Author: Liu Hong, Wen C. Hsi, Faraz Kalantari, Romy Megahed, Ganesh Narayanasamy, Maida Ranjbar, Pouya Sabouri, Zhong Su 👨🔬
Affiliation: University of Arkansas for Medical Sciences, Department of Radiation Oncology, University of Arkansas for Medical Sciences (UAMS) 🌍
Purpose: Quantitative apparent diffusion coefficients (ADC) in diffusion-weighted MRI (dMRI) reflect water diffusivity and thus provide tissue cellular density information. Functional diffusion mapping (fDM), which presents changes in ADC over time, has demonstrated the promise as a sensitive imaging biomarker in assessing early treatment-induced cell necrosis and apoptosis. This preliminary work is to assess short-term radiation response in brain tissues using fDM before and several months after proton radiotherapy.
Methods: Three glioma patients who received definitive radiation or chemoradiation using proton therapy were retrospectively evaluated, each having at least two MRI scans. The dMRI data underwent offline postprocessing, including co-registration of pre- and post-treatment ADC maps, followed by voxel-wise subtraction for fDM calculation. A three-color overlay was generated, which represented regions with unchanged (green), significantly increased (red), or significantly decreased (blue) ADC values, along with scatter plots and percentages of tissues within these categories (Vg, Vr, and Vb, respectively) to quantitatively assess overall changes in tissue ADC values. The clinical-target-volume (CTV) and isodose line regions from the treatment plan were also mapped onto fDM as region-of-interest (ROI) masks for analysis and comparison.
Results: All patients showed notable ADC value changes in CTV region between the pre- and post-treatment (Vr+Vb>15%). Among patients, one who had a follow-up MRI scan 1.5-month after completing proton therapy showed comparable Vr (8.1%) and Vb (8.7%) values, while the other two, who received the post-treatment scan 12-month later, exhibited a higher Vr/Vb ratio (4-7). Furthermore, the largest values (Vr+Vb) were observed in all patients in the 90%-100% isodose regions, with intermediate values in 80%-90% and the smallest in 50%-80% regions, as expected.
Conclusion: The preliminary results showed promise in detecting tissue ADC value changes due to radiation response; however, a larger sample size and statistical analysis are needed to better characterize these findings in future studies.