Study Of Interfractional And Intrafractional Setup Errors In Radiotherapy By Cbct In Pelvic Carcinoma

OBJECTIVE This study was to analyze the interfractional and intrafractional setup errors in radiotherapy for pelvic carcinoma using KV-cone beam CT (CBCT) on IGRT and estimate the margin required for CTV-PTV.METHODS First, 24 patients with pelvic carcinoma were enrolled into the study with the use of ELEKTA Synergy IGRT linear accelerator. 24 patients received CBCT scans after initial setup for a total of 365 times. Translational errors(X, Y, Z) and rotational errors(U, V, W) were obtained. All the setup errors were corrected. Among them there were 13 patients received CBCT scans after initial setup, after re-positioning and after radiation delivery. A total of 600 CBCT scans were obtained. Then analyzing the interfractional and intrafractional setup errors,caculating the margin from CTV to PTV before or after re-positioning. Second, by using self paired design, 8 patients with prostate cancer received definitive radiotherapy with different margins. Group A: the margins were acquired before re-positioning. Groud B: the margins were acquired after re-positioning.RESULTS 1)24 patients received a total of 365 CBCT scans after initial setup. The systemic±random errors on translational X, Y, Z directions were (0.73±1.67) mm,(0.11±4.69) mm, (-1.77±2.60)mm , on rotational U, V, W directions were(0.81±1.11)°, (-0.01±1.18)°, (-0.39±0.88)°, respectively. 2)All the setup errors were corrected, comparing with the initial setup errors,the re-positioning setup errors decreased significantly (P<0.05). 3)Setup errors after radiation delivery exclude V, W directions, the others were increased significantly compared to the re-positioning setup errors(P<0.05). 4)The estimated margin required for CTV-PTV were calculated according to the van Herk formula, before re-positioning which was required 4.93mm in X-axes, 12.63mm in Y-axes, 7.06mm in Z-axes. After re-positioning the margin only required 1.25mm, 2.43mm, 1.67mm in X, Y, Z axes, respectively. 5)Between the group A and group B, the difference of the mean dose of bladder and rectum were significant, the difference of NTCP for the rectum was also significant.CONCLUSIONS This study shows as follows: (1) The spatial distributions of setup errors were smallest in the left-right direction,moderate in the anterior-posterior direction and largest in the superior-inferior direction, the most of rotational errors were lower than 3°. (2)Real-time online correction of setup errors using CBCT could help to reduce the interfractional errors and improve the precise of radiotherapy in patients with pelvic carcinoma. (3)The intrafractional setup errors variation were obvious in pelvic carcinoma and should be taken into account when caculating the CTV-PTV margins. (4)Without IGRT,we suggested the PTV margins were 5mm in left-right direction,15mm in the superior-inferior direction and 10mm in anterior-posterior direction. With the use of CBCT, which could significantly reduce the CTV-PTV margin and decrease the dose of normal tissue, a margin of 3 mm in all directions was adequate.