TORONTO - As a patient, they are one kind of physician you will never see. But the decisions they make could mean the difference between life and death.

They are pathologists, the doctors' doctors who toil away behind the scenes, analyzing vials of blood and urine, swabs from throats and snippets of tissue from skin and organs and muscles and bone.

Used to playing a critical but largely invisible role in patient care, Canada's 1,200-plus pathologists have suddenly found themselves under the public microscope because of several high-profile cases that include botched breast cancer results in Newfoundland and Labrador and the review of tests by a senior pathologist serving six Winnipeg hospitals.

But just what, exactly, is it that pathologists do?

Most patients -- and even some physicians -- have no idea, concedes Dr. Jagdish Butany, president of the Canadian Association of Pathologists.

"One of our problems is that we don't communicate with the patient, the patient never sees us," says Butany. "He thinks we are a little black box - tissue goes in, report comes out, that's it. It isn't quite the case. There's a tremendous amount of art and assembling of information involved."

Depending on the work setting, a pathologist may begin his or her day evaluating a large piece of tissue removed by surgeons because of suspicions of cancer. That could mean all or part of a breast or prostate gland or a portion of colon or lung to decide where it should be sectioned for closer examination.

The tissue then goes to technologists to be prepared for microscopic analysis. A tiny section of tissue is first placed in a tiny mould and embedded in liquid paraffin, refrigerated until hardened, then sliced by a machine with razor-sharp precision into shavings each about four microns thick, far finer than a human hair.

These tissue slivers are then placed on glass slides, exposed to various stains and sent to the pathologist.

What follows are hours of staring through the eyepieces of a microscope at slides, mentally teasing out abnormal cells from normal tissue to make a definitive diagnosis of cancer.

But that is only the first step, says Butany. The pathologist also makes a slew of other determinations, from the exact type of tumour and the stage it has reached to how aggressive it is and whether and how far it has invaded surrounding tissues and lymph nodes.

In the case of breast cancer, for instance, the pathologist must also determine whether it is driven by hormones or not - called estrogen-or progesterone-positive or negative - and whether the tumour is HER2-positive due to a genetic mutation, making it more aggressive and less responsive to hormone treatment.

"These tests help the oncologist to decide what the treatment should be," says Butany, a pathologist at Toronto's University Health Network, who specializes in heart tissue. "In essence, these tests decide the prognosis."

The role pathologists play is critical in deciding the road map for treatment, agrees Dr. Mark Clemons, head of breast medical oncology at Princess Margaret Hospital in Toronto.

"Pathology is essential for the practice of modern oncology, and indeed the pathologist is probably the glue that holds the whole oncology team together," says Clemons.

"And also these days, with some of the incredible new drugs we have for many cancers, the pathologist determines these new therapies as well. They will tell us if a patient has a chance of a particular drug therapy working."

Without them, "I couldn't do my job at all and patient care would be terrible."

But with an aging population, more and more Canadians are developing cancer, putting a growing strain on an already undersupplied and overburdened specialty.

"The workload has kept on increasing and even more than that the complexity has kept on increasing," says Butany. "Instead of looking at say five slides, we now have to look at 12 slides in the case of a prostate, or in a cancer of the breast, maybe when the cancer is very small, 50 slides from that breast."

"You put through virtually the entire tissue that the surgeon takes out and then you spend hours sifting through that trying to find the tumour."

Dr. Raymond Maung, a pathologist in Kamloops, B.C., says it's not uncommon for those in his specialty to stare down a microscope continuously for four or five hours without a break and to put in 10-or 11-hour days to deal with the mounds of slides that come to their tables.

Like air-traffic controllers at busy international airports, pathologists dare not let their minds wander, even for a split-second.

"I have to be concentrating all the time because I'm moving the slide when I'm looking," says Maung, who heads the B.C. Association of Laboratory Physicians. "If I lapse in my concentration once, I may miss something very important."

Fatigue, he says, is one factor that can lead to mistakes. The other is lack of knowledge.

"Knowledge is expanding so fast and new technologies are coming so fast, we have to keep up. But with our workload, many people won't have the time."

In large hospitals in major cities, pathologists may specialize in one area of the body, for instance the breast, gastrointestinal organs or skin. They are also likely to have colleagues expert in the same areas with whom they can double-check their diagnoses.

But in smaller cities or more rural areas, pathologists usually need expertise in all tissue types and may have no one to call on for a second opinion because of understaffing, he says. And that can lead to mistakes.

That appears to have been the case in the misdiagnosis of nearly 400 breast cancer patients in Newfoundland's Eastern Health Authority between 1997 and 2005, says Maung, who delivered an independent report in January 2007 on workload and pay scales across the province.

He found that pathologists in the Atlantic province had the lowest pay rates in the country, with an average turnover rate of 32 per cent in the previous four years, as practitioners left for better-paying jobs elsewhere in the country - taking their experience with them.

Maung and Butany both say more misdiagnoses are inevitable unless provincial governments increase the number of pathologists and determine reasonable workload standards. As well, professional bodies need to develop a national quality assurance program to ensure pathologists working in every setting have continuing education and a means of checking their results against international standards.

Butany hopes the recent spate of misdiagnoses that have come to light will spur such changes.

But Maung worries that new health-care dollars will flow to areas of medicine with higher public visibility and lengthy wait times for patient care, among them joint replacement and cataract surgery.

"Who do you think they are going to fund?" he says.

"Because people don't know what we do and they think we sit on our butts doing nothing - 'Oh, looking down a microscope, that's an easy job' - but they don't know the (medical and legal) responsibility ... and the impact we have on patient care," says Maung.

"They don't seem to understand if we don't do it properly there may be another Newfoundland happening here down the road."