Characterizing the influence of detector density on dosimeter response in non-equilibrium small photon fields

Abstract

The impact of density and atomic composition on the dosimetric response of various detectors in small photon radiation fields is characterized using a 'density-correction' factor, F(detector), defined as the ratio of Monte Carlo calculated doses delivered to water and detector voxels located on-axis, 5 cm deep in a water phantom with a SSD of 100 cm. The variation of F(detector) with field size has been computed for detector voxels of various materials and densities. For ion chambers and solid-state detectors, the well-known variation of F(detector) at small field sizes is shown to be due to differences between the densities of detector active volumes and water, rather than differences in atomic number. However, associated changes in the measured shapes of small-field profiles offset these variations in F(detector), so that integral doses measured using the different detectors are quite similar, at least for slit fields. Since changes in F(detector) with field size arise primarily from differences between the densities of the detector materials and water, ideal small-field relative dosimeters should have small active volumes and water-like density.

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators

“…The overresponse in small radiation fields of silicon diodes, which incorporate silicon of density 2.3 g/cm 3 and possibly other high‐density material in their sensitive volumes and surroundings, has been well documented 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 38 , 39 , 40 , 41 . Diamond detectors, which have a sensitive volume of density 3.5 g/cm 3 , have been reported to overrespond in small fields 16 , 39 , 41 , 42 .…”

“…Diamond detectors, which have a sensitive volume of density 3.5 g/cm 3 , have been reported to overrespond in small fields 16 , 39 , 41 , 42 . Monte Carlo simulations have shown that the magnitude of overresponse is proportional to the mass density of the material in the volume of interest, with a larger overresponse determined for a voxel of the same size in a radiation beam for diamond than for silicon in a 5 mm field for 15 MV (38) . Similarly, air‐filled ionization chambers will show an underresponse compared to water, due to the low density of air compared with water in the sensitive volume.…”

Deriving detector‐specific correction factors for rectangular small fields using a scintillator detector

“…The dose perturbation effect is also minimal with the PSD, considering it has a physical density of 1.0 5 g/cm (39) . One major advantage of PSD over film is its instantaneous readout, which was favorable for clinical dosimetry.…”

“…These measurements are challenging, and are fundamentally linked to the physics of small fields. Unlike traditional large fields, electronic equilibrium breaks down as the radiation field size drops below the range of the secondary electrons, invalidating the Bragg‐Gray conditions 8 , 9 . Source occlusion also takes place at small fields, resulting in a decrease in the dose deposited (4) .…”

“…Source occlusion also takes place at small fields, resulting in a decrease in the dose deposited (4) . In addition, measurements at small fields are limited by the various detector‐specific effects 4 , 9 , 10 . These include the volume averaging around high‐gradient dose distributions, the fluence/dose perturbations introduced by the different physical densities between detector and medium, and the uncertainty in detector geometric alignment.…”