Sci-Thurs PM: Delivery-03: Optical imaging of microscopic radiation dose gradients using a digital microscope

Using superior localization and immobilization methods, stereotactic radiosurgery is capable of delivering spheres of dose as small as a few millimetres in diameter to intracranial targets. For targets abutting critical structures, the most conformal treatments minimize adverse radiation side effects and it is important, therefore, to ensure proper quality assurance prior to delivering high doses of radiation to eloquent brain locations in a single fraction. This work examines the capability of a digital microscope, with translation stage and associated software, to resolve dose gradients in Gafchromic EBT™ film at the micron level. In order to validate the microscope-film system from a radiation physics approach, films were irradiated to produce very steep penumbrae by using very small fields, lower photon energies and minimal geometric penumbra contribution. Orthovoltage film irradiations were done by placing films in phantom beneath pinhole collimators. The experimentally determined off-axis dose profiles were compared with Monte Carlo computer simulations which replicated the irradiation geometry and served to validate our measured data. The measured 80% - 20% penumbral widths were 46 μm ± 26 μm (100 kVp, 2 mm field size) and 69 μm ± 27 μm (300 kVp, 2 mm field size). In the energy range covered, the measured penumbral widths agreed with Monte Carlo computer simulations within experimental uncertainty. The effects of noise originating from both the film and the microscope system are discussed and improvements to this system suggested.