ABSTRACT
This report specifically describes the use of a unique anthropomorphic breast phantom to validate the accuracy of three-dimensional dose calculations performed by a commercial treatment-planning system for intact-breast tangential irradiation. The accuracy of monitor-unit calculations has been corroborated using ionization chamber measurements made in this phantom. Measured doses have been compared to those calculated from a variety of treatment plans. The treatment plans utilized a 6-MV x-ray beam and incorporated a variety of field configurations and wedge combinations. Dose measurements at several clinically relevant points within the breast phantom have confirmed the accuracy of calculated doses generated from the variety of treatment plans. Overall agreement between measurements and calculations averaged 0.998+/-0.009. These results indicate that the dose per monitor-unit calculations performed by the treatment-planning system can be confidently utilized in the fulfillment of clinical dose prescriptions.
Subject(s)
Breast Neoplasms/radiotherapy , Imaging, Three-Dimensional/methods , Phantoms, Imaging , Radiotherapy Planning, Computer-Assisted/instrumentation , Radiotherapy Planning, Computer-Assisted/methods , Tomography, X-Ray Computed/methods , Female , Humans , Radiation Monitoring , Radiotherapy DosageABSTRACT
Recently, the mechanical failure of one of the upper collimator mechanical trimmers on a cobalt-60 unit resulted in large beam asymmetries and unacceptable flatness characteristics. This malfunction was not detected using currently accepted schedules for quality assurance tests. The incident suggests that the frequency of routine beam profile constancy checks should be increased to weekly for cobalt-60 units.
Subject(s)
Cobalt Radioisotopes/therapeutic use , Radioisotope Teletherapy/instrumentation , Radioisotope Teletherapy/standards , Biophysical Phenomena , Biophysics , Equipment Failure , Humans , Quality Assurance, Health Care , RadiometryABSTRACT
An empirical logistic model that linearizes the sensitometric response data of screen-film systems over the entire dynamic range of exposures is presented. This linearization is evident when the net optical densities are scaled as fractions of the net saturation density and plotted on commercial logit, probit, or double-log paper, except under conditions of reciprocity law failure. Weighted linear regression analysis shows that the intercept, but not the slope, depends on the screen-film system used. Previous work indicates that the slope is also independent of development time and photon energy. The model is verified through an analysis of tabulated sensitometric data published by the Center for Devices and Radiological Health.
Subject(s)
X-Ray Intensifying Screens , Logistic Models , Technology, RadiologicABSTRACT
An approximation to saturation curves of parallel-plate ionization chambers for various gases under continuous irradiation is given. The proposed fit asymptotically agrees with Mie's set of equations for the low and the high collection efficiency regions. It is shown that this fit which, for gases containing electronegative atoms, is dependent upon a parameter obtained from physical constants agrees with measurement and with values calculated from Mie's set of equations. However, for gases containing nonelectronegative atoms, it is shown that an additional experimental parameter is required to fit the measurements.
Subject(s)
Gases/radiation effects , Models, Theoretical , MathematicsABSTRACT
An empirically derived logit function is proposed to describe the modulation transfer function of x-ray screen-film systems. Simple and exact weighted linear regression analyses are performed to characterize the function and to describe the data within experimental accuracy. Calculations comparing the proposed function and the complementary error function (probit) representations of the modulation transfer function are given and discussed.
Subject(s)
Radiographic Image Enhancement , X-Ray Intensifying Screens , Models, Theoretical , Radiographic Image Enhancement/instrumentation , Regression AnalysisABSTRACT
Four different techniques for calibrating lithium fluoride thermoluminescent dosimeters were compared at two beam qualities in the diagnostic energy range. The techniques included in-air, saturation backscatter, and Bureau of Radiological Health (BRH) test stand geometry with and without an ionization chamber. The results indicate that there are no statistical differences between any of the calibration techniques evaluated. Hence, the technique using the BRH test stand with an ionization chamber is a simple and reproducible calibration procedure which may be used in a routine diagnostic radiology quality assurance program.
Subject(s)
Lithium Compounds , Radiation Dosage , Technology, Radiologic , Fluorides , Health Physics , Humans , Lithium , Luminescent MeasurementsABSTRACT
The precision and accuracy achieved in the measurement of characteristic curves for radiographic screen/film systems is quantitatively investigated for three techniques: inverse square, kVp bootstrap, and step-wedge bootstrap. Precision of all techniques is generally better than +/- 1.5% while the agreement among all intensity-scale techniques is better than 2% over the useful exposure latitude. However, the accuracy of the sensitometry will depend on several factors, including linearity and energy dependence of the calibration instrument, that may introduce larger errors. Comparisons of time-scale and intensity-scale methods are made and a means of measuring reciprocity law failure is demonstrated.
