Mr P, who is in his late 50s, went to his general practitioner complaining about a weak urinary stream. He was also waking up several times a night to pee.

He underwent a prostate specific antigen (PSA) test, which is a blood test done mainly to screen for prostate cancer.

Results showed an elevated PSA level and he was referred to me to check if he had prostate cancer.

I recommended a trans-rectal ultrasound-guided prostate biopsy.

This involves inserting an ultrasound probe into the rectum and inserting a needle 12 or more times into the prostate gland to take "cores" of tissue samples. These are examined under a microscope for signs of cancer.

Mr P's report showed that one out of 12 cores taken had a small amount of grade-1 prostate cancer. The grade of the cancer refers to how closely the cancer cells resemble normal cells.

The closer the resemblance or the lower the grade, the more the cancer behaves like normal cells - slow-growing with little chance of spreading outside the prostate gland.

Higher-grade cancer cells, on the other hand, tend to behave aggressively, with a high risk of spreading outside the prostate.

Prostate cancer is divided into five grades - grade 1 for the least cancerous to grade 5 for the most aggressive.

Mr P and his wife were shocked by the diagnosis.

I explained that prostate cancer is a complex disease.

It can be a very slow-growing cancer that may never harm a person in his lifetime - what I call docile pussycats - or an aggressive type that quickly spreads outside the prostate to the bones and lymph nodes, like a roaring tiger.

Such aggressive disease can lead to serious symptoms like pain and kidney obstruction, and the risk of death is high.

Most prostate cancer cases discovered today, however, are likely to be pussycats.

Autopsy studies have found that, by the time a man is 80 years old, there is a 60 per cent chance of finding incidental prostate cancer - but these men had died from diseases other than prostate cancer.

With PSA tests more commonly used since the 1990s, the chances of finding these incidental pussycats are increasing.

The chances of being cured of the cancer are also high, in the 80-90 per cent range.

In the past, men with prostate cancer were often diagnosed at a late stage and commonly died from it.

Today, a man has an estimated 16 per cent chance of being diagnosed with prostate cancer in his lifetime, but only a 3 per cent chance of dying from it.

The challenge, then, is to detect the aggressive cancer among the pool of slow-growing ones - catching the killer tigers hiding in a crowd of pussycats.

SAFE TO DELAY TREATMENT

Mr P asked what his treatment options were. For low-risk cancer such as his, radiation therapy, radical prostatectomy (surgical removal of the prostate) and active surveillance are options to consider.

Active surveillance refers to monitoring the disease that is contained in the prostate gland, rather than treating it right away.

This is because treatment for prostate cancer can cause long- term side effects that may have a big impact on a person's life.

Examples are the leaking of urine, difficulty in getting and maintaining an erection during sex, and bowel problems.

The surveillance approach helps men avoid unnecessary treatment, or delay it until tests show that the cancer is getting more severe.

Studies show that nearly all men with low-risk prostate cancer live for at least 10 years after being diagnosed. This shows that, with proper selection, most of these men can be watched over safely.

Mr P found it strange not to have treatment, but I explained that his prostate cancer was likely to be slow-growing and would not cause any problems in his lifetime.

HUNTING FOR HIDDEN TIGER

Six months after his cancer diagnosis, Mr P underwent a multi-parametric magnetic resonance imaging (MRI) of the prostate.

This advanced type of MRI scan can detect cancer within the prostate gland and has a better ability to look for higher-grade tumours that were missed during the initial biopsy.

While this MRI scan cannot detect all prostate cancer cases, those that it misses are likely to be docile pussycats.

The scan detected a suspicious large mass in Mr P's prostate.

As this finding did not match my initial assessment of a low-risk cancer, I advised him to undergo a robotic-assisted, MRI ultrasound fusion-guided prostate biopsy.

In this process, the results of the previously done MRI scan is stored in a robotic device.

The MRI images are merged with real-time ultrasound to create a 3D reconstruction of the gland.

With the model, I could visualise the suspicious mass in Mr P's prostate and, using the robot, aim the biopsy needle into it to retrieve cell samples for analysis.

It is like using GPS to reach your destination rather than driving without directions.

The suspicious mass had a significant volume of aggressive grade-4 cancer.

Therefore, the test had uncovered the hidden tiger masquerading as a pussycat.

After discussing the pros and cons of treatment options with his family, Mr P decided to undergo a robotic-assisted radical prostatectomy to remove the prostate gland.

He was discharged from hospital two days after the operation and was back at work within a week.

A microscopic examination of the removed prostate showed that the cancer was still contained within the gland, so the period of active surveillance had not been detrimental to Mr P.

Two months after, a check-up found that he was cancer-free and had no urinary leakage.

I was happy to tell Mr P that, while he needs to continue the regular check-ups, he is very likely to have been cured of cancer.

Indeed, technological advances in MRI imaging have helped doctors to select men with pussycat prostate cancer for active surveillance, while reducing the chance of missing the tigers among them.

•Assistant Professor Lincoln Tan is the organising chairman of Singapore Urological Association's Prostate Awareness Month, and a consultant at the urology department of the National University Hospital.