John was part of the trials for hypofractionation 7 years ago. The theory was that the cumulative effect of less RT given in higher doses over a shorter period might be more devastating to the cancer but they wanted to see what the optimum dose might be. The standard thinking had previously been that no more than 2Gy could be given in each fraction in the pelvic area but the trial tried a range up to 3.2 or 3.4Gy over 19 or 20 fractions. They concluded that for an otherwise fit male, 3.2Gy could be given without increasing the side effects (in fact, they found that the side effects were actually reduced) and crucially, that it was just as effective.
Cumulatively, 60Gy or 64Gy over 4 weeks delivers the same amount of energy as 74Gy given over 7.5 weeks. Crazy, isn't it!