A very general point about 'omas' all of which sound threatening.
Tumours (Latin = swellings) may be benign or malignant. A benign tumour tends to grow more slowly than a malignant one, and critically, does not spread to other parts of the body. When a growth is malignant and more than locally invasive you see a new terminology, e.g. carcinoma, fibrosarcoma, depending on the tissue of origin.
The doctor sounds like he's seen this before, sounds like you can stop panicking. Best wishes.