Estimation of rectal dose using daily megavoltage cone-beam computed tomography and deformable image registration.
OBJECTIVE
The actual dose delivered to critical organs will differ from the simulated dose because of interfractional organ motion and deformation. Here, we developed a method to estimate the rectal dose in prostate intensity modulated radiation therapy with consideration to interfractional organ motion using daily megavoltage cone-beam computed tomography (MVCBCT).


METHODS
Under exemption status from our institutional review board, we retrospectively reviewed 231 series of MVCBCT of 8 patients with prostate cancer. On both planning CT (pCT) and MVCBCT images, the rectal contours were delineated and the CT value within the contours was replaced by the mean CT value within the pelvis, with the addition of 100 Hounsfield units. MVCBCT images were rigidly registered to pCT and then nonrigidly registered using B-Spline deformable image registration (DIR) with Velocity AI software. The concordance between the rectal contours on MVCBCT and pCT was evaluated using the Dice similarity coefficient (DSC). The dose distributions normalized for 1 fraction were also deformed and summed to estimate the actual total dose.


RESULTS
The DSC of all treatment fractions of 8 patients was improved from 0.75±0.04 (mean ±SD) to 0.90 ±0.02 by DIR. Six patients showed a decrease of the generalized equivalent uniform dose (gEUD) from total dose compared with treatment plans. Although the rectal volume of each treatment fraction did not show any correlation with the change in gEUD (R(2)=0.18±0.13), the displacement of the center of gravity of rectal contours in the anterior-posterior (AP) direction showed an intermediate relationship (R(2)=0.61±0.16).


CONCLUSIONS
We developed a method for evaluation of rectal dose using DIR and MVCBCT images and showed the necessity of DIR for the evaluation of total dose. Displacement of the rectum in the AP direction showed a greater effect on the change in rectal dose compared with the rectal volume.
Department of Radiation Oncology, Osaka University Graduate School of Medicine, Suita, Osaka, Japan; Department of Radiology, Osaka University Hospital, Suita, Osaka, Japan. Electronic address: akino@radonc.med.osaka-u.ac.jp.
Yuichi
Y
Akino
Yasuo
Y
Yoshioka
Shoichi
S
Fukuda
Shintaroh
S
Maruoka
Yutaka
Y
Takahashi
Masashi
M
Yagi
Hirokazu
H
Mizuno
Fumiaki
F
Isohashi
Kazuhiko
K
Ogawa
D000465:Algorithms; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D019583:Dose Fractionation, Radiation; D006801:Humans; D008297:Male; D009068:Movement; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012007:Rectum / Q000000981:diagnostic imaging / Q000528:radiation effects*; D012189:Retrospective Studies; D012984:Software; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000528:radiation effects