Longitudinal MRI is an ideal tool for tracking anatomic changes that occur during head and neck radiotherapy. We have implemented an image tracking system that monitors volumetric changes in gross tumor volumes (GTV) and parotid glands to identify and alert of major changes early in treatment. In a cohort of 91 patients, this system identified systematic shrinkage of GTVs (9.2±8.3% per week) and parotids (3.1±3.7% per week) during treatment. Importantly, GTV changes observed in the first week of treatment were strongly predictive of larger changes that would occur later in the course (P<1x10-5, two-tailed t-test).
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