Influence of respiration on calculation and delivery of the prescribed dose in external radiotherapy.

During the delivery of the prescribed dose in external beam radiotherapy, respiration alters the patient's body outline and consequently the basis for dose calculations. The influence of this effect was studied on both healthy volunteers and patients. In a subgroup of volunteers, we measured that respiratory movements modify a person's thickness, in particular in the antero-posterior direction at the lower abdominal level, by more than 4% implying a variation in absorbed dose of more than 2%. In view of the I.C.R.U. requirements for absorbed dose accuracy, these variations must be taken into account. Extension of the study to a group of 160 patients confirmed that for a subgroup of about 25%, respiration induces an important deviation from the prescribed dose as dose calculations are based on a body outline taken at an unknown stage during the respiratory cycle. Our results indicate that dose calculations should be based on an average body outline which takes the respiratory movements into account.