If you are not asked to give details of your medical history then there is no need to inform your new employer that you have had cancer.

If you are asked about your previous health then you must be honest as if it was discovered later that you had hidden the fact of having cancer in the past then your employer could dismiss you.

If you do have to give details about your previous illness then you can certainly add in any information you have which might help convince your employer that your risk of further problems is small. For example, saying that you had breast cancer five years ago but following treatment then have been well ever since and simply go for a check up every six months would give a far more positive message to your employers than simply saying that you had a history of breast cancer. Once again, though, the information you give must be true.

It may helpful for your doctor to write a letter to your employer explaining the details of your cancer and the chances of any problems occurring in the future.