Adenomas and follicular carcinomas of the thyroid display two major patterns of chromosomal changes.

It was recently shown by flow and static cytometry that a large sub-group of follicular adenomas of the thyroid--fetal/embryonal adenomas--display an aneuploid phenotype. It was also shown that thyroid lesions with a DNA content within the triploid range were either fetal adenomas or follicular carcinomas with a fetal adenoma growth pattern. Follicular tumours with growth patterns other than the so-called fetal adenoma-like pattern were usually diploid or near-diploid. In an attempt to clarify the pattern of chromosomal imbalances in follicular tumours, comparative genomic hybridization (CGH) analysis was performed in a series of 18 follicular neoplasms (ten fetal/embryonal and four common follicular adenomas and four minimally invasive follicular carcinomas). For each tumour, the DNA content was determined by flow cytometry and, in some cases, also by static cytometry. Finally, the copy number of selected chromosomes was determined by interphase fluorescence in situ hybridization (FISH) using centromere probes. With the exception of the single diploid fetal adenoma, all fetal adenomas displayed several DNA copy number changes, with frequent gains of several chromosomes, which were found to be either tetrasomic or trisomic by FISH. This genetic pattern was also present in the single case of follicular carcinoma with aneuploidy and fetal adenoma-like growth pattern. Follicular adenomas other than fetal adenomas, and the remaining follicular carcinomas, showed more losses than gains of chromosomes. These results suggest that follicular tumourigenesis may follow at least two pathways: one characterized by prominent aneuploidy and numerous gains, in which the tumours display a fetal adenoma-like growth pattern; and another accompanied by less obvious aneuploidy or even quasi-diploidy and dominant chromosome losses, in which the tumours display a common follicular architecture. Copyright 2005 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.