Positioning accuracy during VMAT of gynecologic malignancies and the resulting dosimetric impact by a 6-degree-of-freedom couch in combination with daily kilovoltage cone beam computed tomography.
BACKGROUND
To improve the delivery of radiotherapy in gynecologic malignancies and to minimize the irradiation of unaffected tissues by using daily kilovoltage cone beam computed tomography (kV-CBCT) to reduce setup errors.


METHODS
Thirteen patients with gynecologic cancers were treated with postoperative volumetric-modulated arc therapy (VMAT). All patients had a planning CT scan and daily CBCT during treatment. Automatic bone anatomy matching was used to determine initial inter-fraction positioning error. Positional correction on a six-degrees-of-freedom (6DoF) couch was followed by a second scan to calculate the residual inter-fraction error, and a post-treatment scan assessed intra-fraction motion. The margins of the planning target volume (MPTV) were calculated from these setup variations and the effect of margin size on normal tissue sparing was evaluated.


RESULTS
In total, 573 CBCT scans were acquired. Mean absolute pre-/post-correction errors were obtained in all six planes. With 6DoF couch correction, the MPTV accounting for intra-fraction errors was reduced by 3.8-5.6 mm. This permitted a reduction in the maximum dose to the small intestine, bladder and femoral head (P=0.001, 0.035 and 0.032, respectively), the average dose to the rectum, small intestine, bladder and pelvic marrow (P=0.003, 0.000, 0.001 and 0.000, respectively) and markedly reduced irradiated normal tissue volumes.


CONCLUSIONS
A 6DoF couch in combination with daily kV-CBCT can considerably improve positioning accuracy during VMAT treatment in gynecologic malignancies, reducing the MPTV. The reduced margin size permits improved normal tissue sparing and a smaller total irradiated volume.
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. yaolihong133@sina.com.
Lihong
L
Yao
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. zhulihong510@163.comp.
Lihong
L
Zhu
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. junjiewang_edu@sina.cn.
Junjie
J
Wang
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. bysyliulu@163.com.
Lu
L
Liu
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. zhoushun88195@163.com.
Shun
S
Zhou
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. jiangshukun666@163.com.
ShuKun
S
Jiang
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. qqconstance@163.com.
Qianqian
Q
Cao
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. qa11980@sina.com.
Ang
A
Qu
Department of Radiation Oncology, Peking University Third Hospital, Hua-yuan North Road No.49, Haidian District, Beijing, 100191, P. R. China. tsq-1978@163.com.
Suqing
S
Tian
D000230:Adenocarcinoma / Q000532:radiotherapy* / Q000556:secondary; D000328:Adult; D002294:Carcinoma, Squamous Cell / Q000532:radiotherapy* / Q000556:secondary; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D016889:Endometrial Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D008207:Lymphatic Metastasis; D008875:Middle Aged; D009068:Movement; D009367:Neoplasm Staging; D056888:Patient Positioning; D011379:Prognosis; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D061106:Radiotherapy Setup Errors; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D014057:Tomography, X-Ray Computed; D002583:Uterine Cervical Neoplasms / Q000473:pathology / Q000532:radiotherapy*