title: Use of planar kV vs. CBCT in evaluation of setup errors in oesophagus carcinoma radiotherapy.
abstract: OBJECTIVE
  The aim of this study is to evaluate differences in terms of the setup errors observed using kV planar image compared to CBCT for oesophageal cancer patients.
  
  
  BACKGROUND
  Planar kV images are quick to acquire but only allow the observation of bony structures. CBCT allows the evaluation of soft tissues, which includes the oesophagus (and tumour) and OAR, giving a more accurate verification of the positioning.
  
  
  METHODS
  All patients were imaged with both techniques between January 2012 and 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. A correlation study between TNM staging, tumour location and immobilization systems with setup errors was also done. Finally, the calculation of systematic and random errors allowed to determine the CTV-PTV margin.
  
  
  RESULTS
  A significant discrepancy (p < 0.05) between the setup errors observed with kV and CBCT was observed in the lateral direction. No statistical correlation was found between setup errors and tumour location, immobilization system or TNM staging. The CTV-PTV margin 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 influence the correction applied. Allowing a better observation of the volumes of interest, CBCT should be the modality of choice in this pathology. The CTV-PTV margins could be shrunk if CBCT is used.
authors.affiliation: Radiotherapy Department - Escola Superior de Tecnologia da Saude do Porto, Rua Valente Perfeito, 322, 4400-330 Vila Nova de Gaia, Portugal.
authors.forename: Liliana
authors.initials: L
authors.lastname: Martins
authors.affiliation: Radiotherapy Department - Escola Superior de Tecnologia da Saude do Porto, Rua Valente Perfeito, 322, 4400-330 Vila Nova de Gaia, Portugal; Radiotherapy Department - Instituto Portugues de Oncologia do Porto, Rua Dr. António Bernardino de Almeida, 4200-072 Porto, Portugal; Radiography Department - Faculty of Health Sciences - University of Malta, Msida MSD 2080, Malta.
authors.forename: Jose Guilherme
authors.initials: JG
authors.lastname: Couto
authors.affiliation: Radiotherapy Department - Instituto Portugues de Oncologia do Porto, Rua Dr. António Bernardino de Almeida, 4200-072 Porto, Portugal.
authors.forename: Barbara
authors.initials: B
authors.lastname: Barbosa
keywords: CBCT; CTV–PTV margin; Oesophagus carcinoma; Setup errors; kV