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SU-E-T-16: Statistical Variability and Confidence Intervals for Planar Dose QA Pass Rates.

Abstract：

PURPOSE:The most common metric for comparing measured to calculated dose planes is a pass rate generated using percent difference, distance-to-agreement (DTA), or some combination of the two (e.g. gamma evaluation). The grid of analyzed points often corresponds to a dosimeter array with low areal-density of point detectors. This work examines the statistical uncertainty of planar dose comparison pass rates and proposes methods for establishing confidence intervals for pass rates obtained with low detector-density arrays. METHODS:Absolute dose planes were acquired via EPID for twenty intensity-modulated fields of varying complexity. Matching calculated dose planes were created via treatment planning system. Pass rates for each dose plane pair (centered to CAX) were calculated with various %/DTA composite analysis techniques. Software was designed to selectively sample the high-density EPID matrix to simulate many low-density measured grids, each representing a different alignment with respect to CAX. Simulations were repeated (100 positional iterations per field) using grids of varying detector-densities and both random and orthogonal point-detector orientation. For each simulation, pass rates were calculated with various composite analysis techniques. RESULTS:Repositioning simulated low-density grids leads to a distribution of possible pass rates for each measured/calculated dose plane pair, independent of whether the detector grid is random or uniform. Distributions can be predicted using a binomial distribution by which a confidence interval (function of sampling density and observed pass rate) is approximated for each pass rate. For example, 95% confidence intervals for IMRT pass rates (2%,2mm) average +/-5.3% and +/-3.8% with 1-detector/cm2 and 2-detector/cm2 grids, respectively. CONCLUSIONS:Pass rates for low-density array measurements are not absolute and should be reported with both a full description of calculation method and confidence intervals quantifying their uncertainty. Results extend to 3D detector arrays. The concept of fixed 'action levels' for pass rates must be reexamined for low-density array measurements.

journal_name

Med Phys

journal_title

Medical physics

authors

Bailey D,Nelms B,Kumaraswamy L,Attwood K,Podgorsak M

doi

10.1118/1.4735070

subject

Has Abstract

pub_date

2012-06-01 00:00:00

pages

3705-3706

issue

6Part9

eissn

0094-2405

issn

2473-4209

journal_volume

pub_type

杂志文章

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SU-E-T-16: Statistical Variability and Confidence Intervals for Planar Dose QA Pass Rates.