Intra Fraction Organs at Risk Movements in Adaptive Radiotherapy of Upper-Abdominal Stereotactic Body Radiotherapy on 0.35 T MR-Linac

Abstract

The recent development of magnetic resonance guided radiotherapy (MRgRT) has made possible adaptive radiotherapy (ART) especially for abdominal stereotactic body radiotherapy (SBRT). Online ART process allows to adapt the treatment at each fraction by considering the mobility of the organs at risk (OAR) and the target. These volumes are daily delineated and a new treatment plan is reoptimized. This process is multidisciplinary involving therapists, physicians and physicists. Time is a key element because of the presence of the patient on the treatment couch. In spite of having a well-trained team, the fraction duration is quite long, usually equal or higher than 45 min. Consequently, the elapsed time between the acquisition of the MR images used for the ART process and the end of the treatment delivery can be substantial. In this context, we decided to investigate the intra fraction OAR mobility by analyzing and comparing two images of the MR-Linac: one acquired at the beginning of the fraction and used for the ART process and another one immediately acquired at the end of the treatment delivery. The objectives of this study are to investigate the OAR mobility during the fraction and evaluate the possible impact on the dose distribution. Twenty patients treated in 5 fractions for upper-abdominal SBRT (liver, adrenal, pancreas, adenopathy) on the 0.35 T MR-Linac of our institution have been prospectively included in this study between May 2021 and August 2021. For each fraction an additional 3D MR image has been acquired immediately at the end of the treatment delivery. The OARs (colon, small bowel and duodenum) included in the ART process have been delineated on the post-fraction images. After having registered both images of each fraction, OAR volumes and their dose distributions have been compared. A high level of mobility of several OARs has been observed. For instance, a relative mean volume variation (increase or diminution) of 85%, 60% and 24% have been calculated, respectively, for the small bowel, the colon and duodenum. These level of volume variations strongly impacted the OAR dose distributions. For instance, the variation (increase or diminution) of maximum dose for colon, small bowel and duodenum was, respectively, about 4.3 Gy, 3.4 Gy and 2.8 Gy. These modifications led to exceed dose constraints in 2 fractions for colon, 4 fractions for small bowel and 3 fractions for duodenum. Nevertheless, by accumulating all the fractions, the dose constraints were always satisfied. OARs volume modifications during ART process can be significant and lead to important dose variations. At the scale of the treatment, these dose variations respect the dose constraints. In the near future, the correlation of these volumetric variations with the duration of the fraction will be investigated.