Feasibility of Stereotactic Body Radiation Therapy for Non–small Cell Lung Cancer Using a Linear Accelerator-Based Magnetic Resonance Image Guided Radiation Therapy System

Abstract

To describe a linear accelerator (linac) based magnetic resonance image guided radiation therapy system (MR-IGRT) and demonstrate the feasibility of delivering stereotactic body radiation therapy (SBRT) for non-small cell lung cancer (NSCLC) treatment. The linac-based MR-IGRT (MRL) system currently being installed at our institution integrates a 0.35T MRI with a flattening-filter free (FFF) linac delivering 6MV at 600 cGy/min. A novel double-focused and double-stack multileaf collimator (MLC) with an effective leaf width of 4 mm provides modulation for step-and-shoot IMRT delivery. The ability of a similar Cobalt-based system for real time, real anatomy gating has already been demonstrated, and a number of patients with NSCLC were treated. Here, we investigate the feasibility of planning and delivery with the linac-based system for SBRT, primarily focusing on plan quality as compared to conventional linacs (CL) and delivery times. Four MRL plans were generated for NSCLC patients who were previously planned and treated on a CL using 6MV FFF beams with a total dose of 54 Gy in 3 fractions. MRL plans were compared to clinical plans (CP) by evaluating conformity numbers with 100% (CN100), 95% (CN95) of prescription dose (Rx), heterogeneity indices (HI), maximum dose and dose-volume histograms of organs-at-risks (OARs), and delivery times. All MRL plans achieved PTV coverage and OAR sparing within clinical constraints and were comparable to clinically-treated plans as indicated in Table 1. Three plans showed an average of 8% and 12% improvement on PTV conformity CN100 and CN95, respectively. There is no clear trend for treatment time comparison but in general they only differ by 3 mins for a 10-min long treatment. The MRL is capable of generating clinically acceptable SBRT plans targeting NSCLC. With real-time MR gating, novel double-stack MLC design and fast FFF delivery, the system is able to achieve a more conformal and precise dose to the target therefore having great potential for further lung tumor dose escalation and reduction of treated volumes. We expect that a higher dose rate version will be available soon to further reduce the treatment time.Abstract 3536IDPlanTumor LocationVPTV Rx (%)VPTV 95%Rx (%)HICN100CN95Max Cord (Gy)Max Esophagus (Gy)Max Heart (Gy)Max Carina (Gy)Mean Lung-ITV (Gy)V20 Lung-ITV (%)Treatment time (min)1MRLLeft upper lobe97.999.61.160.920.844.4910.5521.828.843.193.110.2CP97.399.91.230.860.785.779.6818.519.223.023.07.22MRLLeft lower lobe99.11001.160.800.641.268.3310.970.452.412.45.7CP98.71001.150.840.690.456.2210.010.281.872.27.83MRLRight upper lobe99.299.91.160.740.665.138.670.730.781.210.711.8CP99.81001.170.720.581.418.160.310.421.081.09.04MRLLeft lower lobe97.299.71.260.890.8313.8714.0712.8912.676.216.26.9CP96.6991.200.780.709.5218.0115.6816.576.319.310.0 Open table in a new tab