Measurement of intrafractional prostate motion using magnetic resonance imaging.
OBJECTIVE
To quantify the three-dimensional intrafractional prostate motion over typical treatment time intervals with cine-magnetic resonance imaging (cine MRI) studies.


METHODS
Forty-two patients with prostate cancer were scanned supine in an alpha cradle cast using cine MRI. Twenty sequential slices were acquired in the sagittal and axial planes through the center of the prostate. Each scan took approximately 9 min. The posterior, lateral, and superior edges of the prostate were tracked on each frame relative to the initial prostate position, and the size and duration of each displacement was recorded.


RESULTS
The prostate displacements were (mean +/- SD): 0.2 +/- 2.9 mm, 0.0 +/- 3.4 mm, and 0.0 +/- 1.5 mm in the anterior-posterior, superior-inferior, and medial-lateral dimensions respectively. The prostate motion appeared to have been driven by peristalsis in the rectum. Large displacements of the prostate (up to 1.2 cm) moved the prostate both anteriorly and superiorly and in some cases compressed the organ. For such motions, the prostate did not stay displaced, but moved back to its original position. To account for the dosimetric consequences of the motion, we also calculated the time-averaged displacement to be approximately 1 mm.


CONCLUSIONS
Cine MRI can be used to measure intrafractional prostate motion. Although intrafractional prostate motions occur, their effects are negligible compared to interfractional motion and setup error. No adjustment in margin is necessary for three-dimensional conformal or intensity-modulated radiation therapy.
Mah
Dennis
D
Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA, USA. dm2066@columbia.edu
Freedman
Gary
G
Milestone
Bart
B
Hanlon
Alexandra
A
Palacio
Elizabeth
E
Richardson
Theresa
T
Movsas
Benjamin
B
Mitra
Raj
R
Horwitz
Eric
E
Hanks
Gerald E
GE
D006801:Humans; D019028:Magnetic Resonance Imaging, Cine; D008297:Male; D009068:Movement; D015588:Observer Variation; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D013997:Time Factors