Radiothérapie asservie à la respiration en combinaison avec l'utilisation d'un faisceau sans filtre égalisateur
Stereotactic body radiation therapy (SBRT) is a technique commonly employed for treatment of lung tumors when surgery is not possible or not accepted by the patient. One complication arising from the use of this method comes from the movement of the tumor during respiration. In this context, respiratory gated radiation therapy (RGRT) can be beneficial. By using a flattening filter free (FFF) photon beam, the increase in treatment time caused by a reduced beam-on time of respiratory gated methods can be compensated by the inherent increased dose rate of FFF beams. This thesis reports on the feasibility of using the RGRT technique in combination with the use of a FFF photon beam on a Synergy S (Elekta, Stockholm, Sweden) linear accelerator with a pneumatic belt, the Bellows Belt (Philips, Amsterdam, Netherlands), to monitor the patient’s respiratory signal. A Synergy S has been modified to deliver a 6 MV FFF photon beam. Dose profile and percentage depth dose measurements were taken in a water tank for different field sizes. Those measurements were used to create a model for the 6 MV FFF beam with the Pinnacle3 treatment planning system from Philips. Measurements were compared with the model using gamma index analysis with a 2%, 2 mm criterion. Then, five SBRT plans with volumetric modulated arc therapy (VMAT) were created in Pinnacle3 with the 6 MV Synergy S model, with and without a flattening filter. A comparison of dosimetric parameters was made between plans with and without a flattening filter to estimate the quality of the FFF plans. Results reveal that it is possible to create SBRT VMAT plans with the 6 MV FFF model of the Synergy S that are clinically acceptable (criteria of the Radiation Therapy Oncology Group 0618 were respected). Also, a RGRT hardware interface was created to fulfill two main functions: read the digital signal from the Bellows Belt pneumatic belt and send an on/off irradiation command to the linac. The activation/deactivation of the beam is regulated by an electromechanical relay. The interface is composed of a homemade printed circuit board that functions with a Raspberry Pi. A RGRT software was also developed to operate on the Raspberry Pi. This software shows the Bellows Belt’s digital signal and gives the option of choosing the upper and lower limits of the gating window. When the respiratory signal of the belt is between those limits, the beam is active, and vice versa when the signal is outside those limits. The software thus effectively sends an on/off irradiation command automatically to the linac depending on the amplitude of the respiratory signal. Finally, a comparison between the delivery of a standard plan without RGRT, with filter, and another standard plan without RGRT, without filter, shows that the treatment time for plans using the FFF beam is reduced by 54.1% on average for one arc. Similarly, a comparison between the delivery of a standard plan without RGRT, with filter, and a gated plan (gating window of 75%), without filter, shows that the treatment time for the gated treatments is reduced by 27.3% on average for one arc. However, it was not possible to deliver RGRT treatments with a gating window smaller than 75%. The linac does not support such a high frequency of beam halting.
Respiration Bellows Belt Synergy S Radiotherapy Respiratory gated radiation therapy Flattening filter free Stereotactic body radiation therapy