Thyroid cancer. Light micrograph (LM) of a section through a papillary thyroid carcinoma. This malignant (cancerous) tumour is the most common form of thyroid cancer. It is more prevalent in women than men. The cause is unknown, but exposure to radiation increases the risk of developing thyroid cancer. There are often no symptoms, but large tumours can cause difficulty swallowing or breathing. Treatment is with the surgical removal of the tumour or gland, often combined with radioactive iodine therapy. Magnification: x50 when printed at 10 centimetres wide. Human tissue.