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MRI-Based Estimation of Water Equivalent Path Length to Detect Changes in Proton Range During Treatment Courses

J Uh

J Uh*, M J Krasin, C Hua, St. Jude Childrens Research Hospital, Memphis, TN


SU-F-601-6 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 601

Purpose: To develop a method using on-treatment MR images for detecting changes in proton ranges from anatomical variations in the abdomen and pelvis.

Methods: MRI-based virtual CT (vCT) was generated to calculate water equivalent path length (WEPL) at the distal surface of CTV. Sequentially acquired T2-weighted MR images with and without fat suppression, minimizing the effects of respiratory motion and susceptibility, were processed by fuzzy c-mean clustering to assign a bulk CT number to each segment of fat, water, and air. Bony structure in vCT was transferred from the planning CT via region-based registration assuming that the anatomical variations were confined to soft tissues. The developed method was validated utilizing images from 3 patients (aged 9, 12, and 17 years old) with pelvic sarcomas who received repeat CT (rCT) for boost planning as well as MRI acquired on the same day. The vCT and the estimated WEPL were evaluated against the rCT and the corresponding WEPL.

Results: The optimized MRI sequences produced sufficient image contrasts to identify the segments. The vCT agreed well with the rCT (mean absolute difference, 39-41 HU), wherein imperfect registration of bones and varied locations of bowel gas contributed to the discrepancy. The mean absolute error of the estimated WEPL was less than 1 mm (range, 0.4-0.6 mm). The percentage of distal CTV having WEPL error less than 2 mm ranged from 98 to 100. Applied to multiple on-treatment MRI of an example patient, the estimated WEPL demonstrated a trend of under-range (~1 mm per 10 days) in accordance with the patient’s gaining weight.

Conclusion: The MRI-derived virtual CT produced highly accurate WEPL estimation, demonstrating the potential of using on-treatment MRI for detecting changes in proton ranges. The developed method provides a nonionizing alternative to frequent on-treatment CT scans for monitoring anatomical changes and tumor response.

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