Not always, it could be an infection or just a benign lump. How do you know it is brown, have you(or the person with it) had an endoscope/oesophagoscopy? If so it would doubtless have been biopsied so you will to wait for the results from Pathology lab before any confirmation.
My father was diagnosed with lung cancer 20 months ago and after chemotherapy is still going strong but we're getting ahead of ouserlves.
Wait for results of the examination first.