Dosimetry system for the irradiation of thin biological samples with therapeutic proton beams

The design and performance of a dosimetric system for the irradiation of thin biological samples with therapeutic proton beams is reported. Protons with initial energies between 40 MeV and 250 MeV are degraded with polystyrene blocks of variable thickness in order to place the sample, an aqueous layer of 10 μm thickness, at various locations on the proton depth-dose curve. The dosimetric system comprises a secondary emission monitor, a Faraday cup and thin ionization chambers, which are located upstream of the sample, and a calcium fluoride scintillator located downstream of the sample for monitoring the position of the sample relative to the Bragg peak. Transverse dose profiles were measured with radiochromic films. System performance was studied and optimized by simulating primary radiation transport through detectors, degrader and sample using the Monte Carlo simulation tool GEANT 3.21. Calculated detector responses and beam profiles agreed well with the measured data. Monte Carlo simulation was also used to evaluate mean values and spectra of linear energy transfer in the sample as a function of initial proton energy and degrader thickness. Long-term experience has shown that the system performance was unchanged after accumulated doses of 10⁵ Gy.