1009 - Quantitative Characterization of Pediatric Ovarian Mobility Using Serial MRI for Fertility-Preserving Radiotherapy

Purpose/Objective(s): Ovarian localization is critical for fertility preservation in pediatric patients receiving highly conformal pelvic RT. However, conventional planning that does not account for interfractional ovarian mobility may compromise reliable sparing. Leveraging weekly on-treatment MRI, we quantified inter- and intra-patient ovarian spatial variability in children and adolescents to inform prospective/retrospective assessment of ovarian dose and PRV design.

Materials/Methods: We analyzed 86 planning/on-treatment MRI studies from 26 patients (aged 6 months–19 years) treated with 3DCRT, IMRT, or proton RT on prospective clinical trials or standard clinical care. All MRIs were registered to the planning CT based on pelvic bony structures (ilium, pubis, ischium, and sacrum). A radiation oncologist contoured bilateral ovaries for each study, primarily on fat-suppressed 3D T2w MRI. To facilitate statistical analysis across varying body sizes, individual pelvic anatomies were aligned to the CT of a median-age patient (11 years) via affine transformation. The results were inversely transformed to report displacement ranges (cm) in individual images. A linear mixed-effects model was employed to analyze the spatial distribution of ovarian centroids, with age and bladder volume included as covariates.

Results: Craniocaudally, the ovarian centers across all patients and studies spanned from the level of posterior superior iliac spine to the superior margin of the ischial tuberosity (~9 cm in an 11-year-old), exceeding the anteroposterior range (pubic symphysis to posterior ischial spine) and the mediolateral range (midline to the outer pelvic brim). In younger patients, ovaries were more medial (P = 0.008), with a marginal anterior trend (P = 0.058). Larger bladder volumes were associated with posterior and superior displacement (P < 0.025). No significant left–right differences were found (P > 0.36). The statistical model estimated a 95% ellipsoid representing the range of intra-patient ovarian variability (Table 1), which was 10-fold greater than the ovary volume (0.3 cc, 1.5 cc, and 3.2 cc for the three age groups, respectively).

Conclusion: Ovarian location varied significantly throughout the RT course across all ages. In modern high precision techniques such as proton RT and VMAT, relying only on initial planning may misestimate ovarian dose, with potential implications for late effects on hormone production and fertility. This study offers a probabilistic mobility range to guide ovarian-sparing planning when on-treatment imaging is not feasible.