Feasibility of quantitative diffusion-weighted imaging during intra-procedural MRI-guided brachytherapy of locally advanced cervical and vaginal cancers

Purpose: To determine the feasibility of quantitative apparent diffusion coefficient (ADC) acquisition during magnetic resonance imaging-guided brachytherapy (MRgBT) using reduced field-of-view (rFOV) diffusion-weighted imaging (DWI). Methods and Materials: T2-weighted (T2w) MR and full-FOV single-shot echo planar (ssEPI) DWI were acquired in 7 patients with cervical or vaginal malignancy at baseline and prior to brachytherapy, while rFOV-DWI was acquired during MRgBT following brachytherapy applicator placement. The gross target volume (GTV) was contoured on the T2w images and registered to the ADC map. Voxels at the GTV's maximum Maurer distance comprised a central sub-volume (GTV_center). Contour ADC mean and standard deviation were compared between timepoints using repeated measures ANOVA. Results: ssEPI-DWI mean ADC increased between baseline and prebrachytherapy from 1.03 ± 0.18 10⁻³ mm²/s to 1.34 ± 0.28 10⁻³ mm²/s for the GTV (p = 0.06) and from 0.84 ± 0.13 10⁻³ mm²/s to 1.26 ± 0.25 10⁻³ mm²/s at the level of the GTV_center (p = 0.03), consistent with early treatment response. rFOV-DWI during MRgBT demonstrated mean ADC values of 1.28 ± 0.14 10⁻³ mm²/s and 1.28 ± 0.19 10⁻³ mm²/s for the GTV and GTV_center, respectively (p = 0.02 and p = 0.03 relative to baseline). No significant differences were observed between ssEPI-DWI and rFOV-DWI ADC measurements. Conclusions: Quantitative ADC measurement in the setting of MRI guided brachytherapy implant placement for cervical and vaginal cancers is feasible using rFOV-DWI, with comparable mean ADC comparable to prebrachytherapy ssEPI-DWI, and may enable MRI-guided radiotherapy targeting of low ADC, radiation resistant sub-volumes of tumor.