Commissioning of a double-focused micro multileaf collimator (μMLC)

Double-focused μMLCs are able to create fields with steeper dose gradients at the field edges and are, therefore, an advancement in delivering stereotactic treatments. A double-focused μMLC has been installed at a Siemens Primus linear accelerator (linac) as a first research installation in Europe. The basic dosimetric parameters, such as leakage, output factors, depth-dose curves and penumbra, have been measured in 6 and 15 MV-mode by use of radiochromic films (GafChromic EBT), ionization chambers and our solid water QA-phantom (Easy Cube). The leakage between the leaves is minimal and lower than that of most commercially available MLCs. Therefore, the field size of the linac can be kept constant while the leaves of the μMLC are creating different aperture shapes. Percentage depth doses (PDDs) generated by the double-focused μMLC are equal to depth-dose curves of the original linac. That means the μMLC affects only the off-axis ratio (OAR). Based on the fact that the μMLC is double-focused and the source-to-collimator distance is larger, the penumbra is sharper than that for fields defined by the original linac MLC. The mechanical and dosimetric investigations show the benefit of the double-focused μMLC attached to a Siemens Primus linear accelerator.