Real-Time Magnetic Resonance Imaging Quantification of Esophageal Motion During Stereotactic Body Radiation Therapy

Abstract

To quantify interfraction esophagus motion during thoracic SBRT using MRI-guided radiation therapy (RT). Materials/Methods: We reviewed computed tomography (CT) simulation and on-treatment MR datasets of 7 patients obtained using a 0.3-T MRguided RT system. Image registration was based on bony anatomy from the cricoid to the gastroesophageal junction and defined as upper, mid, and lower esophagus. The planning CT was compared to 3 treatment MRI scans obtained during initial, middle, and final fractions at slice intervals of 0.9 cm. Esophageal motion was quantified by measuring expansion and displacement in both the right-left (RL) and anterior-posterior (AP) directions and corrected for RL and AP shift. RL shift was defined as the distance between right lateral esophagus on planning CT and on-treatment MR scan. AP shift was defined as distance between posterior aspect of esophagus on planning CT and on-treatment MRI. Expansion was defined as additional length in RL or AP direction. Displacement was defined as movement of central point of RL or AP diameter and was calculated by subtracting CT radius from the sum of the MRI radius and RL or AP shift. Sample means were compared by analysis of variance (ANOVA) and Fisher protected least significant difference test. Results: Twenty-eight imaging datasets from 7 patients undergoing SBRT for primary lung cancer (3) and metastatic lung disease (4) were analyzed. Gross tumor volume locations included right hilum (3), left upper lobe (2), left lower lobe (1), and mediastinum (1). Mean RL expansion was 0.36AE0.02 cm and displacement was 0.29AE0.02 cm, favoring leftward motion (58.2% and 51.3%). Mean AP expansion was 0.37AE0.03 cm and displacement was 0.39AE0.03, favoring anterior motion (68.7% and 71.1%). Magnitude of AP expansion and displacement was less than 0.5 cm in 74.3% and 69.9% of observations, respectively. Motion was greatest in lower esophagus (RL expansion 0.44AE0.04 cm, RL displacement 0.34AE0.03 cm, AP expansion 0.46AE0.04 cm, AP displacement 0.49AE0.04 cm). The difference was significant compared to mid and upper esophagus (P<.0001). A total of 34% of lower esophageal MRI contours showed motion >0.5 cm in either the RL or AP directions. The 95th percentiles of expansion for leftward, rightward, anterior, and posterior directions were 0.68 cm, 0.84 cm, 1.32 cm, and 1.07 cm, respectively. Conclusion: As previously reported, lower esophagus motion is of significantly greater magnitude than mid and upper esophagus. Preliminary analysis of MRI on-treatment imaging of the lower esophagus during thoracic SBRT suggests a margin expansion of 0.68 cm left, 0.84 cm right, 1.32 cm anterior, and 1.07 cm posterior would account for 95% of measurements. Additional work to define individualized esophageal normal tissue structures from personalized observations is ongoing.