Determination of Population-Based Anisotropic Margin for Uterus and Cervix During a Course of Magnetic Resonance-Guided Intensity-Modulated Radiation Therapy
Y Kishigami1*, M Nakamura1, M Nakao1, H Okamoto2, A Takahashi2, H Igaki2, (1) Kyoto University, (2) National Cancer Center Hospital
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Purpose: To determine population-based anisotropic margin for uterus and cervix from daily contours acquired during the course of magnetic resonance (MR)-guided intensity-modulated radiation therapy (IMRT).
Methods: Eleven patients with cervical cancer who underwent MR-guided IMRT were included. A total dose was 46 Gy/23 fr. in one patient, 46 Gy/25 fr. in one patient and 45 Gy/25 fr. in nine patients. Daily MR images were acquired and co-registered to planning MR images based on the bony structure. Organ delineation was conducted manually by a single radiation oncologist. First, the vertices that displaced outside the reference contour determined at planning were identified. Subsequently, patient-specific anisotropic margins that cover the vertices in the nearest X% from the reference contour were defined for 6 directions: right (R), left (L), anterior (A), posterior (P), superior (S), and inferior (I). Thereafter, P-percentile value of patient-specific margin were used as the population-based margin, and coverage probability for uterus and cervix was calculated for each patient.
Results: As the value of X increased, the required margin size became larger for uterus than for cervix. At X of 95, 95-percentile values of patient-specific margins in [R, L, A, P, S, I] directions for uterus and cervix were [18.6, 11.6, 23.0, 20.4, 20.5, 23.0] and [6.8, 5.2, 9.2, 5.0, 9.4, 6.2] mm, respectively. When applying these values to each patient, the coverage probability ranged 89 to 100% for uterus and 94 to 100% for cervix.
Conclusion: This is the first reported study to determine the anisotropic margins for uterus and cervix from daily contours acquired during the course of MR-guided IMRT. The optimum margin sizes proposed in this study can be useful for facilities with adaptive radiotherapy non-available.
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