Can digitally reconstructed radiographs (DRRS) replace simulation films in prostate cancer conformal radiotherapy?
OBJECTIVE
To evaluate the precision of using digitally reconstructed radiographs (DRRs) of either 3 mm or 6 mm slice separation vs. using simulator images for the setup verification of patients receiving CT planned conformal radiotherapy to the prostate. To calculate the transfer error between CT and simulator.


METHODS
Twenty patients were CT scanned (3 mm slice spacing/width). DRRs were generated for both 3 mm (DRR 3) and 6 mm (DRR 6) separations. DRRs and a simulator image of an anterior and a lateral field were used as reference images. Five observers matched each of the reference images to treatment images using the Theraview "Target check" facility. It was assumed that poorer images would lead to a loss of precision of field placement estimations (FPE) between observers. The study was designed to detect a difference greater than 1.5 mm(2) in the precision of image placement. The transfer error was the mean difference in the setup error derived from the DRRs and the simulation films.


RESULTS
The precision of evaluations for simulator films and 3 mm DRRs were similar. There was a trend for the DRR 6 mm to achieve less precise results which was greatest for craniocaudal examinations (variance: simulator 1.5 mm(2), DRR6 2.8 mm(2), p = 0.17), but this did not reach statistical significance. A range of transfer errors was identified, with standard deviations ranging from 1.7 to 4.2 mm. There was evidence of a significant systematic bias in anterior craniocaudal (1.3-1.9 mm, p < 0.004) and anterior posterior (-1.9 mm, p = 0.027).


CONCLUSIONS
The precision of setup evaluations using DRRs is similar to that achieved by using simulator fields when planning conformal prostate radiotherapy. The use of DRRs could reduce systematic errors introduced in the planning process.
Academic Unit of Radiotherapy & Oncology, The Royal Marsden NHS Trust, Sutton, United Kingdom. marc.bollet@curie.net
Marc A
MA
Bollet
Helen A
HA
McNair
Vibeke N
VN
Hansen
Andrew
A
Norman
Una
U
O'Doherty
Helen
H
Taylor
Mark
M
Rose
Rahul
R
Mukherjee
Robert
R
Huddart
D003198:Computer Simulation; D006801:Humans; D008297:Male; D015588:Observer Variation; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011857:Radiographic Image Interpretation, Computer-Assisted; D020266:Radiotherapy, Conformal; D014057:Tomography, X-Ray Computed