Intrafraction motion of the prostate during an IMRT session: a fiducial-based 3D measurement with Cone-beam CT.
BACKGROUND
Image-guidance systems allow accurate interfractional repositioning of IMRT treatments, however, these may require up to 15 minutes. Therefore intrafraction motion might have an impact on treatment precision. 3D geometric data regarding intrafraction prostate motion are rare; we therefore assessed its magnitude with pre- and post-treatment fiducial-based imaging with cone-beam-CT (CBCT).


METHODS
39 IMRT fractions in 5 prostate cancer patients after 125I-seed implantation were evaluated. Patient position was corrected based on the 125I-seeds after pre-treatment CBCT. Immediately after treatment delivery, a second CBCT was performed. Differences in bone- and fiducial position were measured by seed-based grey-value matching.


RESULTS
Fraction time was 13.6 +/- 1.6 minutes. Median overall displacement vector length of 125I-seeds was 3 mm (M = 3 mm, Sigma = 0.9 mm, sigma = 1.7 mm; M: group systematic error, Sigma: SD of systematic error, sigma: SD of random error). Median displacement vector of bony structures was 1.84 mm (M = 2.9 mm, Sigma = 1 mm, sigma = 3.2 mm). Median displacement vector length of the prostate relative to bony structures was 1.9 mm (M = 3 mm, Sigma = 1.3 mm, sigma = 2.6 mm).


CONCLUSIONS
a) Overall displacement vector length during an IMRT session is < 3 mm.b) Positioning devices reducing intrafraction bony displacements can further reduce overall intrafraction motion.c) Intrafraction prostate motion relative to bony structures is < 2 mm and may be further reduced by institutional protocols and reduction of IMRT duration.
Boda-Heggemann
Judit
J
Department of Radiation Oncology, University Medical Center Mannheim, Mannheim, Germany. juditboda@hotmail.com
Köhler
Frederick Marc
FM
Wertz
Hansjörg
H
Ehmann
Michael
M
Hermann
Brigitte
B
Riesenacker
Nadja
N
Küpper
Beate
B
Lohr
Frank
F
Wenz
Frederik
F
D001918:Brachytherapy / Q000295:instrumentation / Q000379:methods; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D019583:Dose Fractionation, Radiation; D006801:Humans; D007091:Image Processing, Computer-Assisted; D021621:Imaging, Three-Dimensional / Q000379:methods; D007496:Iridium Radioisotopes / Q000627:therapeutic use; D008297:Male; D009038:Motion; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D018787:Radiation Oncology / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D015203:Reproducibility of Results; D016896:Treatment Outcome
D007496:Iridium Radioisotopes