Poorly differentiated thyroid carcinoma is still a controversial entity. Some authors have proposed to include, in poorly differentiated carcinoma category, tumours with clinical behaviour, morphology, and biological features intermediate between well- differentiated and undifferentiated carcinomas. A large number of carcinomas has been added over the years, making the poorly differentiated carcinoma category extremely heterogeneous. The tumours nowadays considered to be poorly differentiated include some variants of papillary carcinoma such as tall cell variant, hob nail variant, Hürthle cell carcinoma, and insular carcinoma.
The poorly differentiated carcinoma represents 5% of thyroid malignancies. The mean age of the patients is about 10 years higher than that of patients with differentiated carcinomas, but younger than that of patients with anaplastic cancer. Poorly differentiated carcinoma shows a prevalence for the female sex (male/ female ratio:1/ 2).
The aetiology can develop in three different directions: poorly differentiated cancer can progress from papillary cancer, or from follicular cancer, or ‘de novo’. From a molecular point of view, the most frequent alterations of poorly differentiated cancers are the point mutations. They include very early events that are important for tumourigenesis and that predispose for further mutations (like Ras mutations or BRAF mutations) and also for gene alterations driving de- differentiation (like TP- 53 and β- catenin mutations).
On gross examination, poorly differentiated cancer displays a large size (range 1– 10 cm) with an aggressive and infiltrative behaviour. This neoplasia very often shows an extrathyroidal extension and very rarely shows a partial tumour capsule. Poorly differentiated carcinoma is usually hard to cut, its aspect is firm and yellowish- white- , and it may present haemorrhagic or necrotic areas.
On light microscopy the diagnosis of poorly differentiated carcinoma displays de- differentiated morphological features. On the basis of Torino’s criteria, they result as following :
1. Solid/ trabecular/ insular growth pattern
2. Lack of nuclear features typical of papillary cancer
3. Presence of one of the following features: necrosis; three or more mitoses per high power fields; convoluted nuclei
Poorly differentiated carcinoma commonly shows a bad prognosis with 5- year survival lower than 50%. It generally presents com pression symptoms associated with cough, dyspnoea, dysphagia. Infiltration of the recurrent nerve is frequent and an early event and distant metastases are present at the time of diagnosis in more than 20% of patients. Lymph node metastases are also frequent. Poorly differentiated carcinoma shows a partial response to routine radiometabolic therapy.
