Theoretically, tumors could be destroyed with a single dose of radiation, but in reality, the damage that would accrue to healthy tissue prevents this in X-ray therapy. The affected tissue will recover better if radiation treatments are restricted to certain doses at each session. In conventional radiotherapy, radiation is always administered in individual sessions called fractions. The total number of fractions ranges from 4 to 35, usually with one fraction per treatment day. This results in a relatively long total treatment period and may reduce the chance of a cure, because the tumor also recovers between individual sessions.
The reduced burden placed on surrounding healthy tissue by proton therapy enables the number of sessions to be reduced. With Proton Therapy and it’s high precision scanning system the radiation dose absorbed by healthy tissue is only 1/3 to 1/5 of X-ray therapy, allowing an increased effective dose to be administered in no more than 15 total sessions. In practice, however, the number of sessions is set such that the equivalent daily maximum dose of X-ray radiation—both locally and for the entire body–is never exceeded. Your attending physician will let you know how many sessions are planned and how long the full course of treatment will last.
There are some circumstances where the time compression of proton therapy conveys no advantage: for example, if tumor cells and vital healthy tissue are so closely intertwined that the proton beam targets both types of tissue, as occurs with certain brain tumors. When diseased and healthy cells are commingled, the total dose must be distributed across a larger number of individual sessions, similar to X-ray treatment. However, this situation has become a rarity due to the targeting accuracy of the protons.