Intra-fraction prostate displacement in radiotherapy estimated from pre- and post-treatment imaging of patients with implanted fiducial markers.
OBJECTIVE
To determine intra-fraction displacement of the prostate gland from imaging pre- and post-radiotherapy delivery of prostate cancer patients with three implanted fiducial markers.


METHODS
Data were collected from 184 patients who had two orthogonal X-rays pre- and post-delivery on at least 20 occasions using a Varian On Board kV Imaging system. A total of 5778 image pairs covering time intervals between 3 and 30 min between pre- and post-imaging were evaluated for intra-fraction prostate displacement.


RESULTS
The mean three dimensional vector shift between images was 1.7 mm ranging from 0 to 25 mm. No preferential direction of displacement was found; however, there was an increase of prostate displacement with time between images. There was a large variation in typical shifts between patients (range 1 +/- 1 to 6 +/- 2 mm) with no apparent trends throughout the treatment course. Images acquired in the first five fractions of treatment could be used to predict displacement patterns for individual patients.


CONCLUSIONS
Intra-fraction motion of the prostate gland appears to be a limiting factor when considering margins for radiotherapy. Given the variation between patients, a uniform set of margins for all patients may not be satisfactory when high target doses are to be delivered.
Kron
Tomas
T
Department of Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia. Tomas.Kron@petermac.org
Thomas
Jessica
J
Fox
Chris
C
Thompson
Ann
A
Owen
Rebecca
R
Herschtal
Alan
A
Haworth
Annette
A
Tai
Keen-Hun
KH
Foroudi
Farshad
F
D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011859:Radiography; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal