Practical implications of backscatter from outside the patient on the dose distribution during total body irradiation

Total body irradiation (TBI) sometimes requires the set-up of the patient very close to the wall of the treatment room in order to obtain sufficiently large irradiation fields. Under these conditions, backscattered electrons can become clinically important. In the present study, an attempt was made to quantify the dose contribution to the patient from these electrons. Measurements were performed both in experimental conditions and on patients during their TBI treatment. It is concluded that, with the patient close to the wall, backscattered electrons constitute a significant (up to 20% of the dose obtained under electronic equilibrium at the exit port of the beam) radiation dose which can (under certain conditions) influence measurements of exist dose leading to an overestimation of the midline dose and contribute a superficial irradiation of the patient without therapeutic benefit. This problem can be solved by interposing a 2 cm thick low-Z absorber between wall and patient.