Comparison of Readout Between a New Diode Laser Scanner and a Helium-Neon Laser Densitometer for GafChromic MD-55 Films

GafChromic MD-55 films were irradiated by the Vienna γ-knife unit with a 14 and 8 mm collimator helmet (spherical field of close to 11 mm diameter) at the same isocenter to compare the dose cross-profiles along the x and z directions with different scanning systems. Dose-response curves for GafChromic MD-55 film (Nuclear Associates Model No. 37-041) were measured using two different densitometer systems: a commercial He-Ne laser densitometer (PTW FIPS Plus, Freiburg, Germany) and a newly developed analog diode laser densitometer. For this purpose, we chose the diode laser as the light source, emitting at 675.2 nm with a single-mode surface, measured with a Lock-In Amplifier by a solar cell. The highest uniformity was seen in the change of absorption ranging from 4% to 6%, depending on the polarizing direction of the light with the new scanning system. The reproducibility of the reading method increases from 0.14% in the region of 2 Gy to 0.18% near 16 Gy. Measurements of the dose cross-profiles of the 11 mm collimator with the 675.2 nm laser scanner generally show good agreement with the values of the dose planning program. Reference studies with the He-Ne laser reveal a general tendency between 1.5 and 2.2 mm in diameter as compared to computer-simulated dose profiles and the 675.2 nm laser densitometer values.