As with most things the answer is usually "it depends".
My docs said it would depend on the type of radiation, location and amount of time between radiation treatments. They also said that I could have more HDR brachytherapy procedures if necessary.
My wife had breast cancer in 1996 and treatment included 35 External Beam radiation sessions and was told she could never have radiation again. She again had breast cancer in 2010 in the same breast, but a totally different cancer, not a recurrance of the first. After careful study of the previous radiation, her rad-onc and all her docs agreed that she could and should do radiation again and had another 35 IMRT sessions.
The general rule of thumb is that once you have had IMRT to the prostate area you wouldn't radiate the area again. However, I think the good rad-onc will be guided by what is best indicated by the the situation at hand and the overall history of treatment. As my rad-onc said when I posed the question before I was radiated "it all depends".
PSA 59 on 8-26-2010 age 60. Biopsy 9-8-2010 12/12 positive, 20-80% involved, PNI in 3 cores, G 3+3,3+4,and 4+3=G7, T2b.
Eligard and Jalyn started on 10-7-2010. IMRT to prostate and lymph nodes started on 11-8-2010, HDR Brachytherapy December 6 and 13, 2010.
PSA < .1 and Testosterone less than 3 since February 2011