Distinguishing cellular follicular adenomas (FA) from minimally invasive follicular carcinomas (FC) continues to plague even experienced thyroid pathologists. DNA ploidy analysis has been promoted as a means of making this differentiation; however, the finding of DNA aneuploidy in FA has caused concern that they may demonstrate potential for malignant behavior or should even be reclassified as low-grade noninvasive cancers. In histologically-proven FC, nuclear DNA content has been claimed to have predictive power equivalent to that of all other prognostic factors combined. The aims of the present study, therefore, were to define the DNA ploidy characteristics of FA and FC, to assess the diagnostic potential of cell-cycle parameters, and, in FC, to investigate the prognostic role of such measurements. We measured DNA content of 124 tumors (60 FA, 64 FC). DNA pattern was normal (diploid) in 75% of FA and 45% of FC, tetraploid/polyploid (T/P) in 13% of FA and 25% of FC, and aneuploid in 12% of FA and 30% of FC. FC was histologically verified in 39% of DNA normal, 67% of T/P, and 73% of aneuploid tumors. DNA index, S-phase, G2M, and S-phase plus G2M were analyzed and were not helpful in differentiating between FA and FC. No patient with FA developed tumor recurrence. In FC (excluding the Hürthle cell variant), no significant differences were found among the 3 DNA ploidy groups with respect to either cancer death or tumor recurrence; however, combining the Hürthle cell variant of follicular carcinomas with pure follicular carcinomas, the presence of distant metastases, DNA aneuploidy, and patient age were the only independently significant prognostic variables. Based on these data, while not valuable for determining the presence of malignancy, DNA ploidy determination is of major prognostic value in follicular carcinomas (including the Hürthle cell variant).
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