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Design and experimental testing of air slab caps which convert commercial electron diodes into dual purpose, correction‐free diodes for small field dosimetry
Author(s) -
Charles P. H.,
CranmerSargison G.,
Thwaites D. I.,
Kairn T.,
Crowe S. B.,
Pedrazzini G.,
Aland T.,
Kenny J.,
Langton C. M.,
Trapp J. V.
Publication year - 2014
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 - 10.1118/1.4894728
Subject(s) - diode , dosimetry , air gap (plumbing) , optics , materials science , electron , optoelectronics , x ray detector , step recovery diode , detector , physics , nuclear medicine , schottky diode , nuclear physics , medicine , composite material
Purpose: Two diodes which do not require correction factors for small field relative output measurements are designed and validated using experimental methodology. This was achieved by adding an air layer above the active volume of the diode detectors, which canceled out the increase in response of the diodes in small fields relative to standard field sizes.Methods: Due to the increased density of silicon and other components within a diode, additional electrons are created. In very small fields, a very small air gap acts as an effective filter of electrons with a high angle of incidence. The aim was to design a diode that balanced these perturbations to give a response similar to a water‐only geometry. Three thicknesses of air were placed at the proximal end of a PTW 60017 electron diode (PTWe) using an adjustable “air cap”. A set of output ratios ( OR Detf clin) for square field sizes of side length down to 5 mm was measured using each air thickness and compared toOR Detf clinmeasured using an IBA stereotactic field diode (SFD).kQ clin, Q msrf clin, f msrwas transferred from the SFD to the PTWe diode and plotted as a function of air gap thickness for each field size. This enabled the optimal air gap thickness to be obtained by observing which thickness of air was required such thatkQ clin, Q msrf clin, f msrwas equal to 1.00 at all field sizes. A similar procedure was used to find the optimal air thickness required to make a modified Sun Nuclear EDGE detector (EDGEe) which is “correction‐free” in small field relative dosimetry. In addition, the feasibility of experimentally transferringkQ clin, Q msrf clin, f msrvalues from the SFD to unknown diodes was tested by comparing the experimentally transferredkQ clin, Q msrf clin, f msrvalues for unmodified PTWe and EDGEe diodes to Monte Carlo simulated values.Results: 1.0 mm of air was required to make the PTWe diode correction‐free. This modified diode (PTWe air ) produced output factors equivalent to those in water at all field sizes (5–50 mm). The optimal air thickness required for the EDGEe diode was found to be 0.6 mm. The modified diode (EDGEe air ) produced output factors equivalent to those in water, except at field sizes of 8 and 10 mm where it measured approximately 2% greater than the relative dose to water. The experimentally calculatedkQ clin, Q msrf clin, f msrfor both the PTWe and the EDGEe diodes (without air) matched Monte Carlo simulated results, thus proving that it is feasible to transferkQ clin, Q msrf clin, f msrfrom one commercially available detector to another using experimental methods and the recommended experimental setup.Conclusions: It is possible to create a diode which does not require corrections for small field output factor measurements. This has been performed and verified experimentally. The ability of a detector to be “correction‐free” depends strongly on its design and composition. A nonwater‐equivalent detector can only be “correction‐free” if competing perturbations of the beam cancel out at all field sizes. This should not be confused with true water equivalency of a detector.