Development of a motorized iris collimator for kilovoltage x-ray radiotherapy

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Abstract Objective. To design and build a motorized iris collimator suitable for a novel low-cost kilovoltage dual-robot radiotherapy device and test its reliability and dosimetric capabilities. Approach. A 12-leaflet motorized iris collimator was designed and built weighing 4.8 kg, with 1 mm thick tungsten leaflets, a beam size range of 0.1 cm to 16.5 cm, and a leaflet speed up to 20 cm/s. Dosimetric tests were performed with EBT4 Gafchromic film at depths of 0 to 10 cm in a Solid Water phantom at a source to surface distance of 38 cm using a 225 kVp beam with 2 mm of Al filtration. A Monte Carlo (MC) model of the experimental setup was developed. Beam size was determined by full width at half maximum (FWHM) and penumbra values were calculated for both film and MC. The beam size was measured experimentally using digital radiographs acquired for sizes between 5 and 150 mm to evaluate the robustness of the system. Main results. The collimator demonstrated sharp profiles with penumbra values ≤1.2 mm at 2 cm depth. 2D dose distributions from film measurements found dose leakage within the designed 2% as well as a single offset leaflet as a result of machining tolerance. MC matched film measurements with surface central dose differences ≤1.1%, a beam penumbra match within 0.08 mm, and a FWHM match within 1.1 mm. The linearity and repeatability showed that beam sizes can be set with an accuracy of ≤1.25 mm for beams ≥1 cm. Significance. This work showcases a motorized iris collimator designed for a novel low-cost radiotherapy device, capable of delivering sharp dose distributions. An MC model was successfully developed and matched to film measurements, allowing for accurate simulation of radiotherapy delivery.

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