Prostate motion in magnetic resonance imaging-guided radiotherapy and its impact on margins.

PURPOSE: This study focused on reducing the margin for prostate cancer treatment using magnetic resonance imaging-guided radiotherapy by investigating the intrafractional motion of the prostate and different motion-mitigation strategies. METHODS: We retrospectively analyzed intrafractional prostate motion in 77 patients with low- to intermediate-risk prostate cancer treated with five fractions of 7.25 Gy on a 1.5 T magnetic resonance linear accelerator. Systematic drift motion was observed and described by an intrafractional motion model. The planning target volume (PTV) margin was calculated in a cohort of 77 patients and prospectively evaluated for geometric coverage in a separate cohort of 24 patients. RESULTS: The intrafractional model showed that the prostate position starts out of equilibrium for the anterior-posterior (-1.8 ± 3.1 mm) and superior-inferior (1.7 ± 2.6 mm) directions, with relaxation times of 12 and 15 min, respectively. Position verification scans are acquired at 30 min on average. At that time, the transient drift motion becomes indistinguishable from the residual random intrafractional motion. PTV margins can be reduced to 1.8 mm (left-right), 3.2 mm (anterior-posterior), and 2.9 mm (superior-inferior). Evaluation of the overlap with the clinical target volume (CTV) was performed for a total of 120 fractions of 24 patients. The overlap range between the CTV and the PTV was 93-100% and the applied 3‑mm PTV margin for the CTV had a 99.5% averaged geometric overlap for all patients. CONCLUSION: A PTV margin reduction to 3 mm is feasible. A patient-specific approach could reduce the margins further.