Setup errors and planning target margin by 6D radiotherapy couch combined with cone-beam CT during volumetric modulated arc therapy in gynecological cancer patients

Abstract

Objective To evaluate inter- and intrafractional setup errors by 6-degree-of-freedom(6D)treatment couch in combination with kV cone-beam CT for gynecological cancer patients, and calculate planning target volume(PTV)margins. Methods Twenty postoperative gynecological cancer patients, who were scheduled to undergo volumetric modulated arc therapy(VMAT), were prospectively enrolled in the study. During the treatment, a CBCT was acquired on a daily basis after conventional position and was registered to the planning CT to determine initial inter-fraction error. Then, a second CBCT scan was performed to calculate residual inter-fraction error after the 6D couch online correction. After VMAT delivery, a final CBCT was acquired to assess intra-fraction motion. The PTV margins were calculated from the above setup variations. Results A total of 594 CBCT images were acquired from 20 patients. After the 6D couch online correction, the interfractional setup errors in y, z, Rx, Ry, Rz axis were significantly reduced(t=6.21, -8.60, 2.13, -8.51, -3.48, P<0.05). The total PTV margins(MPTV)accounting for 6D couch online correction and intrafraction errors were 2.20, 3.43, 2.00 mm in the left-right(x axis), superior-inferior(y axis)and anterior-posterior(z axis)directions, respectively. After the 6D couch online correction, the reduction of MPTV ranged from 4.46 to 6.05 mm. Conclusions 6D in combination with CBCT could effectively improve the setup error accuracy of VMAT in postoperative gynecological cancer patients, while providing reliable basis for delineating the MPTV. Key words: Cone beam computed tomography(CBCT); 6-degree-of-freedom treatment couch; Gynecological cancer; Setup errors; Target margin