Quantification of Volumetric Changes Occurring During Radiation Therapy for Esophageal Cancer

Abstract

Many patients undergoing radiation therapy for esophageal cancer have marked anatomic changes during their course of treatment including shrinking of the primary tumor, edematous changes of the treated esophagus, and changes in body habitus. Changes in contour, volume, and location of the esophageal tumor over the total course of radiation therapy could have a potential dosimetric impact if highly conformal treatment techniques are used. We conducted a retrospective repeated measures analysis on daily cone-beam CT imaging to quantify the magnitude of these volumetric changes. Utilizing an institutional retrospective database, fourteen patients were identified for this study. All patients were treated using a linear accelerator with integrated on-board cone beam CT imaging that allows CT imaging during the daily treatment sessions while the patient is immobilized in the treatment position. Gross tumor volumes of the primary tumor were contoured on every axial slice on the primary planning CT and the CT acquired on treatment days 1, 6,11,16,21, and 25. The changes in primary tumor volume over time were analyzed with a repeated measures ANOVA analysis. Gross tumor volumes generally decreased at an average rate of roughly 6% volume loss per week which ultimately resulted in a 23% total average reduction in volume over the total course of therapy. Analysis of variance revealed a statistically significant reduction in tumor volume occurring by cone beam CT 11 with a mean volume of 91% of original (p=0.0011 95% CI 0.89-0.98).The volumes measured on cone beam CTs taken on treatment days 16, 21, and 25 were also statistically significant (p<0.0001) with mean residual volumes of 89% (95% CI = 0.83-0.96), 83% (95% CI=0.79-0.89), and 77% of original volume (95% CI= 0.72-0.83) respectively. Statistically significant volume reductions were noted to begin by radiation treatment day 11 with meaningful dosimetric implications. These data may, therefore, be useful in the implementation of an adaptive radiation therapy planning scheme that periodically adjusts the conformal treatment plan to match these anatomic