This turns out to be more complex than one would hope. They want to give a certain number of Gy Over a certain number of sessions. The thing is, there’s no real consistency to the exact treatment since they use a formula to figure out the Gy and session, and there can be variation to receiving the same result.

How do radiation oncologists make sense of these numbers? First, it is important to note that both the total dose and the dose-per-fraction influence the overall intensity of the treatment. In fact, there is a formula that takes these two factors and computes a single number that quantifies the overall strength of the treatment. For instance, this formula predicts that for breast cancer, giving 45 Gy over 25 sessions is about equal to giving 40 Gy over 15 sessions. In other words, even if radiation clinic A uses the first combination of dose and session number while radiation clinic B uses the second combination, the overall effect of the treatment on the cancer is the same in both cases.