Changes in tumour volume and motion during radiotherapy for thoracic oesophageal cancer

Background and purpose: Variations of target volume and position were important factors in correction of radiotherapy planning. The purpose was to investigate the changes in volume and motion of oesophageal cancer during radiotherapy using four-dimensional computed tomography (4D-CT). Methods and materials: In total, 109 enhanced 4D-CT data sets were acquired for 38 patients throughout treatment. Gross tumour volumes (GTVs) were outlined on each data set. Variations in volume, motion, and position were calculated for GTV and internal GTV (IGTV) during treatment. Results: GTV (25%, P < 0.01) and IGTV (27%, P < 0.01) had decreased significantly when measured at the twentieth fraction. Larger intrafractional GTV centre shifts (P < 0.01) were observed in the superior–inferior direction (median value of 3.1 mm) compared with the right–left and anterior–posterior directions (1.6 mm and 1.4 mm, respectively). The interfractional shift of the IGTV centre was not significant during radiotherapy. The overlap ratios of the targets decreased for both GTV and IGTV during treatment. Conclusions: Variations in GTV and IGTV centre shifts were not significant throughout treatment. However, tumour volume decreased significantly by the twentieth fraction. Finally, changes in oesophageal tumour volume and motion may decrease the overlap ratio for GTV and IGTV during radiotherapy