Set-up errors due to endorectal balloon positioning in intensity modulated radiation therapy for prostate cancer.
OBJECTIVE
To investigate the set-up errors and deformation associated with daily placement of endorectal balloons in prostate radiotherapy.


METHODS
Endorectal balloons were placed daily in 20 prostate cancer patients undergoing radiotherapy. Electronic portal images (EPIs) were collected weekly from anterior-posterior (AP) and lateral views. The EPIs were compared with digitally reconstructed radiographs from computed tomography scans obtained during pretreatment period to estimate displacements. The interfraction deformation of balloon was estimated with variations in diameter in three orthogonal directions throughout the treatment course.


RESULTS
A total of 154 EPIs were evaluated. The mean displacements of balloon relative to bony landmark were 1.8mm in superior-inferior (SI), 1.3mm in AP, and 0.1mm in left-right (LR) directions. The systematic errors in SI, AP, and LR directions were 3.3mm, 4.9 mm, and 4.0mm, respectively. The random (interfraction) displacements, relative to either bony landmarks or treatment isocenter, were larger in SI direction (4.5mm and 4.5mm), than in AP (3.9 mm and 4.4mm) and LR directions (3.0mm and 3.0mm). The random errors of treatment isocenter to bony landmark were 2.3mm, 3.2mm, and 2.6mm in SI, AP, and LR directions, respectively. Over the treatment course, balloon deformations of 2.8mm, 2.5mm, and 2.6mm occurred in SI, AP, and LR directions, respectively. The coefficient of variance of deformation was 7.9%, 4.9%, and 4.9% in these directions.


CONCLUSIONS
Larger interfractional displacement and the most prominent interfractional deformation of endorectal balloon were both in SI direction.
Wang
Chun-Wei
CW
Institute of Electrical Engineering, National Taiwan University, Taipei, Taiwan.
Chong
Fok-Ching
FC
Lai
Ming-Kuen
MK
Pu
Yeong-Shiau
YS
Wu
Jian-Kuen
JK
Cheng
Jason Chia-Hsien
JC
D000368:Aged; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012007:Rectum; D014057:Tomography, X-Ray Computed