Radiotherapy treatment planning

Any subsequent irradiation is generally based on three-dimensional radiotherapy treatment planning. To that end, a computed tomography scan providing a three-dimensional image data set is done.

In this image data set, the target volume to be treated is delineated by a radiation oncologist on a slice-by-slice basis. This is performed for the tumour itself and also for the area of its microscopic (not visible in images) spread, for example along the lymph channels. Not including this area in the treatment would result in an increased risk of relapse for the patient.

There may be slight deviations in the couch position of the patient varying from treatment day to treatment day. Inner organs can shift their position (for example, movement of abdominal organs due to breathing activity, peristaltic motion, variations in bladder filling). Additionally, a safety margin is taken into account to compensate for the entailed positional inaccuracies.

The next step is physical radiotherapy treatment planning. On a computer, medical physics experts select the beam orientation and radiation field shapes that allow optimum dose distribution. The target volume needs to be covered uniformly. At the same time, surrounding organs at risk should be spared as much as possible.