Differential diagnosis

AC may be confused for a TC, paragan-glioma, malignant melanoma, and medullary thyroid carcinoma. The AC is distinguished from the TC by the presence of larger cells, prominence of nucleoli, mitoses, necrosis, pleomor-phism and angiolymphatic invasion. AC is positive for cytokeratin, CEA and calci-tonin, whereas the paraganglioma is negative for these markers. Malignant melanoma is positive for HMB-45 and tyrosinase and negative for synapto-physin and cytokeratin. Separating AC from metastatic medullary thyroid carcinoma (MTC) may be more problematic since both tumours are positive for synaptophysin, calcitonin and CEA. Clinical and imaging studies to detect the presence or absence of a mass in the larynx or thyroid may offer some assistance. Although the serum calcitonin level is almost invariably elevated in metastatic MTC and usually negative in AC, rare cases of laryngeal AC associated with elevated levels of serum calci-tonin have been reported {2409}. Reliance on this test to distinguish between these two tumours is, therefore, not absolute. Knowledge of the serum CEA level (especially if markedly elevated), however, may be helpful. This test is almost universally elevated in MTC, but thus far, has not been reported in association with AC. More recently, thyroid transcription factor - 1 (TTF) has been useful in separating these two tumours. MTC is strongly and diffusely positive for this marker while the AC is typically negative or only focally, weakly positive.

Fig. 3.35 Small cell carcinoma, neuroendocrine type, involving the epiglottis. The tumour (arrows) is ulcerated and indistinguishable from a squa-mous cell carcinoma From L. Barnes et al. {131}, with permission from Marcel Dekker, Inc. New York.

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