A dosimetric calibration method for a double-focused multileaf collimator.

A method based on penumbra measurements and not on light field, is proposed for the calibration of the Siemens 82-leaf Optifocus multileaf collimator (MLC). The measurements were performed using MLCSoft version 1.2.0.4. (PTW-Freiburg, Germany), which is a software tool based on the LA48 measuring system (a linear array of 47 liquid-ionization chambers). The new leaf positions for each of the leaf banks are calculated from the MLCSoft results using an external spreadsheet. Using the proposed method the MLC calibration is directly performed in terms of dosimetry settings with improved precision. The nominal resulting accuracy of 0.5 mm is highly imposed by the limitations of the moving mechanism of the leaves and not by the method itself. To test the results of the MLC calibration, in-water measurements of off-axis dose profiles have been measured in order to verify different field sizes.