Three-dimensional analysis of delineation errors, setup errors, and organ motion during radiotherapy of bladder cancer.
OBJECTIVE
To quantify in three dimensions the geometric uncertainties of bladder irradiation (i.e., uncertainties in target delineation, organ motion, and patient setup).


METHODS
Pelvic CT images were obtained for 10 male bladder cancer patients. Apart from the initial planning CT scan, three follow-up scans were made for each of the patients. The bladder volumes in the planning CT scan were outlined by seven radiation oncologists. One observer also delineated the bladder volumes in the follow-up scans. Two-dimensional scalar maps of the interobserver variation and organ motion of the bladder surfaces were constructed. The setup errors were derived from the portal imaging results of the pooled group of bladder and prostate patients.


RESULTS
All bladder volumes were consistently outlined by all observers. Generally small variations occurred (1.5-3 mm, 1 SD), although in 50% of the patients, larger discrepancies were observed in discriminating the bladder from the base of the prostate. Analysis of the portal imaging data showed setup errors of up to 3 mm (1 SD). Organ motion is the predominant geometric uncertainty in the radiotherapy process (5 mm, 1 SD, at the cranial side of the bladder), although 9 of 10 patients were able to preserve a fairly reproducible bladder volume during the complete treatment course.


CONCLUSIONS
Anisotropic margins between the clinical target volume and planning target volume are needed in conformal radiotherapy of the bladder. Especially at the cranial side of the bladder, larger margins are needed because of the impact of bladder shape variation.
Meijer
Gert J
GJ
Department of Radiotherapy, The Netherlands Cancer Institute, Antoni van Leeuwenhoek Huis, Amsterdam, The Netherlands. gert.meijer@cze.nl
Rasch
Coen
C
Remeijer
Peter
P
Lebesque
Joos V
JV
D000368:Aged; D000369:Aged, 80 and over; D016477:Artifacts; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D008875:Middle Aged; D009038:Motion; D015588:Observer Variation; D011467:Prostate / Q000000981:diagnostic imaging / Q000473:pathology; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed; D001743:Urinary Bladder / Q000000981:diagnostic imaging* / Q000473:pathology; D001749:Urinary Bladder Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*