The Effect of Larynx Motion on Setup/Delivery Error During Radiation Treatment for Head-and-Neck Cancer

Abstract

Setup error is a potentially significant pitfall in head and neck cancer (HNC) radiation therapy. Larynx motion is a potentially significant source of said error. The objective of this study was to determine the extent of setup error in radiation therapy due to larynx motion and the necessary compensatory CTV-PTV correction. This study used images from 7 HNC patients who received radiation therapy with daily CT on rails monitoring. Daily patient CT on rails scans were imported into a treatment planning system and both C5 and the thyroid cartilage were autosegmented. Subsequently, twelve focal points were placed on each scan: the superior-most and inferior-most points of the most anterior portion of both C5 and the thyroid cartilage (4 total), the superior-most and inferior-most points of the most lateral aspects of the C5 articular processes and the thyroid cartilage cornua bilaterally (8 total). Taking the most superior anterior point of the C5 vertebra as a representative isocenter for each scan, displacement vectors to the other 11 points were calculated. Subsequently, using the first scan as a “perfect setup” reference, the magnitude of vector differences for all 11 points for all scans were calculated, and the mean magnitude of vector differences for each point was calculated for each patient. The systematic error for the population, the population random error due to larynx motion, and the necessary compensatory CTV-PTV margin were calculated using Van Herk formula for each focal point. The average mean displacement for each anatomical point was 4.1 mm (range, 2.7-5.1), the average systematic error for each point was 1.2 mm (range, 0.86-1.5), the average random error for each point was 2.1 mm (range, 1.3-2.8), and the average CTV-PTV margin for each point was 4.4 mm (range, 3.3-5.7). Larynx motion is a significant cause of RT set-up/delivery error in HNC both when the larynx is in a CTV or OAR. Each center must measure their own random and systematic error to determine their PTV expansion requirement. We estimate the need for a uniform expansion of at least 6 mm to compensate for larynx motion.Poster Viewing Abstract 3249; TableSummary statistics for distances from isocenter to thyroid cartilage focal pointsPointMean displacement (mm)Systematic error (mm)Random error (mm)CTV-PTV margin (mm)Most superior aspect of the anterior thyroid cartilage4.91.52.85.7Most inferior aspect of the anterior thyroid cartilage4.81.22.84.8Most superior aspect of the left superior cornu of the thyroid4.21.12.34.3Most inferior aspect of the left inferior cornu of the thyroid4.81.02.54.3Most superior aspect of the right superior cornu of the thyroid4.61.52.85.6Most inferior aspect of the right inferior cornu of the thyroid5.11.32.85.3 Open table in a new tab