At the time, I thought I should have my brain scanned. I should have been aware of that. I always told patients and friends not to have brain scans if they had serious problems. I told them that they could not like what they saw.

I was willing to take part in the study. I was curious to see my own brain, even if it was only in the greyscale portion of an magnetic resonance image. The awe I felt as a medical student when I saw brain surgery for the first time had fallen away quickly once I began training as a neurosurgeon. You don't want to distract yourself from the mystery of how our brains generate thought and feeling when you are operating. You don't want to be thinking about the family of the patient while you wait in the theatre. You have to separate yourself from these thoughts and feelings. The operating and self-belief are the most important things. When you are operating, you live very intensely.

It was thought that seeing my own brain would confirm the fascination with neuroscience that led me to become a neurosurgeon. It was not a good look. I assumed that the scans would show that I was one of a small group of older people whose brains are not showing the signs of old age. I can see that I was still thinking of myself as a doctor, even though I had retired, because I was still clever and had a good memory. I did press-ups and ran a lot. All this effort looked like King Canute trying to stop the tide.

I was looking at ageing in action. My 70-year-old brain was shrunken and withered

I was overwhelmed by a feeling of complete helplessness and despair as I looked at the images on my computer monitor, just as I used to look at my patients' scans, slice by slice, working up from the brain stem to the cerebral hemispheres. People had premonitions that they would attend their own funeral. I was looking at aging in action, in black and white, death and dissolution foretold, and already achieved. The shrunken and withered brain of my 70 year old was a sad and worn version of what it used to be. There were signs of ischaemic damage in the white matter and there are various names for them. They looked sick. My brain is getting old. I am getting old. There is a deadline.

Poor eyesight makes it difficult to see the stars at night, but I still feel fear and awe when I look at them. Their cold and perfect light, their incomprehensible number and remoteness, the near eternity of their lives, in such contrast to the short time I have left. I felt the same way when I looked at my brain scans. The desire to not look at my eyes was very strong. I have never looked at the scans again because I had to work through them one by one. It's too scary.

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There is extensive medical literature about the white-matter changes on my brain scans, the white matter being the billions of axons that connect the grey matter. Most of us will have these changes as we get older. The presence of them is associated with an increased risk of stroke. We have a one-in-six chance of developing dementia if we make it to 80. It's true that a healthy lifestyle reduces the risk of dementia, but we can't ignore the effects of aging. If we are fortunate we can delay them. It is not a good thing to live for a long time. Maybe we should not try so hard.

It's hard to accept that our brains are also affected by the same changes as our wrinkled skin, but we accept that. The changes are referred to as degenerative in the reports. As we age, our brains shrink and end up floating in a sea of fluid within our skull. We feel like we are our true selves even though we are diminished, slow and forgetting. Our true self, our brain, has changed, and as a result, we have no way of knowing that we have changed. When I wake up in the morning, I don't know if I'm the same person as yesterday. As for a decade ago?

Hope is one of the most precious drugs doctors have at their disposal

When talking to my patients, I always downplayed the extent of age related changes seen on brain scans, just as I never said that with some operations, you must remove part of the brain. Doctors have to choose their words carefully. It's easy for doctors to forget how patients hold onto everything we say. All sorts of psychosomatic symptoms can be caused by you. White lies were usually told by me. If the scans showed age related changes and nothing more sinister, I would say that the brain looks good for your age. The patients would leave the room with smiles on their faces. It is important to lie to patients in order to be more optimistic than the facts justify. He tells stories of patients who were close to death from heart failure but were still alive because of their positive attitude.

Doctors have hope at their disposal. If there is only a 5% chance of survival, a good doctor will tell you that there is 5% of hope. There is always hope even though there are many horrors in the box. Hope flickers out at the very end. Hope is not a question of chance. Our brains are physically connected to our bodies and our hope is a state of mind. The idea of a brain that isn't real might not be worth anything. This does not mean that being kind and hopeful will cure cancer. The human mind tries to reduce all events to a single cause, but most diseases are the result of many different influences, and the presence or absence of hope is only one among many.

I should have known that I might not like what my brain scan showed, just as I should have known that the symptoms of prostatism that were increasingly bothering me were just as likely to be caused by cancer as by the benign prostatic enlargement that happens in most men as they age. But I continued to think that illness happened to patients and not to doctors, even though I was now retired. Twenty months after I had my brain scanned, I was diagnosed with advanced prostate cancer. I had had typical symptoms for years, steadily getting worse, but it took me a long time before I could bring myself to ask for help. I thought I was being stoical when in reality I was being a coward. I simply couldn’t believe the diagnosis at first, so deeply ingrained was my denial.

