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Distance–dose curve for a miniature x‐ray tube for stereotactic radiosurgery using an optimized aperture with a parallel‐plate ionization chamber
Author(s) -
Biggs P. J.,
Beatty J.,
Yasuda T.
Publication year - 1999
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598792
Subject(s) - aperture (computer memory) , imaging phantom , ionization chamber , optics , materials science , x ray tube , radiosurgery , x ray , nuclear medicine , beam divergence , dosimetry , tube (container) , percentage depth dose curve , physics , ionization , beam diameter , electrode , medicine , radiation therapy , radiology , ion , laser , anode , quantum mechanics , composite material , laser beams , acoustics
A recently developed miniature x‐ray tube operating at 40 kV has been used in a randomized trial for the treatment of small intracranial lesions. The diameter of these lesions ranges from 10 to 30 mm. A thin window parallel‐plate ionization chamber was used to calibrate the output of the x‐ray tube, modified by the addition of a thin platinum aperture to reduce the charge collecting area of the chamber. The effect of such an aperture on the measurement of dose versus distance from the x‐ray tube in a phantom has been examined as a function of aperture diameter. Aperture diameters were varied between 0 and 5 mm and dose measurements were made for distances between the x‐ray source and the front surface of the chamber of 5–30 mm in water. The ratio of doses measured with and without an aperture, when normalized to unity at a distance of 10 mm, differs significantly from unity, for distances between 7.5 and 15 mm, for aperture diameters <1.5 mm and differs from unity, but less significantly, for apertures ⩾3 mm. For intermediate diameters, however, this dose dependence is minimized, indicating an aperture diameter that provides a similar distance–dose curve as the measurement taken without an aperture over this range of distances. This diameter was found to be between 2 and 2.5 mm with a dose variation of less than ±1%. For distances <7.5 mm, measurements made with a 1.5‐mm‐diam aperture agree better with those taken with a 1.7‐mm‐diam chamber compared with a 5.2‐mm‐diam chamber.