Image guidance capability is an important feature of modern radiotherapy linacs, and future cobalt-60 units will be
expected to have similar capabilities. Imaging with the treatment beam is an appealing option, for reasons of simplicity
and cost, but the dose needed to produce cone beam CT (CBCT) 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 is severely limited by the
penumbra cast by the ~2 cm diameter source, it is possible to identify high contrast objects on the order of 1 mm in
width, and bony anatomy in anthropomorphic phantoms is clearly recognizable. Low contrast sensitivity down to
electron density differences of 3% is obtained, for uniform features of similar thickness. The conventional shift-and-add
reconstruction algorithm was compared to several variants of the Feldkamp-Davis-Kress filtered backprojection
algorithm result. The Co-60 DT images were obtained with a total dose of 5 to 15 cGy each.
We conclude that Co-60 radiotherapy units upgraded for modern conformal therapy could also incorporate imaging
using filtered backprojection DT in the treatment beam. DT is a versatile and promising modality that would be well
suited to image guidance requirements.