What are chances of spot in mammogram being cancer?

It's actually: A poor test with too many false positives (finds things that are not cancer) and false negatives (fails to find things that are due to dense breasts, small size) once it finds something, the follow up is u/s to guide biopsy, then MRI to look for other lesions...A slow, inaccurate, costly process begging to be improved.

CHECK BIRADS: Radiologists have adopted a common language to describe mammographic abnormalities, called the birads system (you will see this on your report). A birads 3 abn has a 0.5% risk and is closely watched. A birads 4 (15-30%) or 5 (50-75%) abn should be biopsied to determine if it's a cancer. Birads 0 means that further eval is needed (ultrasound, comparison to prior mamms, etc). I hope this helps!

Probability of Ca: Nodules are characterized on mammo by various features that increase or decrease the probability of cancer. In order to remain accredited to interpret mammo, radiologists are required to render a final interpretation categorizing the lesion into one of the following categories: benign, probably benign, suspicious, or highly suspicious. The last category implies at least a 95% chance of malignancy