Fetal adenomas and minimally invasive follicular carcinomas of the thyroidfrequently display a triploid or near triploid DNA pattern

The ploidy pattern and the percentage of S-phase cells were investigated by means of flow cytometry using fresh or frozen samples in a series of 143 t umors and tumor-like lesions of the thyroid in an attempt to find whether t here is any relationship between the histological characteristics of the le sions and their DNA content. The percentages of aneuploidy cases per catego ry were: nodular goiter, 18.5% (15/81); fetal adenoma (including cases with trabecular/solid growth pattern), 58.3% (14/24); follicular adenoma other than fetal adenoma, 0% (0/18): papillary carcinoma, 11.1% (1/9); and minima lly invasive follicular carcinoma, 57.1% (4/7). Regardless of the histologi cal category, aneuploid lesions had a significantly higher (P <0.001) perce ntage of S-phase cells (7.3%) than diploid lesions (4.1%). All the six case s with a DNA content within the triploid range were fetal adenomas, but one was a follicular carcinoma displaying a fetal adenoma-like growth pattern. The other three follicular carcinomas with an aneuploid DNA pattern also d isplayed foci of fetal adenoma-like growth pattern. Image cytometry of the four aneuploid follicular carcinomas showed similar DNA indexes in the peri pheral, invasive foci of the lesions and in the central fetal adenoma-like areas. These results demonstrate that aneuploidy in benign tumors is restri cted to adenomas displaying a fetal or fetal/embryonal growth pattern and s upport the concept that chromosome instability is a major pathway of tumori genesis in thyroid follicular neoplasms.