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Degree Name



Medical Physics

First Advisor

Mark Yudelev


A 120 leaf collimator of high resolution has been constructed to shape a fast neutron therapy (FNT) beam produced from a superconducting cyclotron using the d(48.5) + Be reaction. The computer controlled multileaf collimator (MLC) replaces an aging, manually operated multirod collimator (MRC). The MLC was built to address two problems: The need to increase the efficiency of FNT at this facility, and the desire to implement intensity modulated neutron radiotherapy (IMNRT) for which a suitable computer controlled beam shaping device of high resolution and rapid shape changing does not currently exist. The specific aims were to build a neutron MLC that would solve these problems and then verify its radiological performance as being clinically acceptable. The MLC leaves project 5 mm in the iso-centric plane perpendicular to the beam axis. A taper has been included on the leaves matching beam divergence along one axis. The 5 mm leaf projection width was chosen to give high resolution conformality across the entire field. To reduce the inter-leaf transmission a 0.254 mm blocking step has been included. End-leaf steps totaling 0.762 mm were also included allowing adjacent leaf pairs to close off within the primary radiation beam. All functions of the MLC operate under the multileaf collimator control system (MLCCS). The MLCCS includes motion control as well as a machine vision system that images optical targets on the leaves to verify set field shape accuracy within +/-1 mm at the level of isocenter. MLC transmission was measured to be 3.9% with a 11% gamma component which is slightly lower than the MRC measured transmission of 4.6% with a 17% gamma component. MLC penumbral measurements in water for a 10 x 10 cm2 field at 5 cm depth were 9.1 +/- 0.2 mm and 11.8 +/- 0.5 mm along the focused and unfocused axes, respectively. The MRC penumbra measured 8.6 +/- 0.3 mm under the same conditions along both of its focused axes suggesting penumbral equivalence between the MLC and MRC along the focused axis with slight degradation in the unfocused direction of the MLC. The many benefits of the fully automatic MLC over the semi-manual MRC are considered to justify this compromise.

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