Poster — Thur Eve — 44: Digital tomosynthesis image quality in a Co‐60 treatment beam

Image guidance capability is an important feature of modern radiotherapy machines. Cobalt‐60 units will require some form of image guidance if they are to be brought up to modern standards. Imaging in the treatment beam is an appealing option, for reasons of simplicity and cost, but the dose needed to produce cone beam CT images in a Co‐60 treatment beam is too high for this modality to be clinically useful. Digital tomosynthesis (DT) offers a quasi‐3D image, of sufficient quality to identify bony anatomy or fiducial markers, while delivering a much lower dose than CBCT. A series of experiments were conducted on a prototype Co‐60 cone beam imaging system to quantify the resolution, selectivity, geometric accuracy and contrast sensitivity of Co‐60 DT. Although the resolution was severely limited by the penumbra cast by the ∼2cm diameter source, it was possible to identify high contrast objects on the order of 1 mm in width, and bony anatomy in anthropomorphic phantoms was clearly recognizable. Low contrast sensitivity down to electron density differences of 3% was obtained, for uniform features of similar thickness. The conventional shift‐and‐add algorithm was compared to the FDK filtered backprojection algorithm using several different spatial filters. The Co‐60 DT images were obtained with a total dose of 5 to 15 cGy. We conclude that, should Co‐60 radiotherapy units be upgraded with image guidance capabilities, filtered backprojection DT in the treatment beam is a versatile and promising modality that would be well suited to the task of patient positioning.