It should be easy, right? I mean, I am a professional with what feels like eons of training. So how can an oncologist with years of experience fail at perhaps the most important question a patient can ask, ”How long do I have, Doc?”

It’s not that I want to avoid the question. Well, okay, sometimes I do. It’s not the most pleasant of conversations to have. But really, I don’t dodge the question. I simply don’t always have an accurate or reliable answer. And this is too important a question to merely guess at.

Prognosis is not so much “how many days do I have left?” but much more so “ how shall I spend my time?” Should I quit my job? Should I move back home to be with my family? Should I start my bucket list? Will I be able to even get to my bucket list?

There is so much is riding on this one question that the answer just can’t be wrong. I can’t be wrong. And believe it or not, there are so many assumptions that go into calculating a prognosis that I generally don’t have all the information I need to be accurate, or sometimes even in the ballpark. And the crystal ball gets a little cloudy. Please consider the following.

1. Primary site of cancer. Where a cancer began is most important in predicting its behavior and response to treatment. Sometimes this is impossible to ascertain. Did it start in the pancreas and go to the ovary? Or the other way around. These two clinical scenarios have completely different prognoses. Biopsies and microscopic patterns are generally helpful. Staining patterns of proteins in the cancer cell can be helpful. Algorithms of the staining patterns can be used to identify where the cancer began. But not always. The most important question can sometimes go unanswered.

2. Stage of cancer. How far a cancer has traveled from its original site is also very important in predicting prognosis. Did it go to the lungs, the lymph nodes, the liver? Multiple technologies can be employed in this endeavor, each with differing reliability. Surgery or biopsy is generally the most accurate but comes at a high cost of being physically well enough to undergo these procedures. Their complications can be very risky including injuring good tissue or sometimes missing the intended biopsy site giving false readings. As a result, less accurate images are often relied upon instead including the best, PET/CT scans with as high as 87% accurate in some lung cancers or as low as 20-40% accurate in early stage breast cancers. CT scanning alone fairs even less well. Physical exam alone or plain x-rays, even worse.

3. Availability of treatment. Sometimes the best treatments are unavailable for use by patients who desparately need them. Shortages of chemotherapy drugs due to multiple forces including economic have resulted in patients receiving less that optimal treatment. Clinical trials, offering cutting edge treatment, are often unavailable for uncommon cancers. Even where you live or your race or ethnicity can factor into the appropriateness of your treatment. And if you think you because you have great insurance so it should be no problem getting treatment paid for, think again. There are many cases where insurance companies have been known to deny or delay payment for proven treatment essentially removing effective treatment options.

4. Effective social support. Marriage has been shown to improve survival of cancer by 20%. Now I’m not saying that everyone with a cancer diagnosis needs to go out and get hitched. But in factoring a prognosis, how much help you get really counts. The burdens of cancer treatment are best managed as a team. Getting to treatments on time, having another set of eyes watching over a cancer patient, just being there and listening all can factor in to a better prognosis.

5. The will to live. Perhaps the least understood and unpredictable variable is a patient’s own intrinsic drive to survive. Those who strongly feel some Earthly bonds, in my opinion, often live a lot longer than expected, even in the face of advanced disease. Those with purpose or goal seem to make it beyond their goal. And those who abandon their desire can seemingly, at will, allow life to ebb.

So to accurately answer your question, I have a lot to consider. Of course it starts with a basic understanding of your disease and its survival statistics, based on site of cancer and its stage. Then you have to figure in general health, treatment availability, social support, and how much you really want to live. And, oh yeah, there are always those 5% outliers, on the edges of the survival curves who do either way better, or worse than expected without any rationale at all. Finally, as people who happen to be physicians, we must factor in our own personal biases. Am I a glass half empty or glass half full person, do I tend to overestimate or underestimate?

I generally think along two lines: curability and palliative treatment. If the disease is potentially curable I’ll say “ based on your disease and its stage you have a ___% cure rate.” If the treatment available will not cure your disease, I guestimate a survival time of weeks, months or years based on available data and if I think you may adhere to, or vary from it. Then I make sure my assumptions are valid by going over them with you.

I think that gets to heart of what you’re asking me. And I strongly believe you have the right to know. My colleagues feel the same. We should all be able to plan our futures. I just hate it when I’m wrong. And I am wrong more often than I care to admit. And therein lies the angst.

Rick Boulay is a gynecologic oncologist who blogs at Journey Through Cancer.