Changes in Rectal Dose Due to Alterations in Beam Angles for Setup Uncertainty and Range Uncertainty in Carbon-Ion Radiotherapy for Prostate Cancer.
OBJECTIVE
Carbon-ion radiotherapy of prostate cancer is challenging in patients with metal implants in one or both hips. Problems can be circumvented by using fields at oblique angles. To evaluate the influence of setup and range uncertainties accompanying oblique field angles, we calculated rectal dose changes with oblique orthogonal field angles, using a device with fixed fields at 0° and 90° and a rotating patient couch.


METHODS
Dose distributions were calculated at the standard angles of 0° and 90°, and then at 30° and 60°. Setup uncertainty was simulated with changes from -2 mm to +2 mm for fields in the anterior-posterior, left-right, and cranial-caudal directions, and dose changes from range uncertainty were calculated with a 1 mm water-equivalent path length added to the target isocenter in each angle. The dose distributions regarding the passive irradiation method were calculated using the K2 dose algorithm.


RESULTS
The rectal volumes with 0°, 30°, 60°, and 90° field angles at 95% of the prescription dose were 3.4±0.9 cm3, 2.8±1.1 cm3, 2.2±0.8 cm3, and 3.8±1.1 cm3, respectively. As compared with 90° fields, 30° and 60° fields had significant advantages regarding setup uncertainty and significant disadvantages regarding range uncertainty, but were not significantly different from the 90° field setup and range uncertainties.


CONCLUSIONS
The setup and range uncertainties calculated at 30° and 60° field angles were not associated with a significant change in rectal dose relative to those at 90°.
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Yoshiki
Y
Kubota
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Hidemasa
H
Kawamura
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Makoto
M
Sakai
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Ryou
R
Tsumuraya
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Mutsumi
M
Tashiro
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Ken
K
Yusa
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Nobuteru
N
Kubo
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Hiro
H
Sato
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Masahiro
M
Kawahara
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Hiroyuki
H
Katoh
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Tatsuaki
T
Kanai
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Tatsuya
T
Ohno
Gunma University Heavy Ion Medical Center, Gunma, Japan.
Takashi
T
Nakano
D000368:Aged; D063193:Heavy Ion Radiotherapy; D006801:Humans; D008297:Male; D008875:Middle Aged; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors; D012007:Rectum / Q000528:radiation effects*; D012189:Retrospective Studies; D035501:Uncertainty; D001743:Urinary Bladder / Q000528:radiation effects