High grade transformation (dedifferentiation) may rarely occur in an adenoid cystic carcinoma. A tumor showing high grade transformation is aggressive and has a poor prognosis.
Histologic features of the tumor:
(1) some areas show an adenoid cystic carcinoma
(2) other areas show a poorly differentiated or undifferentiated adenocarcinoma lacking features of an adenoid cystic carcinoma
The poorly differentiated or undifferentiated areas show:
(1) tumor necrosis
(2) high mitotic rate
(3) enlarged and irregular nuclei
(4) absence of biphasic ductal-myoepithelial differentiation
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