307 - Quantifying Dose-Dependent Changes in Active Bone Marrow Using Multi-Energy CT and Fat Fraction MRI in Pelvic Radiotherapy

Purpose/Objective(s): Hematologic toxicity (HT) is a major dose-limiting toxicity for patients undergoing pelvic external beam radiation therapy (EBRT), especially when combined with chemotherapy. Conventional treatment planning attempts to spare the entire bone marrow (BM) compartment, imposing constraints on the dose distribution. Advanced imaging modalities like multi-energy CT (MECT) and fat fraction MRI (FFMRI) allow identification of regions of active bone marrow (ABM) as benchmarked by our group in an animal model. ABM is particularly radiosensitive and a better understanding of how radiation dose affects the ABM distribution could influence treatment planning strategies. The purpose of this study is to identify and quantify dose-dependent changes in ABM after EBRT using MECT and FFMRI.

Materials/Methods: Patients with gynecological malignancies underwent pre- and post-treatment MECT and FFMRI. ABM was delineated using threshold-based segmentation. ABM was defined as voxels with CT numbers above the mean on MECT-derived virtual non-calcium (VNCa) images and voxels with percent fat fraction (%FF) below the mean on FFMRI. Within the ABM volumes, changes in mean HU/%FF were evaluated as a function of dose. A paired t-test was computed for each dose gradient (significant p <0.05).

Results: In MECT, higher radiation doses corresponded to larger decreases in CT numbers. In FFMRI, higher doses produced greater increases in %FF. Mean pre-treatment CT number and %FF in the BM compartment were 33 HU and 58%, respectively. Post-treatment mean CT number decreased to 6 HU while mean %FF increased to 76%. CT number decreased significantly at doses =10 Gy (p = 0.005), while %FF increased significantly across all dose gradients (p = 0.0017), with both metrics showing strong dose-response behavior at higher doses (p < 10?6).

Conclusion: MECT and FFMRI has been shown to effectively delineate ABM and this work quantified radiation-induced marrow changes in patients receiving pelvic EBRT. Integrating ABM imaging into treatment planning could support improved marrow-sparing strategies and reduce HT. These dose-dependent imaging-based indicators could help identify previously irradiated marrow and detect regions susceptible to further functional decline, thereby enhancing clinicians’ ability to anticipate HT risk and optimize ABM-sparing treatment planning.