesion, and the pigmented cells infiltrated into the tumor tissue alongside the fibrosis (Fig. 8). Isolated clusters of these pigmented cells were also scattered in the tumor tissue (Fig. 9). These pigmented cells displayed low proliferative activity indicated by the absence of mitotic figures and negative immunoreactivity to Ki-67.

Further immunohistochemical analyses showed that spindle-shaped cells focally expressed epithelial membrane antigen (EMA), but not S-100 protein and HMB-45 whereas pigmented cells were immunopositive for S-100 protein, but negative for EMA and HMB-45.

Electron microscopic examination showed that the spindle-shaped cells had nuclei with cytoplasmic invagination, a fine chromatin pattern and conspicuous nucleoli, and abundant intermediate filaments in their cytoplasm. Interdigitation of the cytoplasmic processes and junctional complexes were also noted at the cell-to-cell borders (arrows, Fig. 10). In contrast, pigmented cells contained abundant pigment granules in their cytoplasm (Fig. 11), but displayed no features of spindle-shaped cells.


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