Objective: To investigate interfractional motion of the mesorectum and bladder and to assess dosimetric changes using cone-beam computed tomography (CBCT) during neoadjuvant radiotherapy for locally advanced rectal cancer patients.
Methods: Twenty-one patients who underwent volumetric arc therapy with CBCT imaging protocol were retrieved. The mesorectum and bladder were delineated on every CBCT image, and treatment plans were recalculated for all CBCTs. The organ motion was analyzed as a mean shift on the X-Y-Z axes. The volume changes were evaluated using the DICE index. Mann-Whitney U test was used in pairwise comparison analysis and ANOVA was used to compare shifts in each direction.
Results: A total of 105 CBCTs were evaluated retrospectively. The movement of the total mesorectum was found to be 1.5 mm, 4 mm, and 5 mm on the X-Y-Z-axes, respectively. In the subgroup analysis, the movement of the 1/3 upper mesorectum on the Y-axis was significantly higher (mean movement 8 mm, p = 0.005). Mean bladder displacements were 2 mm, 4 mm, and 8 mm on the X-Y-Z-axes, respectively. In the D2, D95, and D98 doses, there was no statistically significant change depending on the motion.
Conclusion: During radiotherapy planning, the mesorectal movement should not be forgotten and PTV margins should be determined accordingly (Tab. 6, Ref. 22). Text in PDF www.elis.sk Keywords: cone-beam computed tomography, rectal cancer, mesorectum, interfractional organ motion, neoadjuvant radiotherapy.