They are the ninjas of the cancer world — stealthy, fast-moving killers whose victims often don't even know they're there until it's too late.
 

Five of the most aggressive cancers — skin, brain, pancreatic, esophageal, and liver — account for tens of thousands of deaths every year in the U.S., according to the American Cancer Society. Many people they strike never notice the warning signs in time to stop them.
 

But there are precautions you can take to lower your risk, experts say. Being aware of the telltale signs of these fleet-footed killers is also the key to early detection, treatment, and survival.
 

Otis Brawley, M.D., a nationally known cancer specialist and chief medical officer with the ACS, tells Newsmax Health one reason fast-moving tumors are so lethal is because effective screening tests don't exist to identify them early — unlike slow-growing prostate, breast, and colon cancers that are more readily diagnosed in initial stages.
 

"The way to think of it like this: A burglar who breaks into a house, who gets in quickly and gets out quickly, is less likely to be caught than a burglar who breaks into a house and spends a long time there," explains Dr. Brawley, a professor at Emory University in Atlanta.
 

"Breast and colon cancer … are more amenable to screening because they are slower moving and are easier to catch because of that. The thing about pancreatic, esophageal, brain and liver, and some of the skin cancers is … these tumors can grow to quite a large size before we actually realize they are there."
 

Your best defense? Knowing exactly what your risks are, as well as what you steps you can take to prevent and combat such cancers early. With that in mind, here is a primer on five of the fastest-moving cancers.
 

No. 1: Pancreatic cancer. One of the deadliest forms of the disease, pancreatic cancer strikes more than 45,000 Americans each year, a whopping 38,000 of whom die, according to the National Cancer Institute. Because the pancreas lies hidden in the back of the abdomen, tumors of the organ often spread beyond the initial site before it is caught, making it difficult to treat.
 

What you can do: Know the warning signs and see a doctor if you experience abdominal or back pain, unexplained weight loss, digestive problems, light-colored stools, or jaundice. This is especially important if at least two close relatives have been diagnosed with pancreatic, breast, colon, or ovarian cancer — which scientists believe may share common genetic origins. Chronic alcohol consumption — more than one drink a day for women and more than two drinks a day for men over an extended period of time — can increase the risk for pancreatitis and cancer. Diets heavy on red and processed meats have also been linked to the disease, while eating five services of vegetables daily can help prevent it.
 

No. 2: Liver cancer. Only about 1 in 10 liver cancer patients survive at least 5 years after diagnosis. More than 30,600 cases are diagnosed each year, with more 21,600 deaths tied to the disease, NCI statistics show.
 

What you can do: Because obesity is a prime risk factor for liver cancer, your best defense is to maintain a healthy weight by eating balanced diet, getting regular exercise, and keeping belly fat to a minimum. You should also limit alcohol consumption and your intake of sugary foods, which increase your risk for obesity, diabetes, fatty liver, cirrhosis, and liver cancer. Chronic hepatitis is also a known risk factor for this form of cancer, so make sure you have a hep B vaccine. You should also tell a doctor if you experience fatigue, anemia, abdominal pain, jaundice, unexplained weight loss, or digestive problems.
 

No. 3: Brain cancer. More than 23,000 Americans are diagnosed with brain cancer each year; more than 14,000 die, according to the NCI. But the warning signs are often subtle, if they occur at all.
 

What you can do: Be aware that about half of people who have brain tumors report unusual headaches — those that may wake you at night or in the morning — among the first signs. Other potential warning signs: nausea, vomiting, vision or balance problems, tingling in the arms or legs, personality changes, seizures, or extreme fatigue. Although there is no conclusive evidence linking brain tumors to cellphones and CT scans, some experts recommend individuals with a family history of such cancers limit their exposures, by keeping CT scans to a minimum (challenge a doctor who recommends one for minor conditions) and using an ear piece instead of holding a mobile phone to the side of your head.
 

No. 4: Esophageal cancer. Nearly 18,000 new cases of throat cancer are diagnosed — and 14,000 deaths recorded — each year in the U.S., NCI figures indicate.
 

What you can do: Treat reflux (consult a doctor for an appropriate remedy), quit smoking, and limit consumption of alcohol, chocolate, caffeine, mints, raw tomato, and other foods that have high acid content. Be aware that Barrett's Esophagus — caused by chronic acid reflux — can lead to cancer. Dr. Brawley also recommends anyone over 50 suffering from long-term heartburn, throat-burn, coughing, or hoarseness should get a nasal esophagoscopy, which allows a doctor to view the esophagus. Among the warning signs: chest pain, difficulty swallowing, hiccups, sudden unexplained weight loss, and chronic pneumonia.
 

No. 5: Skin cancer. This year alone, more than 76,600 Americans will be diagnosed with melanoma — the deadliest form of skin cancer — and nearly 9,500 will die of the disease, the NCI projects. The biggest risk factor: sun exposure and indoor tanning.

What you can do: Check your skin for irregular moles, growths, and patches. Make sure to limit your sun exposure and apply sunscreen liberally and regularly when your outdoors — particularly if you are light-skinned or have blonde or red hair, which increases skin cancer risk. See a dermatologist regularly for a skin examination, particularly if you had serious sunburns as a child.

Editor's Note:Knowing these 5 cancer-causing signs is crucial to remaining cancer-free for life
 

Dr. Brawley adds that many cancers of all kinds are linked to what he describes as the "unhealthy triad" of obesity, high-calorie diets, and low physical activity.
 

"In the next 20 years we expect tobacco will lower in its impact on cancer and that triad will become the No. 1 cause of cancer in the United States," he explains.