The importance of urethra visualization for preplanned permanent prostate implants.
OBJECTIVE
To assess the potential consequences of using a surrogate urethra on urethral dose estimates in preplanned 125I prostate implants.


METHODS
For n=220 patients, the A-P and L-R extents of prostate and urethra contours were measured in transrectal ultrasound images. Treatment plans were then developed for 6 patients, of which 5 had atypical urethral positions. For each patient, three plan variations were made using the visualized and two different surrogate urethra contours.


RESULTS
The urethra typically remains fixed in the L-R direction and extends slightly below midgland, but may veer off-center and can come within 0.5 cm of the posterior surface of the prostate. Use of a surrogate urethra can potentially result in up to 30% of the urethra receiving doses exceeding a planned limit of 1.5 x 145 Gy over a contiguous length of 2.0 cm.


CONCLUSIONS
The urethra should be visualized for preplanning purposes, because unintended urethral doses arising from the use of a surrogate urethra can approach levels associated with late urinary morbidity. Visualization is also essential in the postimplant setting for accurate collection of dose-toxicity data.
Sloboda
Ron S
RS
Department of Medical Physics, Cross Cancer Institute, Edmonton, Canada. ron.sloboda@cancerboard.ab.ca
Pedersen
John E
JE
D001822:Body Burden; D001918:Brachytherapy / Q000009:adverse effects* / Q000295:instrumentation / Q000379:methods; D006801:Humans; D008297:Male; D009928:Organ Specificity; D011471:Prostatic Neoplasms / Q000150:complications / Q000532:radiotherapy*; D011832:Radiation Injuries / Q000517:prevention & control*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D015203:Reproducibility of Results; D018570:Risk Assessment / Q000379:methods; D012307:Risk Factors; D012680:Sensitivity and Specificity; D014463:Ultrasonography / Q000379:methods; D014521:Urethra / Q000000981:diagnostic imaging* / Q000528:radiation effects*; D014522:Urethral Diseases / Q000000981:diagnostic imaging* / Q000209:etiology / Q000517:prevention & control*