Modeling silicon diode dose response factors for small photon fields

Abstract

The dosimetry of small fields is important for the use of high resolution photon radiotherapy. Silicon diodes yield a high signal from a small detecting volume which makes them suitable for use in small fields and high dose gradients. Unshielded diodes used in large fields are known to give a varying dose response depending on the proportion of low energy scattered photons in the field. Response variations in small fields can be caused by both spectral variations, and disturbances of the local level of lateral electron equilibrium. We present a model that includes the effects from lack of charged particle equilibrium. The local spectra are calculated by use of fluence pencil kernels and divided into a low and a high energy component. The low energy part is treated with large cavity theory and the high energy part with the Spencer-Attix small cavity theory. Monte Carlo-derived correction factors are used to account for both the local level of electron equilibrium in the field, and deviations from this level in the silicon disk cavity. Results for field sizes ranging from 0.5 × 0.5 to 20 × 20 cm² are compared to data from full Monte Carlo simulations and measurements. The achieved dose response accuracy is for the smallest fields 1-2%, and for larger fields 0.5%. Spectral variations were of little importance for the small field response, implying that volume averaging, and to some extent interface transient effects, are of importance for use of unshielded diodes in non-equilibrium conditions. The results indicate that diodes should preferably be designed to have the thin layer of active volume padded in between inactive layers of the silicon base material.

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 .…”

“…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.…”

Dose domain regularization of MLC leaf patterns for highly complex IMRT plans

“…39,40 The accuracy of a planning system depends on the accuracy of commissioning data, which are either unavailable or inaccurate for fields smaller than 2 cm due to the lack of electron equilibrium, volume averaging, and spectral variation. 41,42 Plans with a larger numbers of small fields are more susceptible to the leaf end and tongue-andgroove effects, which were not included in the study but can be modeled using a stair function. 43 It is worth pointing out that although a simple level reduction method was used in the study for fluence stratification and MLC sequencing, the fluence regularization method was F.…”

Monte Carlo validation of the TrueBeam 10XFFF phase–space files for applications in lung SABR

“…These detectors were chosen for this study because they have both been shown to perform well for output factor determination for field sizes down to 1 × 1 cm 2 for 6 MV photons in flat beams from the Varian Clinac. [21][22][23] All the profile and the PDD measurements were performed at a source to surface distance (SSD) of 100 cm. The transverse profiles were measured at depths of 2.5, 5, 10, 20, and 30 cm.…”