Inter-fraction motion and dosimetric consequences during breast intensity-modulated radiotherapy (IMRT)

Abstract

Background and purpose Intensity-modulated radiotherapy (IMRT) can improve dose homogeneity within the breast planned target volume (PTV), but may be more susceptible to patient/organ motion than standard tangential radiotherapy (RT). We used daily cone-beam CT (CBCT) imaging to assess inter-fraction motion during breast IMRT and its subsequent impact on IMRT and standard RT dose homogeneity. Materials and methods Ten breast cancer patients selected for IMRT were studied. CBCT images were acquired immediately after daily treatment. Automatic image co-registration was used to determine patient positioning variations. Daily PTV contours were used to calculate PTV variations and daily delivered IMRT and theoretically planned tangential RT dose. Results Group systematic (and random) setup errors detected by CBCT were 5.7 (3.9) mm laterally, 2.8 (3.5) mm vertically and 2.3 (3.2) mm longitudinally. Rotations >2° in any axis occurred on 53/106 (50%) occasions. Daily PTV volume varied up to 23%. IMRT dose homogeneity was superior at planning and throughout the treatment compared with standard RT (1.8% vs. 15.8% PTV received >105% planned mean dose), despite increased motion sensitivity. Conclusions CBCT revealed inadequacies of current patient positioning and verification procedures during breast RT and confirmed improved dose homogeneity using IMRT for the patients studied.