(CNN) -- In June 2004, Trisha Torrey found a golf ball-size lump in her torso. A surgeon removed it and gave her the grim news: cancer.

If you don't get better after treatment, ask your doctor more questions.

And it wasn't just any cancer but an extremely rare type of lymphoma.

"The oncologist told me that if I didn't begin chemo immediately," says Torrey, "I would be dead by Christmas."

The 52-year-old marketing consultant says she was petrified. But something in her gut told her the diagnosis was wrong.

Her doctor assured her it was right: two labs had confirmed the subcutaneous panniculitis-like T-cell lymphoma.

Against her doctor's orders, Torrey delayed chemo and went to another oncologist, who sent a tissue sample to the National Institutes of Health. The result: Torrey never had cancer.

The lump was a harmless fatty growth.

"On the one hand, I was overjoyed; on the other hand, I was just furious," Torrey says.

She couldn't believe she had been on the verge of having chemotherapy for nothing. What was it in Torrey's gut that told her the diagnosis might be wrong?

It's a lesson worth learning because misdiagnoses are more common than you might think: A 2005 study in the Journal of the American Medical Association says autopsy studies show doctors are wrong 10 percent to 15 percent of the time.

Here, from Torrey and from medical experts, are some red flags -- five reasons for suspecting your doctor might have made the wrong diagnosis.

1. You don't get better with treatment

Sometimes doctors stick to a diagnosis even when multiple treatments aren't working.

As vice president for loss prevention and patient safety at Harvard's Risk Management Foundation, Bob Hanscom remembers one particular lawsuit against Harvard doctors.

A young woman complained of stomach and chest pain. Her doctor prescribed a medicine for gastric reflux. When it didn't work, a second doctor prescribed another drug for gastric reflux. It also didn't work. The woman ended up in the emergency room with acute pancreatitis, which eventually caused kidney failure.

She survived but will be on dialysis the rest of her life.

"In her deposition, she said nobody was listening to her, so she kind of gave up," Hanscom says. "When I read that, I thought, 'Oh God, I wish you hadn't given up.' "

2. Your symptoms don't match your diagnosis

This is where the Internet comes in. You don't have to be a medical professional to Google your diagnosis.

For example, let's say a doctor diagnoses you with tendinitis. Looking it up, you can find out it usually lasts about six to 12 weeks, according to Dr. Saul Weingart, an internist and vice president for patient safety at Dana-Farber Cancer Institute in Boston, Massachusetts.

If you're still in pain beyond that time, the doctor may have made the wrong diagnosis.

3. Your diagnosis is based purely on a lab test

The reality is that labs make mistakes. In Torrey's case, she says two labs made mistakes. When lab results are the sole criteria for a diagnosis, that can be a red flag, says Torrey, who works as a patient advocate. Another red flag is when a diagnosis of a rare disease comes from a lab that doesn't specialize in that disease, Weingart says.

4. Your doctor attributes common complaints to an uncommon ailment

Torrey says her doctor said her night sweats and hot flashes were caused by the extremely rare lymphoma. Actually, they were signs of menopause.

5. Your diagnosis usually involves a test you never received

This is where the Internet comes in handy again. If you find out a specific test can determine the diagnosis you've been given, but you were never given that test, that's a reason to head back to the doctor's office armed with questions, says Torrey.

If you suspect you've been misdiagnosed, you have two choices: You can go back to the doctor who made the original diagnosis, or you can seek out a second opinion (or do both).

In next week's Empowered Patient, we'll tell you how in either case you can help your doctor come up with the correct diagnosis. E-mail to a friend

CNN's Jennifer Pifer contributed to this report.

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