Thyroid cancer. Light micrograph of a section through a thyroid gland with anaplastic carcinoma, a form of thyroid cancer that is most common in the very elderly. The carcinoma is composed of poorly differentiated cells with large round nuclei (dark purple). The cause of thyroid cancer is not known. Symptoms can include swelling in the neck and difficulty swallowing or breathing. Treatment is with the surgical removal of the tumour or gland, often combined with radioactive iodine therapy.