  Recent studies have proposed that adding quinine to water while performing
Cherenkov volumetric dosimetry improves the skewed percent depth dose
measurement. The aim of this study was to quantify the ability of quinine to
convert directional Cherenkov emission to isotropic fluorescence and evaluate
its contribution to the total emitted light. Aqueous solutions of quinine were
prepared with distilled water at various concentrations (0.01 to 1.2 g/L). The
solutions were irradiated with photon beams at 6 and 23 MV. The dependence of
the light produced as a function of sample concentration was studied using a
spectrometer with a fixed integration time. Spectral measurements of the
luminescent solution and the blank solution (distilled water only) were taken
to deconvolve the Cherenkov and quinine contribution to the overall emission
spectrum. Using a CCD camera, intensity profiles were obtained for the blank
and the 1.00 g/L solutions to compare them with the dose predicted by a
treatment planning system. The luminescent intensity of the samples was found
to follow a logarithmic trend as a function of the quinine concentration. Based
on the spectral deconvolution of the 1.00 g/L solution, 52.4% and 52.7% of the
signal in the visible range results from the quinine emission at 6 and 23 MV,
respectively. The remaining fraction of the spectrum is due to the Cherenkov
light that has not been converted. The fraction of the Cherenkov emission
produced between 250 nm and 380 nm in the water and that was absorbed by the
fluorophore reached 24.8% and 9.4% respectively at 6 and 23 MV. X-ray
stimulated fluorescence of the quinine was then proven to be the principal
cause to the increased total light output compared to the water-only signal.
This new information reinforces the direct correlation of the solution
intensity to the dose deposition.
