An assessment of PTV margin definitions for patients undergoing conformal 3D external beam radiation therapy for prostate cancer based on an analysis of 10,327 pretreatment daily ultrasound localizations.
BACKGROUND
We have assessed the planning target volume (PTV) margins required for adequate treatment of the prostate in the absence of daily localization imaging based on the statistical analysis of a large data set obtained from 5 years of use of a two-dimensional ultrasound pretreatment localization device.


METHODS
Data from 387 prostate patients were analyzed retrospectively. Every patient in the study received daily pretreatment localization resulting in a total of 10,327 localizations, each comprising an isocenter displacement in three directions: anteroposterior, right-left lateral, and superior-inferior. The mean displacement for each direction for each patient was computed from daily treatment records, and a mean of the means was used in the analysis.


RESULTS
The mean displacements required to shift the target to the required position were 6.1 mm posterior (4.4 mm SD), 2.1 mm superior (4.5 mm SD), and 0.5 mm right (3.6 mm SD). The 6.1-mm shift posterior is indicative of a systematic uncertainty. Differences in planning conditions between the computed tomography simulation and the treatment room may account for this discrepancy.


CONCLUSIONS
Our study has revealed systematic intertreatment uncertainties that would have required a nonuniform PTV margin ranging in dimensions between 2.7 mm anterior, 14.9 mm posterior, 7.7 mm right, 6.7 mm left, 11 mm superior, and 7 mm inferior to encompass the prostate for 95% of our sample if the ultrasound localization system were not used. In the absence of systematic uncertainties, a uniform PTV margin of 9 mm would suffice.
Poli
M Esmeralda Ramos
ME
Department of Medical Physics, McGill University Health Centre, Montréal, Québec, Canada.
Parker
William
W
Patrocinio
Horacio
H
Souhami
Luis
L
Shenouda
George
G
Campos
Leticia Lucente
LL
Podgorsak
Ervin B
EB
D006801:Humans; D008297:Male; D009068:Movement; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D020266:Radiotherapy, Conformal; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed; D014463:Ultrasonography