Analysis of the intrafraction error in image-guided radiotherapy with thoracic esophageal carcinoma

Objective The purpose of the study was to use the CBCT to evaluate the intrafraction error and its effect on the target volume and organs at risk.Methods Twenty-three patients with thoracic esophageal cancer were enrolled into the study.CBCT scans were performed before and at the end of the irradiation to acquire the intrafraction error.Treatment PLAN2 and PLAN3 were simulated according to the intrafraction error.The primary treatment plan was PLAN1.The dosimetry distribution of target volume and organs at risk of the PLAN2 and PLAN3 were analyzed,and then were compared with PLAN1.The datas was analysed by one-factor ANOVA and matched t-test.Results The median intrafraction error (mm) of the upper,the middle and the lower esophageal carcinoma in x,y,z directions were (1.2 ± 1.5) mm,(1.0 ± 1.0) mm,(1.0±1.0) mm (P=0.138),(1.2±1.0) mm,(1.1 ±1.0) mm,(1.2±1.0) mm (P=0.656) and (1.3±0.1.1) mm,(1.2±1.0) mm,(0.8±0.7) mm (P=0.003),respectively.The frequency of the intrafraction error within 3 mm in x,y,z directions were 95.2％,94.5％,93.9％,respectively.Compared with PLAN1,the volume of the GTV exposed to 100％ prescription dose of the PLAN3 decreased by 5.55％.There were 3 patients whose prescription dose of the PTV exposed to 95％ volume descend more than 5％ in PLAN3.The volume of the lung exposed to 30 Gy was (15.24 ± 2.24) ％ in PLAN3,which was lower than (15.67 ± 2.28) ％ in PLAN1 (P =0.033).There were 4 cases in PLAN2 and 19 cases in PLAN3 whose spinal cord received more than 4500 cGy,the maximum dose were 4517.2 cGy and 5 045.2 cGy in PLAN2 and in PLAN3,respectively.Conclusions The dose distribution of the target volume and organs at risk were influenced by intrafraction error in part.Spinal cord is relatively sensitive to the intrafraction error owing to its tandem belongings.Intrafraction error can result in the over irradiation to the spinal cord for some patients. Key words: Tomography, X-ray computed, cone beam;  Intrafraction error;  Esophageal neoplasms/image guided radiotherapy