Insular carcinoma is a form of poorly differentiated tumor arising from follicular cells, with behavior intermediate between differentiated (papillary/follicular) and undifferentiated (anaplastic) carcinoma . They usually retain some features of differentiated follicular thyroid cells with ability to produce thyroglobulin and retain radioiodine but display a more aggressive clinical behavior [45,46].
Macroscopic features are of an invasive solid tumor typically greater than 5 cm in diameter exhibiting areas of necrosis. Microscopically tumor is characterized by well-defined nests (insulae) of small uniform cells displaying
an infiltrative pattern of growth and vascular invasion.
Insular carcinoma is infrequent comprising 2-4% of all thyroid cancers. Patients are usually middle-aged or elderly, often with a preceding history of goiter. The usual presentation is an enlarging mass (Figure 30.4), although distant metastases at presentation are not uncommon. They have an aggressive behavior with a high recurrence rate of 20-60% and a 10-year cause-specific mortality of 13-41% [47,48]. Tumors with only an insular component do not appear to have such an adverse prognosis, a discrepancy relevant to treatment and outcome [48,49].
Due to its rarity, reports include only small numbers of patients. As many insular carcinomas occur in the elderly who frequently have large tumors it was previously uncertain whether their aggressive behavior was related to these variables rather than the actual histotype. Pellegriti et al. compared patients of similar age and tumor size of which 13 were insular, 18 fol-licular, and 26 papillary . Those with insular carcinoma had the worst outcome with a cause-specific mortality rate of 62% compared with 17% and 15% in the follicular and papillary groups, respectively. Distant metastasis occurred in 85% of insular tumors compared with 50% and 19% of patients with follicular and papillary subtypes. All patients with metastases from insular carcinoma displayed radioio-dine avidity but clinical benefit was seen in only one. Multivariate analysis revealed insular his-totype as being the only variable related independently to cause-specific mortality.
Insular carcinoma demands initial aggressive treatment including total thyroidectomy, lymph node dissection, and radioiodine ablation. External beam radiotherapy may be required in addition and can also provide palliation of symptoms from unresectable disease. Radioio-dine is recommended for treatment of distant metastases but may prove of limited value. Chemotherapy has no proven role.
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