Dear Doctor: I have read that screening guidelines for prostate cancer are changing again. Why does this keep happening, and why should I believe the latest guidelines if they're just going to be rewritten in a couple of years?

Dear Reader: By its very nature, medicine is an ever-advancing discipline. Findings from new and ongoing research not only push the frontiers of understanding, but they also cast light on what is already known. As a result, the latest information gets incorporated into present-day medical practice. This leads to continual -- and sometimes unsettling -- change.

Let's start with what a screening test actually is.

Unlike diagnostic tests, which are performed to pinpoint the reason for specific symptoms, screening tests are performed in individuals who feel well. Their purpose is to catch a disease or condition in its earliest stage, when it is easiest to treat or manage. Hearing tests, mammograms, pap smears, colonoscopies, blood cholesterol levels, urinalysis, HIV tests -- all are examples of various screening tests performed for different reasons.

You're right when you say that screening recommendations seem to be always changing. The recent history of prostate cancer screening can help explain why.

The PSA test, or prostate-specific antigen test, was once routinely used to screen for early-stage prostate cancer. It's a test that measures the level of a certain protein in a man's blood, which can be an indicator of prostate cancer. But elevated PSA levels may also be the result of benign enlargement of the prostate, or an inflamed prostate due to an infection. It can also be the sign of a very slow-growing cancer. This meant that men with elevated PSA levels often underwent invasive procedures like biopsies even when they did not have prostate cancer, or when their disease was growing so slowly that it would never cause symptoms.

Although some patients were helped by PSA testing, the data showed that many underwent treatment without benefit. This led to new screening guidelines in 2008, and again in 2012. To prevent men from having unnecessary procedures, it was recommended that men within certain age groups rely on active surveillance rather than the PSA test to detect prostate cancer.

Now, though, new data and additional studies suggest the need for a higher level of vigilance. As a result, further changes to prostate screening guidelines are under consideration.

If approved, the new guidelines will recommend that men between the ages of 55 and 69 talk to their physicians about whether a PSA test is appropriate for them. They should also discuss how frequently the test should be performed. Risk factors like age, race, lifestyle, family history, environmental factors and inherited gene mutations will play a part in this shared decision-making process.

Despite the confusion that can result from these changing guidelines, we believe that screening is important. The test results, when put into the context of years, create a valuable timeline. And when done properly and appropriately, the right test at the right time can save your life.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Every now and then, I'll see a news story about the benefits of daily aspirin. A recent story linked it to a lower risk of cancer. What gives? Just how healthy is aspirin?

Dear Reader: We've long known that aspirin can reduce the chances of heart attack and strokes in people at high risk for the conditions. In fact, studies on those benefits provided the first inklings of a decreased risk of cancer, especially colon cancer.

Although those studies themselves couldn't verify the benefit of aspirin, what we know about the drug supports such a connection. Aspirin inhibits cyclooxygenase-2, or COX-2, an enzyme important to the formation of inflammatory compounds. Hypothetically, the loss of these inflammatory compounds, called prostaglandins, may lead to decreased cellular replication and decreased recruitment of new blood vessels. That is, with a reduced ability to multiply and grow, cancer cells could have less of a chance to take hold.

One of the earliest assessments of aspirin's impact on cancer came from the Women's Health Study, which randomized 39,876 women to taking a placebo or 100 milligrams of aspirin every other day. The women were followed on average for 10 years, and the majority had additional follow up for 7 1/2 additional years. Researchers found little difference in the rates of breast, lung and many other cancers, but after 10 years, they found a 20 percent reduction in the rate of colon cancer among those taking aspirin. The decrease of colon cancer was even greater among women who continued taking aspirin in the 7 1/2-year follow-up period. Note that there was a 14 percent increase in incidents of gastrointestinal bleeding among those taking aspirin versus placebo, but no change in death rates from bleeding between the two groups.

A 2016 study assessed aspirin use among patients in two large ongoing studies: the Nurses' Health Study and the Health Professionals Follow-Up Study. The combined studies looked at 88,084 women and 47,881 men who had been followed since the 1980s. Those who used aspirin two or more times per week, in either regular or "baby aspirin" doses, had a 19 percent reduction in colon cancer and a 15 percent reduction in stomach and esophageal cancers. However, this benefit was not noted for any other cancer.

Further, the decrease in colon cancer rates became significant only after five years of taking aspirin. The authors note that the population in these two studies was predominantly Caucasian and so may not be applicable to other races.

The same researchers recently presented a follow-up to this study to the American Association for Cancer Research. In this re-evaluation, the authors found a 31 percent decrease in the risk of colorectal cancers -- as well as reductions in breast, prostate and lung cancers. The reasons for such significant improvements are unclear.

Overall, the data point to a 20 percent reduction in colorectal cancer risk with the regular use of aspirin -- specifically 81 milligrams every other day -- but the numbers for other cancers are not convincing.

Be aware, however, of the risk of gastrointestinal bleeding. If you've had an ulcer or stomach problems with aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDS) in the past, daily aspirin use is not for you.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I want to be better about protecting my skin from the sun, but the sunscreen aisle of our drug store is so confusing. Are all sunscreens the same and are they equally effective?

Dear Reader: Ultraviolet radiation (also referred to as UV rays) from the sun is the No. 1 cause of skin cancer, so your resolve to wear sunscreen is right on the mark. And with so many different products on the market, it's no surprise you're feeling overwhelmed.

Sunscreen is any cream, spray or lotion that combines specific ingredients that will prevent UV rays from reaching your skin. Two different kinds of UV rays -- UVA and UVB -- are targeted by sunscreen.

UVB rays cause sunburn. UVA rays, which reach into the deeper layers of your skin, cause premature aging. Both types of ultraviolet rays increase your risk of a variety of skin cancers.

You'll notice that sunscreen is rated by a number system -- SPF 15, for example -- which signifies its level of Sun Protection Factor. The SPF factor of a sunscreen refers to its level of protection against UVB rays only. It does not measure protection against UVA rays, which are equally harmful. To be sure you are getting protection against both UVA and UVB rays, choose a product whose label says "broad spectrum."

What the SPF number tells you about any brand of sunscreen is how it will protect your skin. Just multiply the amount of time it takes for your skin to begin to burn by the SPF number.

Let's say you typically start to turn pink after 10 minutes in the sun. A sunscreen with an SPF factor of 15 will, in theory, give you 150 minutes of protection. However, numerous factors are at play. Did you go swimming? Are you sweating? Did you put on enough sunscreen? Did it rub off on your hands or clothes? The rule of thumb put forth by skin cancer specialists is that an application of sunscreen lasts no longer than two hours. And if any swimming or sweating or rubbing occurred, reapply liberally and often.

The Food and Drug Administration, which regulates sunscreen products in the United States, recommends that you use an SPF of at least 15. But the experts at the American Academy of Dermatology say an SPF of at least 30 is the safest bet.

So which sunscreen should you choose and what's the right way to use it?

-- Go for one that's labeled "broad spectrum," so you're shielded against both UVA and UVB rays. Waterproof formulations are best. And remember -- SPF 30 or higher.

-- Apply the sunscreen 15 minutes before you go out. Yes -- before going into the sun.

-- It takes about 3 tablespoons to coat all exposed skin, including the tops of your ears, the back of your neck, your hands, your feet and any exposed scalp.

-- Don't forget to protect your lips -- products rated SPF 30 or more will do the trick.

-- Remember to reapply as needed.

Even with sunscreen, be sure to wear loose clothing and a hat. And stick to the shade for added protection.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)