Modeling silicon diode dose response factors for small photon fields

• 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.

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