system
Given the following post, respond as if you're commenting on Reddit:
human
How exactly do Oncologists predict how long a patient has left to live ?
gpt
Interestingly, there is a bit of a mismatch between what patients want and what oncologists want when it comes to predicting life expectancy of cancer patients. 

Most patients (>70%) want to know how much time they have left. A realistic sense of expected time left can help facilitate end of life discussions, prioritize expenses, avoid potentially heavy side-effect associated care and improve quality of death.

Most oncologists (>80%), though, don’t provide patients with time left estimates. The reasoning being it is difficult to get right (within a few weeks, often impossible; within a year more feasible), is stressful for doctors and he only limited evidence that making any prediction is actually good for the patient.

As for how the oncologist makes the prediction, it’s mostly trying to make sense of the data we have and statistics. Most major cancers have good data sets on survival rates, disease progression rates etc. stratified by stage and basic health status info. That plus personal experience is how the oncologist will attempt to make an estimate about life expectancy. It’s onviously tricky, though, where patient health and support networks can be hard to quantify but have an outsized impact on survival. Further, the data we have is historical, but the field is always - and often rapidly - changing. What was true five years ago may not be true today.