[Automatic opto-electronic control of patient position in radiotherapy for breast carcinoma]

We report on the technical and clinical validation of a system for real-time analytical control of patient position at simulation and treatment units in radiotherapy.

The positioning control system uses a technology for motion analysis consisting of an optical component (a pair of TV cameras) and of a unit for real-time image processing. The system can provide real-time (up to 100 times a second) three-dimensional (3D) coordinates of a set of passive markers (small plastic hemispheres, 5 mm O) previously positioned on the patient and located within the field of view of the system's TV cameras. The method for positioning quality control is based on the analytical comparison between the current positions of the set of markers placed on the patient's skin and a corresponding reference pattern, the latter acquired at the end of the simulation procedure or at the first irradiation session. The system was used at the Radiotherapy Division of the European Institute of Oncology for the analytical control of repositioning in three patients submitted to irradiation after conservative surgery (quadrantectomy) for breast cancer. This showed the clinical feasibility of both the technology and the method, and permitted to quantify improvement in patient positioning relative to current repositioning procedures. In particular, the possibility to evaluate the patient's breathing phases allowed to distinguish the different factors (repositioning inaccuracies and cyclic and random patient movements) contributing to global localization errors.

This method is independent of physical and geometrical irradiation parameters and permits efficient real-time quantitative control of specific repositioning quality and of actual patient immobility during irradiation. Assessment of the accuracy of conventional repositioning methods based on laser alignment showed a fair performance of the optical centering procedure (with 3D displacements < 5 mm) only for the markers placed close to the skin landmarks used for laser alignment. On the contrary, the markers within the irradiation field but not directly controlled during the repositioning procedure exhibited localization errors > 5 mm; this happened even though the inaccuracies related to patient's breathing movements had been excluded from analysis. Moreover, quantitative assessment of global localization errors confirmed the high influence of breathing movements on the position repeatability and maintenance. In this case, even the markers which were on average well repositioned turned out to be significantly displaced during the radiation dose delivery.

Our results confirm that real-time motion analysis based on opto-electronic techniques can play a crucial role as a means of improving patient positioning and assessing immobility. Thus, the system permits to quantify errors in target volume localization, which permits to adopt suitable countermeasures to reduce uncertainties during actual irradiation. This is a crucial requirement, particularly when the complexity of the irradiation geometry (conformal radiotherapy) or radiation type (hadrontherapy) calls for optimal application of the simulated treatment plan to each actual irradiation session.