Hi Nicki,

I believe that the maximum dose a individual can receive in a lifetime in 81 Gy (gray). Although there are clinical trials going on as we speak for double that amount.

Some more information about radiation with a little background to start off with:

Ionizing radiation is a known cause of cancer and other adverse effects. It is one of the most extensively studied human carcinogens and may account for about 3 percent of all cancers (NAS, 1990). Ionizing radiation is able to remove electrons from atoms and to change the molecular structures of cells. It is these cellular changes that may cause cancer to develop. The genetic DNA in the cell nucleus is thought to be the critical target for radiation-induced damage. (this is one of the true paradoxes of cancer treatment - that one of the things that causes cancer is also used to treat it! It does it by altering the DNA of the cancer cells so they cannot replicate)

Some radiation comes from natural sources, such as that from cosmic rays and radioactive substances in the earth's crust. Each of us is exposed to this "background" radiation at a rate of about 1 to 2 mGy per year (m=10-3 or 1/1000th). (The gray [Gy] is a unit of measurement for the amount of radiation energy absorbed by body tissues. It is equal to 100 rad and is now the unit of dose.)

Very high doses of radiation (tens of gray) received all at once may be fatal, but if spread out over a period of time, a high dose of radiation may be less damaging to healthy tissues. A single, 5-Gy dose of whole-body irradiation, for example, would cause about half the people exposed to it to die within 30 days. But patients who receive daily radiation therapy treatments of 2 Gy directed to a small area of the body can be exposed to tens of gray over a period of weeks.

Source http://seer.cancer.gov/publications/raterisk/risks90.html