Small field diode correction factors derived using an air core fibre optic scintillation dosimeter and EBT2 film

Abstract

There is no commercially available real-time dosimeter that can accurately measure output factors for field sizes down to 4 mm without the use of correction factors. Silicon diode detectors are commonly used but are not dosimetrically water equivalent, resulting in energy dependence and fluence perturbation. In contrast, plastic scintillators are nearly dosimetrically water equivalent. A fibre optic dosimeter (FOD) with a 0.8 mm(3) plastic scintillator coupled to an air core light guide was used to measure the output factors for Novalis/BrainLab stereotactic cones of diameter 4-30 mm and Novalis MLC fields of width 5-100 mm. The FOD data matched the output factors measured by a 0.125 cm(3) Semiflex ion chamber for the MLC fields above 30 mm and those measured with the EBT2 radiochromic film for the cones and MLC fields below 30 mm. Relative detector readings were obtained with four diode types (IBA SFD, EFD, PFD, PTW 60012) for the same fields. Empirical diode correction factors were determined by taking the ratio of FOD output factors to diode relative detector readings. The diodes were found to over-respond by 3%-16% for the smallest field. There was good agreement between different diodes of the same model number.

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

“…The air core FOD is a cylindrical scintillation dosimeter with a sensitive volume of 1 mm diameter and 1 mm length, manufactured for use in external beam dosimetry, has been shown to be dosimetrically water‐equivalent, have no directional dependence, and have sufficient spatial resolution for use in field sizes as small as 4 mm in diameter 10 , 19 , 21 . Scintillation dosimeters are a good choice as a reference detector for small field work 15 , 22 as their response has been shown to be independent of dose rate, energy, and temperature, and they provide a linear response with delivered dose 23 , 24 .…”

“…Scintillation dosimeters are a good choice as a reference detector for small field work 15 , 22 as their response has been shown to be independent of dose rate, energy, and temperature, and they provide a linear response with delivered dose 23 , 24 . Scintillation dosimeters have been benchmarked against film 10 , 21 and Monte Carlo (25) and have a smaller measurement uncertainty than film.…”

“…Alfonso et al (18) recommends radiochromic film for determination of small field correction factors. Although dosimetrically water‐equivalent and suitable for measurement of all field sizes in this study, EBT3 was not used as a reference detector due to the inability to perform real‐time readout and to the larger uncertainty in measurement compared with the FOD 10 , 21 . The film was, however, used to validate the accuracy of the FOD relative output factors in the radiation beams investigated.…”

“…For the smallest fields, volume averaging correction factors were derived, using the methodology described by Ralston et al, (10) and subsequently applied to detector readings. Volume averaging occurs when the dimensions of the detector perpendicular to the beam central axis are large compared to the uniform region of the radiation beam, which can cause a decrease in the measured signal (27) …”

“…Several studies have reported on the response of ionization chambers, diodes, and the microdiamond 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 detector in small radiation fields for flattened beams, using either experiment or Monte Carlo simulations. Where necessary, small field correction factors $false(k_{Q_{clin}{Q}_{msr}}^{f_{clin},f_{msr}}false)$, as defined by Alfonso et al, (18) have been provided.…”