Use of planar kV vs. CBCT in evaluation of setup errors in oesophagus carcinoma radiotherapy

Abstract

Aim: The aim of this study is to evaluate differences in terms of the setup errors observedusing kV planar image compared to CBCT for oesophageal cancer patients.Background: Planar kV images are quick to acquire but only allow the observation of bonystructures. CBCT allows the evaluation of soft tissues, which includes the oesophagus (andtumour) and OAR, giving a more accurate verification of the positioning.Materials and Methods: All patients were imaged with both techniques between January 2012and March 2014 were included in the study (16 patients, 212 kV images and 116 CBCT images).Differences between the setup errors observed on the two images modalities were studied. Acorrelation study between TNM staging, tumour location and immobilization systems withsetup errors was also done. Finally, the calculation of systematic and random errors allowedto determine the CTV–PTV margin.Results: A significant discrepancy (p < 0.05) between the setup errors observed with kV andCBCT was observed in the lateral direction. No statistical correlation was found betweensetup errors and tumour location, immobilization system or TNM staging. The CTV–PTVmargin was smaller with CBCT in the vertical (0.6 cm vs. 0.9 cm) and longitudinal (0.7 cm vs.1 cm) directions and smaller with kV for the lateral directions (0.8 cm vs. 0.9 cm).Conclusions: The chosen modality influences the setup error observed which will influencethe correction applied. Allowing a better observation of the volumes of interest, CBCT shouldbe the modality of choice in this pathology. The CTV–PTV margins could be shrunk if CBCTis used.