Inter- and Intrafractional Tumor and Organ Movement in Patients With Cervical Cancer Undergoing Radiotherapy: A Cinematic-MRI Point-of-Interest Study

Purpose: Internal tumor and organ movement is important when considering intensity-modulated radiotherapy for patients with cancer of the cervix because of the tight margins and steep dose gradients. In this study, the internal movement of the tumor, cervix, and uterus were examined using serial cinematic magnetic resonance imaging scans and point-of-interest analysis. Methods and Materials: Twenty patients with Stage IB-IVA cervical cancer underwent pelvic magnetic resonance imaging before treatment and then weekly during external beam radiotherapy. In each 30-min session, sequential T₂-sagittal magnetic resonance imaging scans were obtained. The points of interest (cervical os, uterine canal, and uterine fundus) were traced on each image frame, allowing the craniocaudal and anteroposterior displacements to be measured. The mean displacements and trends were analyzed using mixed linear models. Prediction intervals were calculated to determine the internal target margins. Results: Large interscan motion was found for all three points of interest that was only partially explained by the variations in bladder and rectal filling. The intrascan motion was much smaller. Both inter- and intrascan motion was greatest at the fundus of the uterus, less along the canal, and least at the cervical os. The isotropic internal target margins required to encompass 90% of the interscan motion were 4 cm at the fundus and 1.5 cm at the os. In contrast, smaller margins of 1 cm and 0.45 cm, respectively, were adequate to encompass the intrascan motion alone. Conclusion: Daily soft-tissue imaging with correction for interfractional motion or adaptive replanning will be important if the benefits of intensity-modulated radiotherapy are to be maximized in women with cervical cancer.