A kV cone-beam CT based analysis of the setup errors and the corresponding impact on the dose distribution of intensity modulated radiotherapy for thoracic esophageal carcinoma

Abstract

Objective To measure the setup errors in thoracic esophageal carcinoma treated with intensity modulated radiotherapy (IMRT) using kilo-voltage cone-beam CT (kV-CBCT),and to evaluate the impact of the setup errors on the dose distributions in tumor target volume and the peripheral normal tissues.Methods Twenty-one patients with thoracic esophageal carcinoma undergoing image guided IMRT (IG-IMRT) were included in this study. Using kV-CBCT,173 setup errors of these patients were acquired before treatment.By shifting the isocenters,these setup errors were simulated in the 3-dimentional treatment planning system and the corresponding impact of the dose distribution in tumor target volume and peripheral organs were evaluated.Results The translational deviations of lateral,longitudinal and vertical directions were ( 2.73 ± 1.85 ),( 3.19 ± 2.71 ),and ( 2.35 ± 1.71 ) mm,respectively.The dose of gross tumor volume (GTV) was not impacted significantly by the setup errors. However,comparing with the standard plan without setup errors,the setup errors in the simulated plan reduced the dose of 95％ planning target volume (PTV) by 3.38 Gy.The dose to PTV Dmin and PTV Dmean were also reduced by 9.83 Gy and 0.65 Gy respectively. The correction of setup errors improved the conformity index (CI) and the homogeneity index (HI) for PTV.The C1 and HI for the standard plan were 0.74 ± 0.10 and 1.07 ±0.02, respectively. The C1 and H1 for the simulated plan were 0.69 ± 0.08 and 1.13 ± 0.07,respectively.Statistically significance was observed in these differences ( t =3.43 and -3.91 respectively,P ＜ 0.05 ). No statistical significance was observed in the dose differences in lungs,spinal cord and heart between the two plans ( P ＞ 0.05).The mean maximum dose of the spinal cord was (42.20 ± 4.97 ) Gy in the simulation plan,which was (41.37 ± 2.75 ) Gy in the standard plan.For some patients,the maximum dose of the spinal cord exceeded the tolerance level of 45 Gy in the simulation plan,and one case even reached 52.8 Gy.Conclusions Using the image guidance of kV-CBCT,the setup errors can be reduced significantly for patients with esophageal carcinoma receiving IG-IMRT.The correction of setup errors can increase the treatment precision and enhance the dose in PTV.No significant dose changes were observed in the lung,spinal cord and heart as a result of setup error correctio. Key words: Esophageal neoplasm; Intensity modulated radiotherapy; Image guided radiotherapy; Setup error; Dosimetry