Yes. In most cases, you can donate if you remain free of cancer five years after completing treatment. This is to protect your own health by ensuring, as far as possible, that the cancer is gone and won’t recur. Five years is the period most often used by doctors to define a cancer as presumed 'cured'.
For some cancers (or pre-cancers) of the skin, carcinoma in situ (CIN and CIS) of the cervix, prostatic intraepithelial neoplasia (PIN) of the prostate, or ductal carcinoma in-situ (DCIS) of the breast, you may be eligible to donate as soon as treatment is complete.
However, if you have a history of leukaemia, lymphoma and myeloma, which involve the blood production system, you can't donate blood. It’s to protect your health and the health of patients who receive donated blood.
Don't be disappointed though, because there are other ways you can help.