Volumetric changes of the primary esophageal tumor during radiation therapy.

Abstract

134 Background: Management of esophageal cancer with radiotherapy requires weeks of daily treatments. The objective of this study is to identify edematous/obstructive changes in the esophageal tumor occurring during therapy and quantifying its change relative to the original tumor volume. Methods: A total of 66 patients were reviewed on a retrospective IRB-approved protocol between July 2012 – July 2016. The gross tumor volume (GTV) targeted with radiation (GTVsim) was constructed on a CT-simulation image. Cone beam CT images (CBCTs) were taken daily during treatment. GTVs (GTVCT#) were contoured on the CBCT on treatment days 1, 6, 11, 16, 21, and 25. Volumes for the GTVsim and each GTVCT# per patient were calculated. A conformality index (CI) was determined by visual confirmation of overlap and calculation of GTVCT#/GTVsim. The standard deviation (SD) was calculated. A theoretical max volume (GTVCTMAX) was determined per patient based on the upper range of 2SD from the average GTVCT# value. A theoretical PTV created from a 1cm (PTV1) and 0.5cm (PTV0.5) expansions were also calculated with associated CIs for PTVCTMAX/PTV1 and PTVCTMAX/PTV0.5. Results: Twenty-one patients had a total of 94 CBCT volumes. The median age was 67 years (53-83), sixteen patients were male, 5 female. T-stage was 1/2 in 5 patients and 3/4 in 15 patients. Chemotherapy was administered concurrently in 17 patients. Median duration of radiotherapy was 35 days (2-50). Results were analyzed by absolute volume comparison and CI calculations as noted in table 1.The median CI for PTVCTMAX/PTV1 was 0.40 (0.15-0.84) and PTVCTMAX/PTV0.5was 0.65 (0.34-0.89). Conclusions: Review of the weekly CBCTs demonstrated a larger tumor volume in the first day of therapy relative to the subsequent weeks, and even relative to the volume at the time of simulation. However, in projecting potential changes in maximum size of the individual tumors, 100% remained within standard PTV volumes constructed with a 1cm and 0.5cm expansions. [Table: see text]