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Irradiation should deliver the necessary and sufficient dose to a given target volume and, at the same time, offer the best protection possible to surrounding healthy tissue. Treatment execution involves several important steps.

Decision to irradiate

This decision is taken during the multidisciplinary meeting with physicians of various specialities: radiotherapist, surgeon, medical oncologist, pathologist and radiologist after a clear description of the tumour and the general patient status.

Certain preventive measures should be discussed according patient status:

• Stomatology advice (treatment of dentition, and fluoride rinses for head and neck cancers),
  
• Nutritionist advice when needed,
  
• Corticosteroid therapy (before brain irradiation),
  
• Haematological values, especially for large volume irradiation after chemotherapy,
  
• All radiographical examinations (scanner, RMI) necessary for the precise identification the target volume.

The patient should not be agitated or ache to the extent that he/she cannot remain calm during treatment fractions: sufficient quality sedation should be obtained beforehand.

One should also verify that healing of scars from previous surgery has been obtained.

Determining the target volume

It depends on:

• the size of the tumour (necessary radiological analysis),

• the natural history of cancer (systematic irradiation of supraclavicular nodes in oesophageal cancer of the upper third),
  
• the surgical operative report, the histological report (complete excision or not, capsular invasion in the node sample, and so on...)

IRCU Standards

According to IRCU standards (International Commission on Radiation Units and Measurements), various volumes are defined for irradiation:

GTV: or gross target volume, related to the apparent volume of the tumour (in red)

CTV: or clinical target volume, related to the usual tumour extension to surrounding tissue (local tumour invasion) (oin orange)

PTV: or planning target volume : (in blue ) related to the patient and tumour movements and beam imperfections (the two latter factors are called IM for internal margin and SM for set-up margin). Note that organs at risk OR are represented in dark green.

Note that healthy sensitive tissue may be found in the PTV and constitutes a major treatment risk.

The diagram shows the various volumes to be considered. GTV is the volume as seen by physicians. CTV is the volume calculated according to the pathological knowledge of usual local invasion by cancer. PTV is a compromise taking into account various physiological phenomena (respiration, difficulties in positioning the patient, patient movement) IM and SM. Normal tissue (in dark green) is included in the PTV. The arrows show various attempts to reduce parasitical irradiation, the ideal being of course the smallest volume.

The precise delimitation of the volume to be irradiated, taking into account the organs at risk and the dose delivered within such volumes is the responsibility of the radiotherapist helped by the medical physicist. Below is an example of treatment planning for a case of prostate cancer (the cancer volume is too small to be irradiated, therefore the whole prostate is irradiated (in orange). The rectum is one of the main organs at risk (in green) for which irradiation should be reduced as far as possible.

This diagram shows that with 4 fields, approximately 85% of the dose (60 to 65 Gy) is received by the rectum and approximately 50% by the femoral heads.