Radiation therapy (or radiotherapy) is one of the main treatment modalities for cancer, along with surgery and chemotherapy. About 50% of the newly diagnosed cancer patients (more than 3 million each year in Europe and 12 million globally) are treated with radiotherapy. Because each patient is anatomically unique, a personal treatment plan is generated. A treatment plan contains information on how the dose, and consequently the probability of physical damage from irradiation is distributed inside the patient. The goal is to deliver a sufficient amount of dose to the tumour for eradication, while minimizing the (unavoidable) dose to the other structures (organs-at-risk), where each structure has a different radiosensitivity.

Healthy tissue damaged by irradiation can cause severe short and long-term complications. For example, reduced functionality of salivary glands after radiotherapy treatment of a head-and-neck tumour may result in xerostomia (dry-mouth syndrome). Xerostomia urges the patient to drink some water every half-an-hour (day and night). Side-effects after radiotherapy of prostate cancer may be rectal bleeding, urinal and erection problems. It goes without saying that each complication has a significant impact on the quality-of-life of the patient, and it is thus of utmost importance to reduce the probability of developing treatment-induced complications.

Improving the treatment plans is one of the ways to achieve this. "Improving" implies reduced dose to the healthy tissue, or a better trade-off between doses to different organs, while still delivering a sufficient amount of dose to the tumour for erradication.

Radiotherapy treatment is performed by irradiating the tumour from several directions by ionizing beams. The treatment device has many degrees of freedom, and the result of the treatment planning are the machine parameter settings required to deliver the optimal treatment. This is illustrated in the following figure.

Prior to treatment planning, a CT-scan (Computed Tomography) is made to delineate the organs-at-risk and other structures. For some tumours to be identified it may additionally required to obtain an MRI, PET or SPECT scan as well.

The following animation gives an idea on how the dose is delivered inside the patient, when irradiating from 9 directions. The final image is the "treatment plan". The next section goes deeper in the typical characteristics of the dose itself.