A simple method for the evaluation of side doses in radiotherapy

Abstract

Radiation protection is taken into account in radiotherapy practice from the point of view of the medical aspect in order to achieve the best ratio between the tumour control probability (TCP) and the normal tissue complication probability (NTCP). But any radiotherapy treatment implies a dose delivered to the patient's body also outside the beams. Therefore, there is a certain interest important to quantify that which can be considered a negative impact of radiotherapy also if it cannot absolutely be avoided and, in any case, the benefit of radiotherapy should prevail. The dose delivered to the patient by the radiation scattered outside the direct beams has been measured in general in phantoms. This procedure has the advantage of avoiding the direct involvement of the patient but, on the other hand, a phantom implies a certain degree of approximation in the reproduction of the real characteristics of a human body. In particular rather inaccurate phantoms are normally used. This simple solution supplies reliable results to some specific questions as, e.g., the measurement of the collimator scatter or the dose distribution in the penumbral region. But when the dose to the whole body or to a single organ has to be evaluated, such phantoms are totally unreliable. In fact the dose distribution within the body varies notably with the distance from the target volume and the average value can be identified with difficulty if only one value is available. The scope of this study is the description of a simple method for the evaluation of the dose, outside the beams, delivered to the body of a patient submitted to external radiotherapy by means of direct measurements. The radiation scattered by the target was monitored by means of pen dosimeters distributed along the body. A virtual model was obtained by dividing the body into about 800 small volumes where the doses could be assumed uniform. The power functions obtained by the best fit of the experimental values were used to calculate the effective dose delivered to the body and single organs due to scattering only. © 2006 Springer.