SU‐FF‐T‐397: Characteristics of a Double‐Focused μMLC

Purpose: Micro multi‐leaf collimators (μMLC) are widely used as add‐on to linear accelerators for the generation of small irregular shaped beams. The purpose of this paper is to measure the dosimetric characteristics of a double‐focused μMLC. The dose field generated by a double‐focused μMLC has a steep dose gradient at all field edges and is therefore able to spare the healthy tissue better than a single‐focused μMLC Method and Materials: At our department we have mounted a double‐focused μMLC on a Siemens Primus linear accelerator. Base on the separate gantry sensor, this μMLC is independent from the type or manufacturer of the linac. The basic dosimetric properties, such as transmission or leakage, depth‐dose curves and penumbra, have been measured by use of radiographic and radiochromic films, pinpoint ionization chambers and our multi‐purpose QA‐Phantom (Easy Cube). Results: Transmission and leakage between the leaves are due to their special focusing design very low. The depth doses distributions for different field sizes generated by the μMLC are identical to the depth doses for the same field sizes generated by the MLC integrated in the linac. Therefore the μMLC affects only the off‐axis ratio. Due to the double‐focusing characteristics the penumbra is small and identical for the field length and width. The penumbra gradient does not differ significantly with different field sizes. Conclusion: The mechanical investigations show that the μMLC can securely be used with a Siemens Primus linear accelerator. The measured dosimetric properties show a small penumbra that is identical in both directions (inplane and crossplane). This is important if the μMLC is applied for stereotactic and intensity modulated radiotherapy.