The medical colleague I was going to see about my prostatic symptoms was put on hold as a result of the lock down. After the Pandemic, I forgot my symptoms and didn't get an appointment until seven months later. I didn't have private medical insurance, so I went privately.

After washing my bottom, I rode into Harley Street and drank a liter of mineral water. I was told to do this in order to have my urine flow measured. The nurse made me drink a lot of water. She'd put her head around the door a lot. She wanted to know if he was bursting yet.

Henry Marsh, at home in Oxford.

Henry Marsh, at home in Oxford. Photograph: Sebastian Nevols/The Guardian

When I reached this point, I was directed to a urinal that would measure me and record my sad attempt to empty my bladder, a problem I had been living with for many months, perhaps even years. I was taken up a carpeted staircase to the consulting room.

The double oak doors of the room were so tall and imposing that I was not sure if they were just for a medical consulting room. This was not a hospital. The room was large and my colleague, Ken, was sitting behind a desk. Mussolini had a big desk in his office. The cabinet ministers had to run a long way to his desk to deliver their reports. Ken isn't like Mussolini. Two years ago he operated on me for a stone and I had made inquiries as to who I should see. Being able to do this is the best benefit of being a doctor. Being a doctor is helpful when you are sick. It was a disaster in my case.

We talked for a bit. He said the Covid crisis had been good for him because his hospital had come to understand that stones were important. During the lockdown, patients still needed urgent treatment for kidney stones. We talked about my symptoms and I found myself playing them down.

He apologized, saying he needed to examine you. I used to dread having a rectal examination, but now it's unremarkable. He said that my prostrate was a little firm.

I didn't want a PSA. Many aging men are concerned by the abbreviation PSA, which means "prostate-specific antigen." The test can be used to measure the amount of a specific hormone in the urine. One in seven men will be diagnosed with cancer at some point in their lives. Cancer is not the only cause of a raised PSA and a slightly raised level in an older man can be perfectly normal. Many men die from cancer rather than from it, which is a good thing. It's hard to decide whether to treat the cancer in every case or not, as no treatment is without some risk. If the cancer has not spread beyond the capsule of the gland, it is possible to remove it with surgery, but there is a risk of incontinence and impotent men. It will probably kill the man if the gland has spread beyond the prostrate.

I am amazed at how wilfully blind I was, how I had been so frightened by my symptoms that I had now probably left it too late

Ken was able to convince me to take the test. I came to seek his advice, and I couldn't deny it. I told him I wouldn't want any treatment if it's cancer.

He said that putting your head in the sand wasn't a good idea.

I was so scared of my symptoms that I probably left it too late, but I am amazed that I was so blind.

When I was in practice, I wanted to operate all the time. I loved caring for patients and it meant a lot to me. Now that I have finished, I don't miss it at all. Are you a fan of honey? I gave him a small pot of honey that I kept in my garden, because he said he had honey every morning for breakfast.

It was almost certainly due to chronic prostatitis that I had intermittent prostatic symptoms for 25 years. I was embarrassed by them and didn't seek professional help because I felt that illness happened to patients and not to doctors like myself. There is a world of illness and death when we are a medical student. We learn about frightening diseases and how they begin. Many students become convinced that they have developed a catastrophic illness after a few minor ailments. They have to think that diseases only happen to patients and not to themselves. A few doctors remain hypochondriacs throughout their careers, but most of us carefully maintain a self-protective wall around ourselves, which separates us from our patients.

Doctors with cancer often dismiss the early symptoms of the disease and treat it with advanced disease. I knew about this phenomenon, but I fell victim to it myself. Prostatism affects older men in a number of ways. The diagnostic sieve is taught in medical school. Cancer in its early stages can't be distinguished from inflammation of the prostrate. For a long time I had buried my head in the sand.

When the PSA blood test showed that I had a PSA of 127, I wasn't sure what to think. Most cases of cancer will be less than 20 if the man has a PSA over 100. Most men will die within a few years if they have a PSA of over 100, according to a panicked, panic-ridden search on the internet.

The Royal Marsden is a renowned cancer hospital in London. I walked to the hospital after reading that cyclists can put up their PSA from the pressure of the saddle on their bottom. I was hoping that this would show that the first PSA reading was a mistake.

Hospitals remind me of prisons. Your clothes are taken away and you are put in a confined space. You must obey orders

Much of what goes on in hospitals is about trying to bridge the gap between staff and patients. Helping patients isn't what it's about. The hospitals remind me of prisons. There are places where your clothes are taken away, you are given a number, and you are put in a small space. Obeying orders is something you have to do. It is possible that you are subjected to a rectal examination.

