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Int J Radiat Oncol Biol Phys. 2007 Apr 1;67(5):1430-7. Epub 2007 Jan 8.

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.

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1
Department of Medical Physics, McGill University Health Centre, Montréal, Québec, Canada.

Abstract

INTRODUCTION:

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 AND MATERIALS:

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.

CONCLUSION:

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.

Comment in

PMID:
17208385
DOI:
10.1016/j.ijrobp.2006.11.004
[Indexed for MEDLINE]

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