Purpose: To assess the effect of a rectal enema on interfraction prostate movement in bone alignment (BA) for prostate radiotherapy (RT), we analyzed the spatial difference in prostates in a bone-matched setup.
Materials and methods: We performed BA retrospectively with data from prostate cancer patients who underwent image-guided RT (IGRT). The prostate was identified with implanted fiducial markers. The setup for the IGRT was conducted with the matching of three fiducial markers on RT planning computed tomography images and those on two oblique kV x-ray images. Offline BA was performed at the same position. The coordinates of a virtual prostate in BA and a real prostate were obtained by use of the ExaxTrac/NovalisBody system, and the distance between them was calculated as the spatial difference. Interfraction prostate displacement was drawn from the comparison of the spatial differences.
Results: A total of 15 patients with localized prostate cancer treated with curative hypofractionated IGRT were enrolled. A total of 420 fractions were analyzed. The mean of the interfraction prostate displacements after BA was 3.12±2.00 mm (range, 0.20-10.53 mm). The directional difference was profound in the anterior-posterior and supero-inferior directions (2.14±1.73 mm and 1.97±1.44 mm, respectively) compared with the right-left direction (0.26±0.22 mm, p<0.05). The required margin around the clinical target volume was 4.97 mm with the formula of van Herk et al.
Conclusions: The interfraction prostate displacement was less frequent when a rectal enema was performed before the procedure. A rectal enema can be used to reduce interfraction prostate displacement and resulting clinical target volume-to-planning target volume margin.
Keywords: Displacement; Enema; Image guided radiotherapy; Prostate neoplasms.