Tumour regression refers to a tumour becoming smaller, or decreasing in size, over time. Regression may occur without treatment and it is then called spontaneous. The most common type of tumour to show spontaneous regression is melanoma, a type of skin cancer. Doctors believe that spontaneous regression occurs when cells from the immune system start to attack the tumour. For some patients, the entire tumour may disappear.
Tumour regression more commonly occurs after treatment, such as chemotherapy or radiation therapy. Pathologists determine the amount of regression by comparing the size of the tumour after treatment to the size of the tumour before treatment. Partial regression means that part, or some, of the tumour, has disappeared after treatment. With complete regression, the entire tumour has disappeared after treatment.