tumours [4, 6, 9, 15]. Immunohistochemistry is useful in the characterization of these tumors. Staining for thyroglobulin is positive in almost all cases confirming the follicular cell origin of the tumors [4, 9, 15]. Stains for keratin are posi­tive in 75% of cases, and negative staining for calcitonin enables differentiation of these tumors from medullary thyroid carcinoma.

anaplastic carcinoma in the SEER database had disease confined to the thyroid gland at presen­tation [25]. Local invasion beyond the thyroid capsule is seen in greater than 82% of cases, involving surrounding structures including overlying strap muscles, the trachea and larynx, esophagus, common carotid artery, adjacent nerves, great vessels of the superior mediasti­num, and overlying skin [22, 37–39, 41]. A study of adjacent structures involved by UTC found recurrent laryngeal nerve involvement in 59%, trachea or larynx in 50%, esophagus in 23%, carotid artery in 16%, and skin in 7% [39]. Direct invasion ofstructures within the superior mediastinum can produce superior vena cava syndrome in a small number of cases (Fig. 9.2) [28, 37, 40]. Metastatic involvement of regional lymph nodes can be documented in 21–38% of patients at presentation and is seen in 83% of cases at autopsy [25, 37, 38]. Hematogenous dissemination of the tumor to distant sites can be demonstrated in 43–64% of patients at the time of initial assessment and is present in 87% of cases at autopsy [22, 25, 27, 28, 38, 43, 44]. The most common site of metastatic spread is to the lungs, seen in 75–88% of patients with meta­static disease [27, 28, 37, 44]. Less common sites

appear normal or contain evidence of preexist­ing nodular disease [47]. Extracapsular invasion of the tumor into surrounding structures in also commonly observed.

The histological appearance of UTC varies considerably among cases, and criteria for his­tological diagnosis have been defined by the World Health Organization [1, 49]. Three histo­logical patterns are commonly identified: giant cell, spindle cell, and squamoid cell types (Fig. 9.4) [46–48]. In the giant cell form, large rounded neoplastic cells are seen with abundant eosinophilic cytoplasm, which display bizarre arrangements of hyperchromatic nuclei or be multinucleated. In the cases where spindle cells predominate, elongated fusiform cells with hyperchromatic nuclei form fascicles, often with a dense collagenous stroma, and the appearance can resemble that of sarcoma. Squa­moid cellular patterns are seen less commonly than giant and spindle cell forms, and display flattened cells with abundant eosinophilic cyto­plasm that form tumor nests and islands that resemble squamous carcinoma. It is rare for one pattern to predominate within a tumor, with most cases of UTC containing regions of varying histological appearance. In all forms, tumor cells have a high mitotic rate and fre­quent mitoses are demonstrated within sec­tions. Large multinucleated cells resembling osteoclasts may also be seen scattered in some tumors [47]. Areas of necrosis and hemorrhage within the tumor substance are common, and there is frequently an inflamma­tory cell infiltrate within the stroma. In all cases, UTC displays a distinct propensity for metastasis, with areas of invasion into vascular structures and lymphatic channels routinely identified in specimens. Histological variants of UTC have also been described and display similarly aggressive tumor behavior and poor prognosis. In the paucicellular variant of spin­dle cell UTC, prominent fibrosis is seen with few atypical spindle-shaped cells seen within a dense collagenous stroma with scattered inflammatory cells [47, 50]. This lack of cellu­larity makes diagnosis by fine needle aspiration (FNA) difficult, and the histological appearance can closely resemble Riedel’s thyroiditis [50]. An angiomatoid variant of UTC has also been reported and occurs very rarely, with histolo­gical features similar to angiosarcoma but with

immunohistochemical staining consistent with a thyroid follicular cell origin [51].

Imaging has an important role in the evalua­tion of patients with UTC. Cross-sectional ima­ging of the neck and mediastinum using computed tomography (CT) can correctly define the extent of invasion into adjacent struc­tures such as the trachea, esophagus, and car­otid sheath in a high proportion of cases, and can assess the extent to which invasion or exten­sion of the tumor into the superior mediasti­num and its contents has occurred (Fig. 9.5)