MR-Guided Radiotherapy for High-Grade Glioma in Pregnancy: Planning, Validation, and Adaptive Fetal Dose Monitoring

Abstract Introduction Managing high-grade gliomas during pregnancy requires balancing maternal oncologic control with fetal safety. While radiation therapy (RT) is essential for adjuvant treatment, it carries dose-dependent risks of teratogenicity and fetal demise. Current multidisciplinary guidelines recommend keeping cumulative fetal doses as low as reasonably achievable (ALARA), ideally <5-10 cGy. This case report details the planning, dosimetric validation, and adaptive treatment of a pregnant patient with a WHO Grade III astrocytoma using a 1.5T MR-LINAC. Case Presentation A 21-year-old female at 14 weeks gestation presented with seizures. MRI revealed an 8.3 × 4.3 × 5.1 cm expansile T2/FLAIR hyperintense mass in the left frontal lobe. Pathology following awake craniotomy confirmed a WHO Grade III IDH R132H mutant astrocytoma. Methods To minimize fetal dose, a 1.5T Elekta Unity MR-LINAC was selected for its self-shielding cryostat, flattening-filter-free (FFF) beam, and elimination of ionizing imaging radiation. A "zero CT-sim" workflow was implemented. Pre-treatment validation utilized a custom "MAMA" phantom and in-vivo TLDs. Results Initial phantom measurements projected a total fetal dose of 0.803 cGy. However, weekly MR-stitching revealed a 7 cm superior migration of the uterine fundus over the 6-week course. Adaptive re-measurements yielded a final, weighted cumulative fetal dose of 1.54 cGy. Conclusion This is the first reported case of a pregnant patient with a high-grade glioma treated on an MR-LINAC. The case demonstrates that pre-treatment static dose estimates are insufficient due to fetal growth. The MR-LINAC's non-ionizing imaging capability enabled a novel, safe workflow for weekly adaptive fetal dose monitoring.