Detection of Tumor Volume Regression and Motion Changes During Radiation Therapy for Primary Thoracic Esophageal Cancer Based on 4DCT Scans: Mesothelioma, Thymic Malignancies, and Other Thoracic Malignancies

Abstract

To investigate the magnitudes of the changes in volume and the GTV intrafractional displacement of the primary thoracic esophageal cancer in response to conventional fractioned radiation therapy with repeated 4-dimensional computed tomography (4D-CT) during the course of treatment. The 4D-CT data sets were acquired at the time of treatment simulation and after every ten fractions for each of the 38 patients throughout treatment. Scans were registered to the baseline (simulation) 4D-CT scans by using bony landmarks. The gross tumor volumes (GTVs) were delineated on each data set, and the GTVmean was the average of all 10 GTVs of each 4D-CT scan. All 10 GTVs were combined to form the internal gross tumor volume (IGTV). Coordinates and volumes of the GTV and IGTV were acquired from the treatment planning system (TPS). The intrafraction variations in the displacement of the GTV, the volumes of the GTVmean and the IGTV were computed during radiation therapy. The tumor volumes decreased significantly at the twentieth fraction, which was up to 26.09% (P<.001) in GTVmean and 27.78% (P<.001) in IGTV, respectively. The 95th percentile values from the cumulative distributions were used to define minimum margins to account for gross tumor volume motion during treatment planning. According to this study, 2.5 mm RL, 2.3 mm AP, 5.0 mm SI, 5.5 mm 3D vector in the total, 2.3 mm RL, 2.1 mm AP, 4.9 mm SI, 5.4 mm 3D vector in the upper, 2.4 mm RL, 2.3 mm AP, 4.1 mm SI, 4.7 mm 3D vector in the middle, 2.5 mm RL, 2.5 mm AP, 5.4 mm SI, 6.3 mm 3D vector in the distal thoracic esophagus provided coverage of tumor motion in 95% of the cases in our study population. The intrafraction GTV centroid displacements in the SI direction were greater than in the LR and AP direction during treatment. However, no significant displacement was observed during radiation therapy in any direction (P=.616, RL; P=.471, AP; and P=.134, SI). The intrafractional variations of GTV centroid displacement were not significant throughout treatment, but the tumor volume decreased significantly at the 20th fraction. Repeated 4D-CT scans might be beneficial for target correction and planning modification.