Why might a person die suddenly 3 weeks after having a successful brain tumor surgery?

Many possibilities. Many possibilities exist. Pulmonary embolus, a blood clot to the lung, is something that brain tumor patients have higher chance of having. However, heart attack, bleeding in the brain and a myriad of other reasons exist.
Many reasons. The most common cause is pulmonary embolus but if someone dies suddenly an autopsy is generally a good way to find out what happened.

Related Questions

After brain tumor surgery, is that going to cause a permanent amnesia?

Location. It depends on the location of the tumor. If the tumor is close to memory centers then amnesia might happen, otherwise, if tumor is located in the motor center or sensory center of the brain then brain surgery might affect motion, sensation, speech etc based on the area affected. Always consult with neurosurgeon. Read more...

How long will I have to wait after having brain tumor surgery to ride roller coasters?

Ask your doctor. Your surgeon who operated on your tumor must be the best person to guide you. So ask him. But in general most operations on the brain take 3-4 weeks to heal unless you have paralysis which can take much longer. I would ask you to go slow and temper your enthusiasm for ride roller coasters too soon. Read more...