Can docs diagnose cancer in a parotid gland from a single cat scan with contrast? or would biopsy and/or additional tests/scans be needed to confirm? next appointment is with oncologist (before seeing ent) after finding growth on cat scan. do i worry?

Pursue the advised consultative pathway, as the specialists can help pinpoint the problem and provide a therapeutic approach to deal with this. If you wish further information regarding your CAT scan, contact the radiologist and have her(him) review the findings directly with you.

No doctor will ever diagnose cancer without a tissue diagnosis with biopsy. The contrast enhanced CT result may suggest a malignancy but it is not 100% specific. You should not see an oncologist until you have a biopsy done by the ENT doctor, otherwise, it is a waste of your time. If I was you, I would postpone the appointment with the oncologist until the parotid cancer is confirmed.