Target motion variability and on-line positioning accuracy during external-beam radiation therapy of prostate cancer with an endorectal balloon device.
OBJECTIVE
To prospectively define the setup error and the interfraction prostate localization accuracy of the planning target volume (PTV) in the presence of an endorectal balloon (ERB) device.


METHODS
Weekly portal images (PIs) of 15 patients undergoing external-beam radiotherapy were analyzed. Displacements of the isocenter and the center of the ERB were measured. The setup and target motion variability were assessed with regard to the position variability of the ERB.


RESULTS
The setup error was random and target motion variability was largest in the craniocaudal direction. The mean displacement of the isocenter was 2.1 mm (+/-1.2 mm SD [standard deviation]), 2.4 mm (+/-2.2 mm SD), and 3.8 mm (+/-4.0 mm SD) in the left-right, craniocaudal, and anteroposterior directions, respectively (p=0.1). The mean displacement of the ERB was 2.0 mm (+/-1.4 mm SD), 4.1 mm (+/-2.0 mm SD), and 3.8 mm (+/-3.3 mm SD; p=0.03). Setup margin and internal margin contributed equally to the PTV margin. Cumulative placement insecurity of the field and the ERB together was 4.0 mm (+/-2.1 mm SD) laterally, 6.4 mm (+/-2.5 mm SD) craniocaudally, and 7.7 mm (+/-7.0 mm SD) anteroposteriorly. The 95% CIs (confidence intervals) were 2.9-5.2 mm, 5.1-7.8 mm, and 3.8-11.5 mm. In 35% of cases, the estimation of the dorsal margin exceeded 1 cm.


CONCLUSIONS
Margin estimate dorsally may exceed 1 cm and on-line position verification with an ERB cannot be recommended for dose escalation>70 Gy.
El-Bassiouni
Mazen
M
Radiation Oncology, Zurich University Hospital, University of Zurich, Zurich, Switzerland.
Davis
J Bernard
JB
El-Attar
Inas
I
Studer
Gabriela M
GM
Lütolf
Urs M
UM
Ciernik
I Frank
IF
D000368:Aged; D002404:Catheterization / Q000379:methods; D016001:Confidence Intervals; D006801:Humans; D008297:Male; D009068:Movement; D011187:Posture; D011446:Prospective Studies; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal / Q000295:instrumentation; D012007:Rectum / Q000528:radiation effects*; D014057:Tomography, X-Ray Computed