“What do you want?”

“Whatever you say, doctor.”

“It would be helpful for me to know about your goals in life.”

“Goals?” He is nonplussed. A look of concern spreads over his face as if he has disappointed me.

The patient has just turned 85. He stopped chemotherapy six months ago when he was hospitalised with deconditioning. Following a stint in rehabilitation and devoted care from his sons, he is back in shape. It helps that he is not overweight, doesn’t smoke, and has avoided any major comorbidities such as heart disease, diabetes and cognitive impairment.

But in some ways the absence of these problems has created a dilemma. On the face of it, he presents well and, on the basis of age alone, there is no compelling reason to rule out resuming a less intensive form of treatment. But his recovery has been arduous so I hesitate to rush headlong into treatment that is ultimately not curative. Having known him for a long time, the dilemma feels like mine but it’s ultimately also his.

I want him to say that in the twilight years of his life, what matters most of all is quality of life that is spent away from the complications of chemotherapy and the resultant hospitalisation – and I would happily concur, agreeing to keep an eye on him but protecting him from the things he fears. Or I want to hear that having listened to the pros and cons that I have discussed at length, he believes that resuming treatment is in his best interest, in which case I would respect his choice and promise to keep an extra close eye on him. Either way, there would be informed consent, a tenet of a good doctor-patient relationship.

But when he says, “you decide, I will follow”, I can’t help feeling a little caught.

The ageing of the population has had a profound effect on cancer incidence and mortality. Half of all cancers are now diagnosed in people over 70 and more than half of cancer deaths occur in those over 75.

When it comes to physical fitness, cognitive capacity and independent function, there is great variation among the elderly. The danger of ignoring this is that the fit elderly may not receive appropriate advice, which could lead them to forego cancer treatments that can benefit both quality and quantity of life. A 91-year-old golfer was told that bowel cancer at his age didn’t even merit an oncology consultation. But his daughter insisted and three months later, he was back on the golf course.

On the other hand, the exceedingly high rate of chronic illness means that, for many elderly patients, cancer is the final insult. According to the Australian National Health Survey, 80% of elderly patients have three or more chronic conditions (such as arthritis, heart disease, diabetes and mental illness) that require active management and have an impact on their health outcomes and mortality. Toxic cancer treatment has the power to make already difficult lives miserable.

Polypharmacy, usually described as the use of five or more medicines, is rife. An estimated two thirds of Australians over age 75 take more than five medicines, with 20% taking over 10. The probability of adverse drug reactions rises from 13% for two drugs to a staggering 82% for seven drugs. For each additional drug, the risk of falls increases by 7%. Similar burdens have been described in other western countries.

In this context, the addition of aggressive cancer therapy can prove an insurmountable challenge. An 85-year-old widow with mild cognitive impairment, who was living in her own home and had a pleasant social life in a tight-knit community, was recommended surgery for an incidentally diagnosed cancer. The family was apprehensive about her cognitive health but the surgery was deemed low-risk. Indeed, the cancer was cured but the complications led to her placement in a nursing home in a new city, leaving the patient and family shattered by the experience.

As cancer diagnoses in the elderly increase, this scenario will become more common unless greater judgment and thorough caution is exercised by providers and patients. But what does this actually mean?

For patients who are diagnosed with cancer when they are elderly or already have significant other health problems, this means understanding the crucial importance of truly shared decision-making. This starts with understanding the implications of a cancer diagnosis and asking for all treatment options, ranging from the most aggressive to the least. As counterintuitive as it might sound, a routine question should be: “What happens if I do nothing?”

Everything about cancer seems urgent but there is usually time to digest information, seek another opinion and discuss advice with trusted family. A diagnosis of cancer holds different meaning across the lifespan but evidence shows that doctors make decisions with insufficient attention to the perspective of patients. Patients fear that by raising their doubts and opinions they may be perceived as being disloyal, but they would be wise to understand that it is very hard for doctors to do nothing. When the entirety of a doctor’s education is spent on learning how to “defeat” disease, eschewing anything but the most aggressive care can feel like withholding care.

Most patients are so overwhelmed by their diagnosis and the ensuing labyrinth of investigations that they stumble from one instruction to another without having the energy to question why. Everything about cancer seems steeped in mystery and it’s very easy to feel disempowered.

A starting point is for patients to think carefully about whether they value quality of life or length of life. Evidence based on the experience of young, fit patients cannot be extrapolated to the elderly, frail or vulnerable. Patients commonly overestimate their life expectancy but when the burden of treatment and importantly the probability of a positive outcome in their circumstances, is discussed the elderly strive to protect quality of life and independence over longevity.

In one study, published in JAMA, when patients understood that there was as little as a 10% probability that they would not be alive in six months, they were more likely to favour comfort measures over chemotherapy. But even in an era where we celebrate patient autonomy, nearly half of elderly patients in another study preferred to play a passive role in decision-making. This resulted in the worrying downside of doctor preferences not being concordant with patients’ stated preferences.

For cancer professionals, it’s time to recognise the quiet revolution under way in the area of geriatric oncology. Geriatric oncology is a subspecialty that combines geriatricians and oncologists to deliver care that is patient- rather than disease-directed. Few cancer clinics have embedded geriatricians but the tools they have developed are nonetheless free to use.

How a patient manages daily life is one of the most important predictors of cancer outcome, yet it is rarely established through the typical brief interactions in a cancer clinic. A more comprehensive assessment deliberately looks for emotional distress, cognitive decline, malnutrition and isolation, which are absolutely critical to the cancer experience. This kind of medicine is not only sensible, it’s humane.

Tools that calculate remaining years of life, the probability of cancer recurrence, the benefit of chemotherapy, and the probability of chemotherapy toxicity can put evidence on the table to direct conversations. Despite the hiccups in implementation, participation in advance-care planning discussions can stimulate thinking about what really matters to the patient. Lack of time, resources and comfort are frequently cited as a reason for not inviting such conversations but in the long run, the cost of avoiding them is greater to the patient and society. Healthcare planners must act on the concerns of professionals who are trying to do their best under mind-bogglingly bureaucratic organisations.

My patient asked for a week’s grace. On his return, he apologetically told me that while he didn’t know about chemotherapy, his life’s greatest remaining wish was to go back to where he was born and see his siblings one more time. That was all the instruction I needed to assume my role as his oncologist and tell him that, in that case, chemotherapy would not serve his best interest. He went home, packed his bags, and flew to his home town the next day.

• Ranjana Srivastava is an oncologist and Guardian Australia columnist