Is radiation a cause of cancer?

Yes: Environmental exposure (as in hiroshima, chernobyl) can lead to blood and thyroid cancer decades later. Occasionally, radiation used for treatments or frequent scanning (x-rays, ct scans) can lead to cancer later in life.

Potentially, yes: Environmental radiation exposure may cause cancer. Radiation treatment for cancer may also lead to other cancers (called "second cancers") years after the treatment was received. While this is uncommon, it is one of the potential risks of receiving radiation treatments.

Genetics: Radiation can damage dna. Damaged dna can alter the genetics of any cell in the body and cause it to lose it's natural regulatory processes that make it a normal cell. Usually the body is good at repairing this damage itself, unless it gets overwhelmed. Then an unregulated cell is the building block for cancer, and when replicated it will eventually form a tumor.