Anaplastic carcinomas occur in late middle age and in the elderly. Most tumours arise from a differentiated papillary or follicular carcinoma, sometimes with a long-standing history of goitre.
The tumours are undifferentiated (Fig. 16.11) with varying proportions of large spindle cells and giant cells, resembling a sarcoma. Mitoses are numerous, many being abnormal. Focal necrosis is often present. An undifferentiated small-cell type has also been found, although current opinion is that most tumours in this group are lymphomas.
