Cancer insurance
Your major medical policy will generally provide benefits regardless of the type of sickness or injury you have including cancer. Typically a major medical policy will meet most of your health insurance needs.
However, there are usually deductibles and co-insurance provisions with a major medical policy and there are non-medical costs associated with the treatment of cancer that a supplemental policy could provide some assistance for.
One should always carefully analyze his/her overall insurance needs prior to making any decision on additional insurance purchases.