The potential impact of the tension of the pelvic muscles on set-up errors in radiotherapy for pelvic malignancies

Abstract

The purpose of the study reported here was to evaluate the potential impact of the tension of pelvic muscles on set-up errors. Twenty-nine consecutive patients with rectal cancer were included. The treatment simulation of the lateral beam in prone position was performed twice–with relaxed and next with maximally tense pelvic muscles. During the second simulation, the couch was moved so as to align the centre of the beam with the actual position of the skin mark tattooed during the first simulation. The bony landmarks on both images of corresponding lateral fields were matched. The beam's centre displacement and the rotation were measured using the beam image taken in relaxed position as a reference. The absolute values were used in calculation of the mean. For the anterior–posterior direction, the mean value of displacements was 15.3 mm, standard deviation (SD) 6.9 mm and the maximal value 37 mm. For the cranial–caudal direction, the mean value was 4.4 mm, SD 4 mm and the maximal value 17 mm. The mean rotation of the pelvis was 5.3 degrees, SD 2.4 degrees and maximal rotation 11 degrees. The majority of displacements were in the posterior (86%) and caudal (55%) directions. The majority of rotations were clockwise (76%). It was shown that pelvic muscle tension was the reason for anal verge displacements and mispositionings of the shielding block. This results in set-up inaccuracy, especially in the anterior–posterior direction, shielding block mispositioning and anal verge displacement.