Three‐dimensional assessment of interfractional cervical and uterine motions using daily magnetic resonance images to determine margins and timing of replanning

PURPOSE: This study was conducted to determine the margins and timing of replanning by assessing the daily interfractional cervical and uterine motions using magnetic resonance (MR) images. METHODS: Eleven patients with cervical cancer, who underwent intensity‐modulated radiotherapy (IMRT) in 23–25 fractions, were considered in this study. The daily and reference MR images were converted into three‐dimensional (3D) shape models. Patient‐specific anisotropic margins were calculated from the proximal 95% of vertices located outside the surface of the reference model. Population‐based margins were defined as the 90th percentile values of the patient‐specific margins. The expanded volume of interest (expVOI) for the cervix and uterus was generated by expanding the reference model based on the population‐based margin to calculate the coverage for daily deformable mesh models. For comparison, expVOI(conv) was generated using conventional margins: right (R), left (L), anterior (A), posterior (P), superior (S), and inferior (I) were (5, 5, 15, 15, 10, 10) and (10, 10, 20, 20, 15, 15) mm for the cervix and uterus, respectively. Subsequently, a replanning scenario was developed based on the cervical volume change. ExpVOI(ini) and expVOI(replan) were generated before and after replanning, respectively. RESULTS: Population‐based margins were (R, L, A, P, S, I) of (7, 7, 11, 6, 11, 8) and (14, 13, 27, 19, 15, 21) mm for the cervix and uterus, respectively. The timing of replanning was found to be the 16(th) fraction, and the volume of expVOI(replan) decreased by >30% compared to that of expVOI(ini). However, margins cannot be reduced to ensure equivalent coverage after replanning. CONCLUSION: We determined the margins and timing of replanning through detailed daily analysis. The margins of the cervix were smaller than conventional margins in some directions, while the margins of the uterus were larger in almost all directions. A margin equivalent to that at the initial planning was required for replanning.