Interfractional reproducibility of lung tumor location using various methods of respiratory motion mitigation

Purpose: To determine interfractional reproducibility of the location of lung tumors using respiratory motion mitigation. Methods and Materials: Free-breathing four-dimensional computed tomography (CT) data sets and CT data sets during breath hold were acquired weekly for 17 patients undergoing treatment for non-small-cell lung cancer. Distances between the center of the gross tumor volume (GTV) and a reproducible bony reference point under conditions of breath hold on end inspiration (EI) and end expiration (EE) and during free breathing on the 0% phase (corresponding to EI) and 50% phase (corresponding to EE) were analyzed for interfractional reproducibility. Systematic uncertainties in tumor location were determined as the difference in distance between the GTV center on the first CT data set and the mean location of GTV centers on the subsequent data sets. Random uncertainties in tumor location were determined as the standard deviation of the distances between the GTV centers and the bony reference points. Margins to account for systematic and random interfractional variations were estimated based on these uncertainties. Results: Mean values of interfractional setup uncertainties were as follows: systematic uncertainties - EI, 0.3 cm; EE, 0.2 cm; 0% phase, 0.3 cm; and 50% phase, 0.3 cm; and random uncertainties - EI, 0.3 cm; EE, 0.3 cm; 0% phase, 0.3 cm; and 50% phase, 0.3 cm. There does not appear to be any correlation between uncertainties and GTV size, but there appears to be a weak positive correlation between uncertainties and the magnitude of GTV excursion. Conclusions: Voluntary breath hold and gating on either EI or EE appear to be equally reliable methods of ensuring the reproducibility of lung tumor position. We recommend setup margins of 0.3 cm if using cone-beam CT or kilovoltage X-ray with fiducials and aligning directly to the tumor and 0.8 cm when aligning to a nearby bony surrogate using cone-beam CT or kilovoltage X-ray.