Precise target localisation in conformal and fractionated radiotherapy: accuracy assessment of a positioning algorithm.

The goal of radiotherapy treatments is to irradiate as well as possible the lesion while minimizing the undesirable effects onto the neighbouring healthy tissue. Two popular techniques are based in this idea. For the fwst one conformal Radiotherapy, CRT) the irradiation dose applied at the tumour centre Is increased, whereas the target volume is shrunken. For the second technique Eractioned Radiotherapy, FRV the irradiation dose is applied in several sessions, the regeneration of healthy cells being better than that of carcinogenic ceb when low irradiation energy is employed. Patients positioning in therapy room is one of the crucial steps of a treatment for both CRT and FRT techniques. In CRT, the method usually used is based in stereotactic frames. This method is invasive and represents a real trauma for the patient. The positioning methods in FRT are generally complicated and expensive. This paper describes a positioning method which is non innsive, simple to implement and precise. This method authorizes the combinations of FRT and CRT in order to take advantage of the two techniques. The image processing technique used for the tumour localization in the treatment room is based on the registration of the patient??s head data given by a CT-scan, witb the patient??s face data acquired with a 3D sensor in the therapy room. The paper describes also the tests performed to evaluate the real positioning accuracy of the proposed method. The test were done with a radiographic system of a simulation room and with pre-operative data (CT-scan). The mean positioning error measured is less than 1 mm.