Applicability and dosimetric impact of ultrasound-based preplanning in high-dose-rate brachytherapy of prostate cancer.
OBJECTIVE
Analyses of permanent brachytherapy seed implants of the prostate have demonstrated that the use of a preplan may lead to a considerable decrease of dosimetric implant quality. The authors aimed to determine whether the same drawbacks of preplanning also apply to high-dose-rate (HDR) brachytherapy.


METHODS
15 patients who underwent two separate HDR brachytherapy implants in addition to external-beam radiation therapy for advanced prostate cancer were analyzed. A pretherapeutic transrectal ultrasound was performed in all patients to generate a preplan for the first brachytherapy implant. For the second brachytherapy, a subset of patients were treated by preplans based on the ultrasound from the first brachytherapy implant. Preplans were compared with the respective postplans assessing the following parameters: coverage index, minimum target dose, homogeneity index, and dose exposure of organs at risk. The prostate geometries (volume, width, height, length) were compared as well.


RESULTS
At the first brachytherapy, the matching between the preplan and actual implant geometry was sufficient in 47% of the patients, and the preplan could be applied. The dosimetric implant quality decreased considerably: the mean coverage differed by -0.11, the mean minimum target dose by -0.15, the mean homogeneity index by -0.09. The exposure of organs at risk was not substantially altered. At the second brachytherapy, all patients could be treated by the preplan; the differences between the implant quality parameters were less pronounced. The changes of prostate geometry between preplans and postplans were considerable, the differences in volume ranging from -8.0 to 13.8 cm(3) and in dimensions (width, height, length) from -1.1 to 1.0 cm.


CONCLUSIONS
Preplanning in HDR brachytherapy of the prostate is associated with a substantial decrease of dosimetric implant quality, when the preplan is based on a pretherapeutic ultrasound. The implant quality is less impaired in subsequent implants of fractionated brachytherapy.
Aebersold
Daniel M
DM
Department of Radiation Oncology with Division of Medical Radiation Physics, University of Bern, Inselspital, Bern, Switzerland.
Isaak
Bernhard
B
Thalmann
George
G
Behrensmeier
Frank
F
Kolotas
Christos
C
Kranzbühler
Helmut
H
Mini
Roberto
R
Greiner
Richard H
RH
D001918:Brachytherapy / Q000379:methods*; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D016896:Treatment Outcome; D014463:Ultrasonography