The aim was to measure the threshold diameter for detection of masses and calcifications in synthetic 2D images created from planes of digital breast tomosynthesis (DBT) of a mathematical breast phantom. The results were compared to those for 2D images and DBT. Simulated ill-defined masses and calcification clusters were inserted into mathematical breast models with a thickness of 53mm. The images were simulated as if acquired on a Siemens Inspiration X-ray system. Acquisitions of 2D and DBT images of the breast phantom at a mean glandular dose (MGD) of 1.6mGy were simulated using ray tracing with allowance for unsharpness and the addition of scatter and noise. The resultant images were processed using the manufacturer’s software to create 2D, DBT planes and synthetic 2D images. Image patches with or without the lesion were extracted. These patches were used in a 4-alternative forced choice study using 5 observers to measure the threshold diameter for each imaging mode. The threshold diameters of the masses and microcalcifications were 7.0mm, 6.3mm, 7.1mm and 4.9mm (masses) and 395μm, 211μm, 220μm, and 357μm (calcifications) for synthetic 2D, 2D (1.6mGy), 2D (1.1mGy) and DBT respectively. The threshold diameters were 10% (p=0.4) and 47% (p<0.0001) smaller for 2D images compared to synthetic 2D images for masses and calcification respectively at a MGD of 1.6mGy. At the same dose, the threshold diameter for small calcifications was larger for synthetic 2D images than 2D images, but no significant differences were found for masses between 2D and synthetic 2D.