Thank you for your query at DoctorSpring.com
Incidentalomas are as the name suggests an extremely incidental benign swelling, which is present in the brain, without causing any symptoms. It could be a result of past infections, or because of other vascular problems.
This is nothing related to cancer and you should not worry.
The Arnold-Chiari malformation again is a vasculature disorder, and I request you to kindly upload the MRI reports for better correlation. I can assure you, it is not fatal.
Feel free to discuss further.