Premkumar Elangovan, Alistair Mackenzie, David R Dance, Kenneth C Young, Kevin Wells

This work investigates the detection performance of specialist and non-specialist observers for different targets in 2D-mammography and digital breast tomosynthesis (DBT) using the OPTIMAM Virtual Clinical Trials (VCT) Toolbox and a 4-alternative forced choice (4AFC) assessment paradigm.
 
 Using 2D-mammography and DBT images of virtual breast phantoms, we compare the detection limits of simple uniform spherical targets and irregular solid masses. Target diameters of 4mm and 6mm have been chosen to represent target sizes close to the minimum detectable size found in breast screening, across a range of controlled contrast levels. The images were viewed by a set of specialist observers (five medical physicists and six experienced clinical readers) and five non-specialists.
 
 Combined results from both observer groups indicate that DBT has a significantly lower detectable threshold contrast than 2D-mammography for small masses (4mm: 2.1% [DBT] vs 6.9% [2D]]; 6mm: 0.7% [DBT] vs 3.9% [2D]) and spheres (4mm: 2.9% [DBT] vs 5.3% [2D]; 6mm: 0.3% [DBT] vs 2.2% [2D]) (p<0.0001). Both observer groups found spheres significantly easier to detect than irregular solid masses for both sizes and modalities (p<0.0001) (except 4mm DBT). The detection performances of specialist and non-specialist observers were generally found to be comparable, where each group marginally outperformed the other in particular detection tasks. Within the specialist group, the clinical readers performed better than the medical physicists with irregular masses (p<0.0001). 
 
 The results indicate that using spherical targets in such studies may produce over-optimistic detection thresholds compared to more complex masses, and that the superiority of DBT for detecting masses over 2D-mammography has been quantified. The results also suggest specialist observers may be supplemented by non-specialist observers (with training) in some types of 4AFC studies.&#13.