Poorly differentiated thyroid carcinoma: 5 years after the 2004 WHO classification of endocrine tumours.

Poorly differentiated thyroid carcinoma (PDTC) was originally described in 1983 but included in the WHO classification of thyroid tumours in the 2004 edition, only. The diagnostic problems encountered in the 5 years of application of the WHO criteria are reviewed here. A long debate is still going on about the nature of PDTC, its morphological diagnostic features, its clinical significance and its optimal therapeutic approach. A consensus conference held in Turin in 2006 confirmed the geographical differences among claimed classical forms of PDTC and suggested a diagnostic algorithm based on the presence of a solid/trabecular/insular growth pattern and of high-grade features, in line with the WHO definition of PDTC, capable to select tumours with a distinct aggressive clinical behaviour. This worked well for PDTC cases from mountain areas (e.g. Northern Italy), where most, if not all, thyroid carcinomas having high-grade features also share a solid/trabecular/insular pattern of growth. However, this scheme might be less easily applicable for American and Japanese cases, possibly due to heterogeneous architectural and cytological features; indeed some Authors still prefer to base their diagnostic work up on the recognition of high-grade features only, including mitotic index and necrosis, irrespective of the growth pattern.