Validation des petits champs de photons pour une utilisation en radiothérapie stéréotaxique appliquée aux régions pulmonaires

Abstract

The Stereotactic Body Radiation Therapy (SBRT) is a highly conformational external beam radiation therapy technique which can treat tumors with an extreme precision. Doses in stereotactic conditions are very high and delivered in a hypo-fractionated schedule. At the CHU Tivoli, this method is used to treat pulmonary tumors, normally by delivering 48 Gy to the target, in 4 fractions. This work pursues a double objective. The first one is to carry out the necessary measurement and verifications to perform pulmonary SBRT with fields sizes from 4 x 4 to 2 x 2 cm . Nevertheless, the dosimetry of small beams represents a major challenge caused mainly by the loss of lateral electronic equilibrium as well as the volume averaging effect. Therefore, it is important to choose the right detectors and to proceed to manipulations with high precision. The second objective consists in defining the procedure to be followed and the choices that will facilitate the commissioning of small fields on the future linear accelerators of CHU Tivoli, scheduled for 2021. After comparing the lateral dose profiles, percent depth doses and output factors with some small fields specific detectors, we modeled the beam in the TPS and configured the AAA algorithm from the data acquired with the diode E 60017 from the firm PTW. The match between the TPS calculations and the dose delivered by the accelerator was first verified by comparing the latter with measurements under simple conditions, identical to the manipulations. The satisfactory results of the gamma analysis ensure the correct configuration of the algorithm. Then, to validate the clinical use of small beams in VMAT, stereotactic treatment plans, based on a computed tomography acquisition of a homemade antropomorphic thoracic phantom, were verified with the aS500 portal imager, the OCTAVIUS 4D and via irradiation of the phantom. The results allow to treat clinically tumors located in lungs’ center areas in VMAT using small fields. However, if a small tumor is placed at the edge of the lung parenchyma, the complexity of the treatment plan combined with the equipment and algorithm used didn’t provide the desired results. Consequently, it would be interesting to repeat some verifications with the new TrueBeam accelerators and the AcurosXB algorithm, in 2021.