Atypical Teratoid/Rhabdoid Tumor

Fig. 41.1

Typical rhabdoid cells with granular chromatin, prominent nucleolus and eosinophilic cytoplasm, brisk mitoses, and apoptotic bodies (HE, ×100)

Reticulin may be abundant in some AT/RTs, especially in the perivascular regions [10]. The mitotic count is high, and necrosis is common. Microvascular proliferation in AT/RT very rarely shows the endothelial proliferation characteristic of some gliomas [1, 2].

Cytologic examination by scraping and smearing squash preparation or fine-needle aspiration offers a useful alternative to frozen section during intraoperative consultation. Cytomorphologic features are unique and lead to an accurate diagnosis in the right clinicoradiologic context [12]. Cytomorphologically, the smears are hypercellular with primitive-type small, round neoplastic cells admixed with large-sized rhabdoid cells in varying proportions. The rhabdoid cells have abundant eosinophilic cytoplasm, eccentrically located hyperchromatic nucleus with prominent nucleolus and irregular nuclear contour [12, 13]. Mitosis, necrosis, and dystrophic calcification may be present.

Ultrastructurally, the rhabdoid cells, which can be prominent but often not dominant, show intracytoplasmic, paranuclear whorls of intermediate filaments [3, 12]. Rhabdoid cells contain multiple nuclei with invagination. In small cells, cytoplasmic organelles are poorly seen. Epithelial cells formed nests separated from the stromal elements by basement membrane material [3, 14].

41.3 Immunohistochemical Findings

Immunohistochemical staining shows diffuse expression of vimentin. Both rhabdoid and epithelial cells are immunoreactive for epithelial membrane antigen, and cytokeratins are positive in most cases [1, 3, 10, 14] (Figs. 41.2 and 41.3). In more than half AT/RT cases, the rhabdoid cells/pale cells and scattered spindle cells are positive for smooth muscle actin [3, 10] (Fig. 41.4). They may also express GFAP, neurofilament protein, and synaptophysin (Figs. 41.5 and 41.6). Germ cell markers such as human chorionic gonadotropin and placental alkaline phosphatase are generally negative [1, 3, 10]. However one series found limited alpha-fetoprotein staining in large cells [10]. Supplementing these immunohistochemical characteristics, the absence of nuclear expression of INI1 has emerged as a critical tool for accurate AT/RT diagnosis [1].

Fig. 41.2

Most of tumor cells show strong expression of membranous and cytoplasmic EMA (streptavidin-biotinylated complement; ×400)

Fig. 41.3

Tumor cells show strong expression of cytoplasmic pan CK (streptavidin-biotinylated complement; ×400)

Fig. 41.4

Some tumor cells show strong expression of cytoplasmic synaptophysin (streptavidin-biotinylated complement; ×400)

Fig. 41.5

Focal reactivity of cytoplasmic GFAP (streptavidin-biotinylated complement; ×400)

Fig. 41.6

Focal cytoplasmic smooth muscle actin alpha 8 [SMAA] (streptavidin-biotinylated complement; ×400)

AT/RTs in children have marked MIB-1 labeling indices that are often more than 50 %, but in some cases the labeling index may be significantly lower [1, 9, 14] (Fig. 41.7).

Fig. 41.7

Ki67 labeling index of 40 % (streptavidin-biotinylated complement; ×400)

41.4 Differential Diagnosis

In most cases, when the neoplasm occurs in the cerebellum of an infant, the principal differential diagnostic issue is MB. This distinction may be difficult to resolve if a small specimen contains only the small cell component. In contrast to MB, the AT/RT has a high incidence of necrosis with calcification, frequent broad fibrovascular septa, a population of large cells, and a lack of nodularity. Immunoreactivity for a broad range of antigens and the presence of chromosome 22 alterations are additional points of distinction [10].

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