Quantitative Relaxometry for Target Localization and Response Assessment in Ultra-Hypofractionated MR-Guided Radiotherapy to the Prostate and DIL

Abstract

<h3>Purpose/Objective(s)</h3> MR-guided systems, such as hybrid MR-Linacs, provide a novel platform for the acquisition and development of image-based biomarkers that can provide crucial information during daily online plan adaptation. Relaxometry has been shown to be sensitive to changes in tissue composition or function during radiotherapy. We investigate the added value of longitudinal relaxometry for target localization and response assessment. <h3>Materials/Methods</h3> The study included 42 unfavorable intermediate risk prostate patients undergoing ultra-hypofractionated MR-guided adaptive therapy to the prostate and seminal vesicles CTV with an integrated boost to the dominant intraprostatic lesion (DIL). The prescription was 800/900 cGy x 5 fractions, every other day. The longitudinal (T1) and transverse (T2) relaxation times were measured in the MR-Linac. T1 maps were acquired using the variable flip-angle method: TR/TE=20/2.3 ms, FA={4°, 22°}, FOV=320 × 320 × 200 mm, voxel = 0.7 × 0.7 × 3 mm³, scan time = 1.5 min. T2 maps were acquired using the multi-echo spin-echo method: TR/TE/ΔTE = 4000/22/11 ms, 8 echoes, FOV = 320 × 320 × 100 mm, voxel = 0.5 × 0.5 × 3 mm³, scan time = 5.5 min. The bias and reproducibility of T1/T2 mapping was assessed over the course of eight weeks using a NIST-traceable phantom. Longitudinal phantom measurements were also used to establish baseline values for T1/T2. The analysis was implemented in the Computational Environment for Radiotherapy Research. Daily T1 and T2 maps of prostate patients were rigidly registered to the pre-beam online-planning T2w images and T1/T2 values were analyzed within the CTV/DIL contours. <h3>Results</h3> The median bias with respect to nominal values for T1, T2 was -1.1%, 3.4%. When compared to baseline, the median bias for T1, T2 was 0.6%, 0.1% and the coefficient of variation was 2.1%, 0.9%, respectively. Maximum translation and rotation corrections within the T1/T2 maps were 1.9±0.3mm and 0.9°±0.2°. Across all patients and fractions, T1_CTV=1766±150ms, T1_DIL=1559±245ms, T2_CTV=127±19ms, T2_DIL=111±19ms. T1 and T2 were significantly longer in the CTV. Variations over fraction number are shown in Table 1. Linear regression showed there is a significant increase of mean T1 in the DIL (p=0.05) and CTV (p=0.05). The increase in T1 from fraction 1 to fraction 5 was 12.3% and 6.3% in DIL and CTV, respectively. The trend for T2 was not significant for either DIL or CTV. <h3>Conclusion</h3> T1 and T2 quantitative relaxometry is a surrogate measure of unbound water content and motility and shows the potential to serve as a predictor for changes in the local tumor microenvironment during MR-guided adaptive radiotherapy.