Impact of intrafractional bowel gas movement on carbon ion beam dose distribution in pancreatic radiotherapy.
OBJECTIVE
To assess carbon ion beam dose variation due to bowel gas movement in pancreatic radiotherapy.


METHODS
Ten pancreatic cancer inpatients were subject to diagnostic contrast-enhanced dynamic helical CT examination under breath-holding conditions, which included multiple-phase dynamic CT with arterial, venous, and delayed phases. The arterial-venous phase and arterial-delayed phase intervals were 35 and 145 s, respectively. A compensating bolus was designed to cover the target obtained at the arterial phase. Carbon ion dose distribution was calculated by applying the bolus to the CT data sets at the other two phases.


RESULTS
Dose conformation to the clinical target volume was degraded by beam overshoot/undershoot due to bowel gas movement. The D95 for clinical target volume was degraded from 98.2% (range, 98.0-99.1%) of the prescribed dose to 94.7% (range, 88.0-99.0%) at 145 s. Excessive dosing to normal tissues varied among tissues and was, for example, 12.2 GyE/13.1 GyE (0 s/145 s) for the cord and 38.8 GyE/39.8 GyE (0 s/145 s) for the duodenum. The magnitude of beam overshoot/undershoot was particularly exacerbated from the anterior and left directions.


CONCLUSIONS
Bowel gas movement causes dosimetric variation to the target during treatment for radiotherapy. The effect of bowel gas movement varies with beam angle, with greatest influence on the anterior-posterior and left-right beams.
Kumagai
Motoki
M
Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba, Japan.
Hara
Ryusuke
R
Mori
Shinichiro
S
Yanagi
Takeshi
T
Asakura
Hiroshi
H
Kishimoto
Riwa
R
Kato
Hirotoshi
H
Yamada
Shigeru
S
Kandatsu
Susumu
S
Kamada
Tadashi
T
D000368:Aged; D000369:Aged, 80 and over; D002250:Carbon Radioisotopes / Q000627:therapeutic use; D003287:Contrast Media; D005260:Female; D005740:Gases; D006801:Humans; D007422:Intestines / Q000000981:diagnostic imaging / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009068:Movement; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012119:Respiration; D036542:Tomography, Spiral Computed / Q000379:methods*
D002250:Carbon Radioisotopes; D003287:Contrast Media; D005740:Gases