This is the question I get asked the most: “So Doc, how do I know that this is working?” Sometimes my patients come to me with visible or palpable disease—something on the skin that they can see fading away, an enlarged lymph node in the neck that shrinks visibly during treatment, a lump or a bump that disappears, much to the gratification of both patient and doctor. But most of the time, this is not the case. Most of the time, the tumors are either deep inside, and not seen or felt, or the tumor has been removed, and we radiation oncologists are called in to do “clean up” work after the surgeon. As disturbing as it might be to a patient, most of the time, we don’t actually know that “it”, meaning the radiation, is working.

I’m old enough to know that life is not black or white, right or wrong, on or off. But still, as an optimist, I am a person who likes absolutes—I have always believed that if you play by the rules, you deserve to win. I dot all of my “I’s” and I cross my “T’s”. I was the kid who NEVER colored outside the lines in my coloring book, and now that I am a grown up, everything should be in place: my patients will attest to the fact that I am likely to rearrange the furniture in the consultation room if the cleaning people have set anything off kilter. I don’t see this as obsessive-compulsive—I see it as maintaining order in a disordered world. I like to see justice served, the plates cleared off after dinner, and I do not eat dessert first. In my linear world, the beginning is the consultation, the ending is the cure. The daily radiation treatments are the means to that end. Why should my patients expect less?

So what do I tell my patients who ask tentatively, half way through treatment, “Is it working?” when they have the invisible tumors, the ones deep inside, or the ones where the surgeon took most of it and we’re seeking out and destroying those microscopic stragglers? One of my teachers once said, meaning to be humorous, “Radiation works best when there is no disease!” Even the patients with the palpable masses that melted away—how can we be sure that every last malignant cell is gone? At the end of treatment, my patients want to be told that their disease has been vanquished and will never come back. Some doctors will oblige. They will say “We got it all”. Or they say, “You are cancer free.” This is despite the fact that there is not a single diagnostic test on the planet that can support that claim.

We oncologists prefer to use the word “remission.” Or “complete response.” As in, “You are in remission.” Or “You have had a complete clinical and radiographic response to treatment.” We would love to say, “Your cancer is cured,” because that is ever so much more satisfying than stating the truth, which is that we do not and cannot know for sure. Sometimes, somethings, some days—you just have to take it on faith and try to move on. Even if you are not a believer.

Here is what I tell my patients. I tell them that first the side effects will fade from their bodies and their memories. And then there will come a day when they will actually miss the camaraderie and support that they got from their chemotherapy and radiation teams. I tell them that the sun will rise and the sun will set, and they will bravely put one foot in front of the other. And one day, before they know it, they will wake up and stretch and smile and they will have forgotten, just in that moment, that they ever had cancer. And that’s when they will know, it worked.

Miranda Fielding is a radiation oncologist who blogs at The Crab Diaries.