Assessing intra- and interfraction motion and its dosimetric impacts on cervical cancer adaptive radiotherapy based on 1.5T MR-Linac

Purpose: The purpose of this study was to quantify the intra- and interfraction motion of the target volume and organs at risk (OARs) during adaptive radiotherapy (ART) for uterine cervical cancer (UCC) using MR-Linac and to identify appropriate UCC target volume margins for adapt-to-shape (ATS) and adapt-to-position (ATP) workflows. Then, the dosimetric differences caused by motion were analyzed.

Methods: Thirty-two UCC patients were included. Magnetic resonance (MR) images were obtained before and after each treatment. The maximum and average shifts in the centroid of the target volume and OARs along the anterior/posterior (A/P: Y axes), cranial/caudal (Cr/C: Z axes), and right/left (R/L: X axes) directions were analyzed through image contours. The bladder wall deformation in six directions and the differences in the volume of the organs were also analyzed. Additionally, the motion of the upper, middle and lower rectum was quantified. The correlation between OAR displacement/deformation and target volume displacement was evaluated. The planning CT dose distribution was mapped to the MR image to generate a plan based on the new anatomy, and the dosimetric differences caused by motion were analyzed.

Results: For intrafraction motion, the clinical tumor volume (CTV) range of motion along the XYZ axes was within 5 mm; for interfraction motion, the range of motion along the X axis was within 5 mm, and the maximum distances of motion along the Y axis and Z axis were 7.45 and 6.59 mm, respectively. Additionally, deformation of the superior and anterior walls of the bladder was most noticeable. The largest magnitude of motion was observed in the upper segment of the rectum. Posterior bladder wall displacement was correlated with rectal and CTV centroid Y-axis displacement (r = 0.63, r = 0.50, P < 0.05). Compared with the interfractional plan, a significant decrease in the planning target volume (PTV) D98 (7.5 Gy, 7.54 Gy) was observed. However, there were no significant differences within the intrafraction.

Conclusion: During ART for UCC patients using MR-Linac, we recommend an ATS workflow using isotropic PTV margins of 5 mm based on intrafraction motion. Based on interfraction motion, the recommended ATP workflow uses anisotropic PTV margins of 5 mm in the R/L direction, 8 mm in the A/P direction, and 7 mm in the Cr/C direction to compensate for dosimetric errors due to motion.