Development of a Filtimator for Pediatric Image-Guided Radiation Therapy with Low Imaging Dose

Abstract

We propose to use the filtimator (a filter and a collimator) for CBCT-based image-guided radiation therapy (IGRT) optimized for pediatric patients to minimize the imaging dose but not to sacrifice the imaging quality required for bony-based image fusion. The filtimator was made of Tin (0.6 - 1 mm) and Cu sheets (0.6 to 1.2mm) for filter and collimation with adjustable collimation with 4 to 6 cm fields at the isocenter. The thickness of the filter in the central and the peripheral regions were determined using vendor-provided image registration software and to ensure that the rigid image registration results from the CBCT were with and without the filtimator. The image quality of the filtered CBCT was compared with the regular CBCTs. Image registration accuracy was investigated by creating a < 2° shift in pitch, roll, and rotation and < 3 cm shifts in the lateral, vertical, and longitudinal directions, using commercial head and house-made phantoms. The imaging dose reduction factor of the filtimator was measured using a CT dose index phantom. The contrast-to-noise ratio substantially improved in the opening region of the filter, which provided better visualization of normal anatomy and target volumes. The slim filter reduced the imaging dose by > 98% in the filtered region, and the visualization of bony structures was well preserved, allowing for accurate rigid image registration with the filter. The imaging registration difference was < 0.2° shift in pitch, roll, and rotation and < 0.5 mm shift in the lateral, vertical, and longitudinal directions compared to the regular CBCT. The proposed dose reduction with the filtimator was demonstrated to be efficient and effective in considerably reducing the patient imaging dose while yielding accurate registration results. This novel technique can become a valuable tool for generating 3- and 4-dimensional images with a much-reduced dose to improve the precision of target localization in radiation therapy.