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2006 Rajko Topolnjak

The six-bank multi-leaf system : A large field size, high resolution collimator for advanced radiotherapy

A linear accelerator (linac) is the most commonly used device for treatment of patients with cancer in external beam radiotherapy (EBRT). The Linac delivers a high-energy ionizing radiation (photons or electrons) to the region of the patient's tumor. The absorption of radiation in the treated area damages the diseased cells. To minimize irradiation of healthy tissue beams should be shaped. Commonly, this is achieved by using a multi-leaf collimator (MLC). The motivation for this work was given by the fact that a conventional Linac/MLC, currently available on the market, was designed for conformal radiotherapy, but is nowadays also used for intensity-modulated radiotherapy (IMRT). In this thesis the first goal was to understand and to quantify the impact of Linac/MLC design parameters on IMRT treatment plans. The investigated parameters were: leaf width of the MLC, leaf transmission related to the thickness of the leaves, and penumbra related primarily to the source size. For this purpose, various head and neck IMRT plans were evaluated using the Plato and Pinnacle treatment planning systems. Lead by those findings, and a desire to obtain a collimator which could deliver large field size with high resolution field shaping, we present an alternative design of a multi-leaf collimator, called a six-bank MLC. The MLC system consists of three layers of two opposing leaf banks. The layers are rotated 60 degrees relative to each other. The leaves in each bank have a standard width of 1 cm projected at the isocentre. This is a novelty compared to the existing systems which can achieve either large field size with low resolution field shaping, or small field size with high resolution field shaping. For the six-bank MLC which would function as a multi-purpose collimator, suitable for all types of treatments, two methods for delivering IMRT were developed. In a low-resolution mode similar segments can be delivered as with a conventional two-bank MLC with a leaf width of 1 cm. The performance in high-resolution mode is comparable to that of a mini-MLC, but a trade-off had to be made between accuracy and number of segments. Finally, an analytical model of the optimal MLC leaves design was made, and was used to find the optimal leaf design parameters for a six-bank MLC. In conclusion, Linac/MLC design is of great importance for IMRT treatment of patients. By further improvement of image-guided radiotherapy (IGRT) and application of on-line position verification on a daily basis, additional reduction of margins will be possible. This would lead to a great benefit enhancement of a six-bank MLC over the conventional MLCs.