Radiotherapy sessions (fractions)
Theoretically, tumours could be destroyed with a single radiation dosage. However, damage to healthy tissue prevents this in x-ray therapy. The affected tissue will have a more effective recovery if the tumour dosages are restricted to specific radiation amounts each day. In conventional radiotherapy, the radiation dosages are always carried out in individual sessions called fractions. The total number of fractions is between 4 and 35, generally comprising one session a day. This not only results in a relatively long total treatment period but also has a tendency to impair the chances of a successful treatment, because the tumour naturally recovers as well during the intervals between individual sessions.
Fewer sessions are beneficial
The lower burden on healthy tissue in locations of proton therapy means there can be a reduction in the number of sessions. In the case of Voxel-to-Voxel Modulated Scanning Proton Therapy (VVM-SPT), the radiation dosage in healthy tissue is 50% to 70% lower than in x-ray therapy. The effective dosage can therefore be increased, so that the maximum number of sessions is often only 15 sessions. In practice, the number of sessions is selected so that the equivalent daily maximum dosages in x-ray radiation--locally and to the entire body--are certainly exceeded. Your attending physician will keep you informed about the number of sessions planned and how long the overall treatment will last.
The time benefit of proton therapy is not useful in only a few cases: namely, if tumour cells and vital healthy tissue are so intimately interwoven that the proton beam is targeted at both types of tissue, as in the case of certain brain tumours. In a case where diseased and healthy cells are comingled, the total dosage is distributed over a larger number of individual sessions in a similar way to x-rays. However, this situation has become a rarity due to the targeting accuracy of the protons.