I sat down next to the pamphlets about living with a wide variety of cancer at the Marsden after I had been checked in. There were pictures of healthy looking elderly people on their covers. I didn't know if they were models or patients. I was weighed andmeasured by a nurse. I noticed that I was almost two inches shorter than when I was a young man, and that my bathroom scales had been underestimating my weight. I was told to do it again on the device. I was put in a small side room and given many plastic cups of water, which I drank before being taken to a special toilet.

I emerged a few minutes later and held the printed statement that showed my urinating difficulties. The nurse looked at it disapprovingly. I got the impression that I didn't try enough. I felt like I was about to enter my second childhood and that I was being potty trained all over again.

The nurse told me to go back to the side room. Her hair was long and dark.

I liked your hair.

She said she was growing it for charity to make wigs for women going through cancer treatment.

She told me that the doctor was coming to see me after she left the room.

The oncologist came after a while. He apologized and said how sorry he was. I suppose it was meant, but I found it rather depressing and felt a bit uneasy. He told me that he would fast-track the various scans that were needed to determine if my cancer was spreading or not.

Given my PSA, how likely is that? I wanted to know.

He looked away from me and said that 70% was his answer. I wanted to know if bicycling would affect my PSA. He said it would take 100 miles on a bike to raise it.

I was a doctor and anxious patient at the same time, and found it hard to ask him about my future, even though I hoped for a better outcome. I asked him to speak to me as a doctor. I used to have to tell my patients about their cancer in order to cheer them up.

He said that wasn't how things are done here.

He was sitting on the edge of a chair, with a piece of paper on his knee, as if he was going to leave immediately. I felt awkward and tongue tied. I blurted out the question that all of us ask when we first meet the oncologist, "How long have I got?" I already knew how much it was. He didn't tell me this.

He said that you needn't write your will for five years. He didn't know what would happen to me. I hoped he would tell me that I would be okay. He only had the power to quote probabilities. Doctors can only deal with uncertainty

He wanted to know a little about you. I stated in my letter that I valued being physically fit.

He said that if you write one book a year you will be able to write five more. I think he underestimated the difficulty of writing.

I read that hormones can have cognitive effects.

He said that he might be a little less sharp. I was too scared to take them in if he had told me more about the possible side-effects. Patients only hear a small part of what a doctor tells them at the first visit.

He talked about my meeting and then left.

There are problems for patients. Some oncologists told me that they wouldn't give patients percentages. The doctor can only reply to 100 patients with the same diagnosis if the patient wants to know what will happen to him or her. A certain percentage of people will live after a certain number of years. There is no way of knowing which patients will fall. Your doctor can't tell you how long you'll live.

I realized that I too rarely talked in terms of percentages when I was a surgeon. Malignant gliomas have a mortality of 50% at one year, and only 5% or so of patients are still alive five years later.

I look back in wonder at how little I thought about the effect I had on my own patients after I had spoken to them

I told patients with these tumors that if they were unlucky, they might be dead in six months, and if they were lucky, they might live for several years. I would say that for most people the tumours would recur between the two extremes, and that further treatment might be possible, without admitting that further treatment usually achieved very little. I thought this was a good way of dealing with the problem, and of balancing hope and realism. I don't know if this is still true in the days of internet and search engines.

I look back at how little I thought about the effect I had on my patients when I spoke to them, because the uncertainty about my own future threw me into torment. I was worried that if my voice was too pessimistic the poor patient would just wait to die. I wanted to tell the truth but not give them hope. I didn't know if the way I spoke to the family was appropriate or not after a patient died. It is difficult for the doctor to know how to balance hope and truth when talking to patients with diseases like mine, as false hope is better than no hope at all.

I thought I was going to meet the team when the nurse told me that I could go. The nurseReluctantly went into the next room after looking dubious at me. The oncologist was sitting in front of a computer monitor, laughing and talking with a few of his colleagues. A nurse came back.

She only said that you can leave.

I crossed to the other side. I knew I didn't have a right to claim that I deserved more than I got. The cancer of Henry Marsh is no longer present. He is waiting for his PSA test results to come back. He went to Ukraine to teach and advise on medical cases. There is a family living with him in London.

There is an edited extract from And Finally: Matters of Life and Death by Henry Marsh. You can order your copy at There may be delivery charges.

Will Self and Henry Marsh will discuss And Finally at a Guardian Live online event. You can book tickets on the Guardian website.

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