[Malignant transformation of thyroid cancer; poorly differentiated cancer and anaplastic cancer transformation].

The prognosis of thyroid cancer depends largely on histological differentiation and clinical stage. The prognosis in anaplastic cancer is worst, in well differentiated cancer it is best; poorly differentiated cancer takes a middle position. The prognosis of patients with differentiated cancer is generally good, however, there are cases with malignant transformation such as poorly differentiated, anaplastic and squamous-cell cancer transformation. We studied the frequency and the inducement factors of malignant transformation in patients with thyroid cancer. The frequency of poorly differentiated, anaplastic and squamous-cell cancer transformation was 13.6%, 6.4% and 0.7%, respectively. In anaplastic cancer transformation, irradiation is an important factor.