Objective To analyze setup errors for irradiation of pelvic carcinoma by online conebeam CT (CBCT) scanning and to calculate the external margins from clinical target volume (CTV) to planning target volume (PTV) in treatment planning. Methods Twelve patients with rectal or prostate cancer were enrolled in this study. Translational errors (x,y,z) and rotational errors (u,v,w) were obtained by using CBCT in radiotherapy. Results The set-up errors were gathered from 229 sets of CBCT in 12patients. The systemic ± random errors on x,y,z, u,v and w axes were (0.49 ± 1.18) mm, (-0. 11 ±3.45) mm, (-2. 00 ± 1.59) mm, 1.14°±0. 67°, 0. 42°±O. 94°and -0. 32°±±0. 68°, respectively. Setup errors in the left-right, anterior-posterior, and superior-inferior directions were 4. 6 mm, 12. 5 mm, and 6. 2 mm, respectively. Conclusions Set-up errors were unavoidable in pelvic carcinoma irradiation. To minimize the influence of set-up errors, we suggest a PTV margin of 5 mm, 15 mm and 10 mm in the leftright, anterior-posterior and superior-inferior directions, respectively.

Objective To explore the clinical and imaging factors influencing the patients' prognosis after preoperative radiotherapy for local advanced rectal cancer.Methods We retrospectively analyzed 106 locally advanced rectal cancer patients from June 2004 to September 2015 in our institution.All patients underwent preoperative radiotherapy.According to the Mandard score,patients were divided into 5 groups (TRG1-5).All patients were divided into two groups according to the TRG,which including good responder (TRG1 + 2) and poor responder (TRG3 + 4 + 5) groups.All of the tumor ADC values of post-RT were measured by Diffusion-weighted MRI technology,and the relationship between tumor ADC values of post-RT and TRG was analyzed.Results In univariate analysis,age,chemotherapy,pT,pN,differentiation degree,vascular invasion and TRG were significantly associated with overall survival (x2 =3.945-8.110,P ＜ 0.05).Multivariate analysis indicated that differentiation degree and TRG were the independent prognostic factors for OS (x2 =5.221,6.563,P ＜ 0.05).No significant difference was found between long-course and short-course radiotherapy group (P ＞ 0.05) in OS.The good responder group had a favorable survival in 5-year OS compared to the poor responder group (x2 =8.110,P ＜ 0.05).Preoperative radiotherapy,preoperative chemotherapy,pathological type,differentiation degree and gross type,vascular tumor thrombus and tumor ADC values of post-RT were significantly associated with TRG (x2 =4.189-18.139,P ＜ 0.05).The best critical point of tumor ADC values of post-RT was 1.7 x 10-3 mm2/s by using ROC curve.The accuracy of tumor ADC values of post-RT in predicting TRG1 + 2 was 70％.Conclusions The TRG can predict the efficacy of preoperative radiotherapy in patients with locally advanced rectal cancer based on the Mandard score.There was no significant difference in OS between long-course radiotherapy group and short-course radiotherapy group.The tumor ADC values of post-RT might become a potential factor to predict TRG in patients with locally advanced rectal cancer after preoperative radiotherapy.