Histologic transformation of polymorphous low-grade adenocarcinoma of salivary gland.

Polymorphous low-grade adenocarcinoma of salivary gland origin (PLGA) was initially described in 1984 and has since become an established clinicopathologic entity. Owing to the indolent nature of PLGA and its relatively recent description, the full clinicopathologic spectrum of this entity has not been elucidated fully. Transformation to a histologically different-appearing lesion or progression to a higher histologic grade has not been reported. We describe 2 PLGAs arising in the palate and associated with multiple locoregional recurrences that were treated with excision and radiation therapy. This was followed by histologic transformation to a higher grade neoplasm after 17 and 26 years, respectively. The histologic appearance after transformation was characterized by a predominantly solid and cystic growth pattern, nuclear atypia with prominent nucleoli, and foci of necrosis. High-grade transformation of PLGA may occur after a protracted clinical course with multiple recurrences of typical PLGA. The possible role of radiation therapy as an initiator of this transformation merits further study. Tumors with these histologic features should not be included under the rubric of typical PLGA. Segregation of these neoplasms will allow further study of their biologic potential, particularly with regard to possible increased rates of local recurrence and metastasis.