Which test is better for finding cancers, a mammogram or an ultrasound?

Mammogram: Mammograms are excellent for finding any abnormality in the breast, and thus are best as a screening test for cancer. Ultrasounds are most useful after finding an abnormality on mammogram or examination; we can then direct the ultrasound to the area in question to help determine if it is worthy of biopsy or observation.

Few factors: Depends on your age, density of your breast tissue and also if anything is felt on a breast exam. Family history also plays a big role in determining your risk and therefore what modalities are best.

Mammogram: Typically a mammogram would be the first imaging modality used to screen the breast for any areas of concern. After identifying troublesome areas, ultrasound is often used to hone down on that specific area. Mammogram is better at picking up areas of micro calcifications, though.

In the screening: setting, mammography detects about 3-5 cancers per 1000 patients, and screening ultrasound about 2-3, so overall mammography is a little better, and remains the standard of care for screening. However, ultrasound performs better in dense breasts, and mammography performs better in fatty breasts.