Many insurers are reticent to cover customers with pre-existing medical conditions including cancer, as they are considered more likely to need medical treatment whilst abroad and are therefore more likely to make a claim. However, travellers do not just require insurance for medical treatment but also for lost baggage, cancellation, delayed travel, etc, so it is important to have travel insurance to cover all eventualities.
Some mainstream insurers will insure cancer patients but will not include any claim that is related to needing any treatment for cancer or for any illnesses brought on by the condition. If customers do require some form of medical treatment abroad for their cancer then they will have to cover the expenses themselves.
If travellers require more comprehensive cover then it is advisable to use an insurance company which specialises in covering customers with pre-existing medical conditions. This is likely to be more expensive, particularly if you are travelling to the US, but you will be covered for all medical requirements. Do make sure you have details of your condition to hand; including your prognosis and treatment that you have already had, as you will be asked a series of comprehensive medical questions. It is also recommended that you obtain a letter from your doctor to say that you are well enough to travel.
In the first instance, it is worth contacting charities like Cancer Research, as they will have up-to- date advice and may even be able to recommend specialist insurers.