High resolution radiochromic film dosimetry: Comparison of a microdensitometer and an optical microscope

Purpose: Microbeam radiation therapy is a developing technique that promises superior tumour control and better normal tissue tolerance using spatially fractionated X-ray beams only tens of micrometres wide. Radiochromic film dosimetry at micrometric scale was performed using a microdensitometer, but this instrument presents limitations in accuracy and precision, therefore the use of a microscope is suggested as alternative. The detailed procedures developed to use the two devices are reported allowing a comparison. Methods: Films were irradiated with single microbeams and with arrays of 50 mu m wide microbeams spaced by a 400 mu m pitch, using a polychromatic beam with mean energy of 100 keV. The film dose measurements were performed using two independent instruments: a microdensitometer (MDM) and an optical microscope (OM). Results: The mean values of the absolute dose measured with the two instruments differ by less than 5% but the OM provides reproducibility with a standard deviation of 1.2% compared to up to 7% for the MDM. The resolution of the OM was determined to be similar to 1 to 2 mu m in both planar directions able to resolve pencil beams irradiation, while the MDM reaches at the best 20 mu m resolution along scanning direction. The uncertainties related to the data acquisition are 2.5-3% for the OM and 9-15% for the MDM. Conclusion: The comparison between the two devices validates that the OM provides equivalent results to the MDM with better precision, reproducibility and resolution. In addition, the possibility to study dose distributions in two-dimensions over wider areas definitely sanctions the OM as substitute of the MDM.