Detection Of The Variation Of The Target Volume And The Target Displacement During Radiotherapy For Primary Thoracic Oesophageal Cancer Based On Repeated Four-dimensional Computed Tomography

Objective: To investigate the displacement characteristics and the tumor volume variation ofprimary thoracic esophagus carcinoma with enhanced four dimensional computedtomography (4DCT) scanning during fractionated radiotherapy. To analyze the correlationbetween the displacement of the gross tumor volume（GTV）and volume, length and largestdiameter of the primary tumor using repeated4DCT during radiotherapy.Methods:4DCT data sets were acquired at the time of treatment simulation and every tenfractions for the patients with esophagus cancer throughout treatment. Scans were registeredto baseline (simulation)4DCT scan by using bony landmarks. The gross tumor volumes(GTVs) were delineated on each data set. Coordinates of the GTV centroids were acquired oneach respiration phase. The magnitudes of peak-to-peak motion in three dimensions werecalculated. In addition, the three dimensional tumour motion vectors were obtained usingmotion data in the different axes. The average length of the GTV, the largest diameter of theprimary tumor was aquired from the treatment planning system. The GTVmeanwas the averagevolume of all10phases of each GTV, and all10GTVs were combined to form the internalgross tumour volume (IGTV10), IGTVMIPwas the contour delineated from the maximum intensity projection. To investigate the variation in target displacement, tumor volume, and thecorrelation between target displacement and parameters of tumor volume of the esophagealcancer in response to conventional fractioned radiotherapy during treatment.Results: For the middle segment, significant differences were found among the threedirections for the first positioning and the twentieth fraction, but for the tenth fraction, thedifferences were only existed between SI and radial directions. For the distal segment, thedisplacements were different between SI and AP direction for each time. There were nostatistical significance on the same direction among all fractions in total, upper, middle anddistal thoracic esophagus cancer displacement during radiation course.The majority of the GTVmean, IGTVMIPand IGTV10were decreased with increasingfractions during radiotherapy based on repeated4DCT scanning, but the volume change wasdifferent in different positions or different fractions. GTVmeanincreased by4.20%～39.42%(1.31cm3～7.44cm3) at the tenth fraction for21.87%(7/32) patients, and the differences weresignificant (t=-4.753, P=0.003). Except the upper esophageal tumors, statistical significancewere existed in GTVmean, IGTVMIPand IGTV10for all patients between the twentieth fractionand the first positioning (P<0.05).At twentieth fractionation, the statistically positive correlation were observed betweenthe displacement of the GTV in LR direction and the volume of the primary tumor for wholegroup patients and the subgroup patients with distal thoracic esophageal cancer (r=0.475,P=0.012; r=0.731, P=0.040), between the displacement of the GTV in SI direction and thelength of the primary tumor for whole group patients and the subgroup patients with upperand middle thoracic esophageal cancer (r=0.559，P=0.003；r=0.714，P=0.031；r=0.646，P=0.044), and between the displacement of the GTV in LR direction and length of theprimary tumor for the subgroup patients with distal thoracic esophageal cancer (r=0.765，P=0.027); At first4D-CT scanning before radiotherapy, the statistically positive correlationwere observed between the largest diameter and the displacement of the GTV in LR, SIdirection and3D vector for whole group patients (r=0.373，P=0.036；r=0.377，P=0.033；r=0.415，P=0.018), but when evaluation was separated for subgroup patients, the statisticallypositive correlation was observed only between the largest diameter and the displacement ofthe GTV in LR direction for the subgroup patients with distal thoracic esophageal cancer (r=0.794, P=0.011).Conclusions: For the upper location, the displacements among the three directions are notsignificant, but for the middle and distal locations, the displacements in the SI direction arelarger than in the LR and AP directions. Under free breathing, the magnitude of oesophagealGTV displacement in the same direction is not significant during radiotherapy.Repeated4DCT scanning could not only observed the volume variance of primary tumorGTV without motion information, but also observed IGTV volume change. For primary middleand distal esophageal cancer, the best time to reset position should be selected at twentiethfraction when the primary tumor target volume changed significantly, and it was preferable toguide target correction and planning modification.At different time period during radiotherapy, no significant correlation between thedisplacement of the GTV in AP direction and the volume, length and largest diameter ofthoracic esophageal cancer regardless of every segment; however, the correlation between thedisplacement of the GTV in LR and SI directions and the volumetric parameters of thoracicesophageal cancer depended on the different locations of the tumor and different time periodduring radiotherapy.