I might add to Marks response that different cell types have varying times to recover as well. The red blood cells are replaced by the body every 120 days so the ones that are destroyed, passing through the carotid during RT, will get to anemic levels or close to it. The bone marrow is affected as well (mainly by certain chemos which affect white cell production) and takes a long rime to recover. Salivary glands, not directly in the beam field, take 18 months or more. Taste buds start returning in a couple of months but can take a year or more for more sophisticated taste sensation to return. Skin cells heal very quickly. Sometimes neck muscles are made fibtrotic by the radiation and never return to normal. (The fastest healing cells in the body are the cells in the cornea). The way that radiation (and most chemo as well) works, simply, is that it alters or damages the DNA in cancer cells so it can't replicate itself so it simply shrivels up, dies and is absorbed into the body (that's the theory anyway).

Another thing I might add is that a linear accelerator (LINAC), used for medical RT use, actually IS a gigantic microwave source that, when the beam is directed into a water cooled (usually titanium or tungsten) target, is transformed into high energy level x-radiation, anywhere from 6 MeV to 20 MeV (MeV = Millions of electron Volts), depending on the power source and penetration depth. Most OC is treated in the 6 MeV range.

One more thing, they don't measure dosage in "rads" anymore - that stopped decades ago. Like "cycles" were changed, in the 60's, to honor the scientist "Hertz", "rads" were changed to "Greys" (Gy, to honor the scientist "Grey") [100 rads = 1 Gy]. The maximum total dose rate, given to OC patients can range between 60-72 Gy. Anything beyond 66 Gy is considered a "boost" and is sometimes done to slam certain areas a little harder.

I also understand that the word rad was also changed to disassociate it from nuclear energy, as in the type, used in weapons and atomic bombs, and/or older cobalt 60 machines, all sources of "gamma" radiation. LINAC RT differs in that it is "ionizing" radiation and the LINAC must be on, and generating a beam, for radiation to be present. Gamma sources are always generating radiation (cobalt 60 machines are rarely if ever used in the US today but are still used in some 3rd world countries). The gamma source is simply controlled by mechanically moving 2 or 4 lead bricks (like doors or better, a variable aperture) on top of a lead box covering the radioactive source. The downside of gamma is that the source has a 1/2 life, so as the radiation loses it's potential, dose times must be increased. Often times this would result is many undesireable side effects, such as severe and permanent skin damage from the "dark" radiation found in gamma type sources.

RT is given in "fractional" doses, typically not to exceed about 1.8 - 2.0 Gy/day. If you were to be given the total dose rate at one time it would be lethal. This also determines how many treatment days you will actually have. Some new theories are being tested to split the dose into 2 sessions of .9 - 1.0 Gy. Typical treatment is 5 days a week with the weekends off. The daily dose rate was calculated on the bodys natural ability to repair healthy cell tissue.

These are the primary reasons that we say each patient responds differently to treatment and my responses listed above are generalizations.

Sorry to get so technical but I worked for the nuclear physicist who invented the Siemens LINAC and a beam bending technique that minimizes harmful radiation.