Evaluation of intrafraction motion of the organs at risk in image-based brachytherapy of cervical cancer.

Abstract

To assess the variation in the doses received by the organs at risk (OARs) that can occur during treatment planning of cervical cancer by image-based brachytherapy. After intracavitary application, two sets of images-CT and MRI-were obtained. The two sets of images were fused together with respect to the applicator. Contouring was done separately on CT and MR images. Dose received by the OARs on CT images with respect to the plans made on the MR images was estimated and compared with those on the MR images. Although there was always a difference between the dose received by the OARs based on the CT and MRI contours, it was not significant for the bladder and rectum; 2cc doses differed by 0.49Gy (±0.44) p= 0.28 for the bladder and 0.30Gy (±0.29) p= 0.16 for the rectum. The 1cc and 0.1cc differences were also not significant. However for the sigmoid colon, there was significant intrafraction variation in the 2cc doses 0.61 (±0.6) p= 0.001, 1cc doses 0.73 (±0.67) Gy p= 0.00, and 0.1cc dose 0.97 (±0.93) Gy p= 0.009. The variation in the doses to the OARs must be considered while weighing target coverage against overdose to the OARs. Although not significant for the bladder and rectum, it was significant for the sigmoid colon. Estimated doses to OARs on the planning system may not be the same dose delivered at the time of treatment.