Tanya Kairn
Royal Brisbane & Women's Hospital
Medical Physicist

Zac Pross
Masters Student
Queensland University of Technology

Sanna Nilsson
Medical Physicist
Prince of Wales Hospital

Jodi Dawes
Mould Room Technician
Royal Brisbane & Women's Hospital

Tanya Kairn
Medical Physicist
Royal Brisbane & Women's Hospital

Craig Lancaster
Medical Physicist
Royal Brisbane & Women's Hospital

Personalised brachytherapy applicators can provide improved treatment quality for some patients with gynecological malignancies. The use of non-water-equivalent applicators for these treatments can result in a dose reduction, compared to calculated dose, when not using a dose calculation algorithm capable of modelling non-water-equivalent materials. One material used for custom applicator moulds is a substance marketed as Fricotan, which has been reported to reduce dose by approximately 10% at depths of 1-2 cm compared to Virtual Water. This study aimed to characterise the radiological properties of the Fricotan moulding material using Monte Carlo simulations and evaluate the dosimetric impact of its use as an applicator.

The elemental composition of the moulding material for Monte Carlo simulation was determined by x-ray fluorescence analysis and inductively coupled plasma mass spectrometry. Simulations were performed with dose determined at varying depths in water surrounding Fricotan applicators of varying thicknesses. Film measurements were performed to verify single dwell position measurements, using fabricated slabs of Fricotan with thicknesses of 0.5-3.5 cm. For single dwell positions, dose reductions varied between 1% (for 0.5 cm Fricotan) and 5% (for 3 cm Fricotan). Film and simulation results were consistent within calculated uncertainties.

The simulation results were used to determine correction factors for treatments containing multiple dwell positions with a spacing of 0.25 cm, with varying treatment length, applicator radii and depth of reference dose. Specifically, the dose at a reference position for each dwell position was determined using simulated dose in water with TG-43 anisotropy and Monte Carlo simulated single dwell position corrections applied. The differences between single dwell-based corrections and multiple dwell-based corrections varied by 0.5–4%, increasing with treatment length (by approximately 0.5% for each 1 cm of treatment length for any given applicator radius or dose depth) and decreasing with distance from applicator.

While the correction factors were calculated using an approximated treatment geometry, they could be simply applied to correct dose calculations in lieu of an algorithm featuring accurate modelling of scatter conditions. The method developed in this study could more broadly be used to develop correction factors for other non-water-equivalent moulding materials, for a TG-43 dose calculation environment.


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