Who told you that that?!

Selenium is so undervalued today. It is an antioxidant that binds with heavy metals making them less dangerous to us.

Se is needed to convert T4 to the more active T3 thyroid hormone. It also provides antioxidant protection for the thyroid gland.

The supplements you mentioned are some of the best for fighting or avoiding cancer, however you should consider the positives and negatives of various *forms* of each of them.

Sadly I don't have the reference for this, but it probably is either from Life Extension magazine--lef.org, or wellnessresources.com.

"A gold standard study sponsored by the National Cancer Institute involving 1300 people (J.A.M.A. Dec.1996) found 200 mcg of high selenium yeast daily can reduce the all cancer incidence rate by 39% and the overall cancer death rate by half. Prostate cancer incidence was reduced 63%. Colon cancer incidence was reduced 58%. Lung cancer incidence was decreased 46%. All carcinoma incidence was reduced 45%"

I have much more for you below; however, I suggest that you watch this video now because it will only be available online for free for a few more hours.
https://cbtc.clickfunnels.com/optin-1183909811841377

In Episode 10 Chris discusses the RCGG Onconomics Plus Test, also known as oncostat, or the Greek test. They test your cancer and tell you which natural and Pharma substances will likely be helpful in the fight against your particular cancer.

Lef.org has quite a bit of information on prostate cancer. Do a search. For example check out the issues related to the various forms of Se and various forms of vitamin E.
http://www.lifeextension.com/Protocols/Cancer/Prostate-Cancer-Prevention/Page-05

Selenium: What Forms Protect Against Cancer?
http://www.lifeextension.com/Magazine/2012/SS/Selenium-Protect-Against-Cancer/Page-01

Notice the discussion of the "select" study of Vitamin E and Selenium
http://www.webmd.com/prostate-cancer/news/20130409/can-selenium-lower-risk-of-advanced-prostate-cancer#1

These are two very harsh critiques of the so called "select" study! It is an example of how studies can be designed to promote a particular agenda--such as Big Pharma profits--rather than promote human health! Then pro Pharma sites such as WebMD and mainstream media that receives 50% of its ad revenue from Pharma repeat the Pharma propaganda.
http://www.wellnessresources.com/health/articles/flawed_select_study_attacks_vitamin_e/

http://www.lifeextension.com/Featured-Articles/2008/12/Response-to-Selenium-Vitamin-E-Cancer-Prevention-Trial-Study/Page-01

Consider taking mixed tocopherols and or mixed tocotrienols or a combination of those forms of vitamin E. Taking just the cheap alpha form most commonly found in pharmacies is unnatural and can lead to lower levels of the gamma form.

If I had time I'd share links to studies on some esoteric forms of vitamin E causing apoptosis--natural cell death--in cancer cells.

There is SO much information out there on supplements and diets as alternative or adjunct treatments for cancer. Sadly the best information on this is not likely to come from the US but from overseas. Pharma has a stranglehold on scientific development for the benefit of their profits, not our health.

check out

https://www.cancertutor.com/
he seems to really like the Budwig protocol.

This guy lost both parents and several other relatives to cancer. He goes around the world interviewing researchers. Most are natural or semi-natural. One, Rigvir, is an approved natural medical treatment in Eastern Europe and is covered by insurance there. It is available in Mexico
https://thetruthaboutcancer.com/

This is an into to one of his video series.
https://www.youtube.com/watch?v=KqJAzQe7_0g&t=898s

This article on GcMAF will help you to understand what is at stake for Pharma if they don't squash the wave of alternative cancer treatments. When used in correct combinations, they can be remarkably effective
http://www.naturalnews.com/050582_nagalase_GcMAF_cancer_industry_profits.html

Stories from a British guy who ran a GcMAF clinic in Switzerland where he was fairly free from harassment from Pharma and their surrogates in government.
https://www.youtube.com/watch?v=058Xfexop7E&feature=em-subs_digest-vrecs

You might learn about the liposomal form of vitamin C.

Could your source be referring to a risk from taking excessively high doses of selenium? In contrast to the 800mcg that was used in this study, 200mcg is more commonly sold in pharmacies.
http://www.wellnessresources.com/studies/selenium_supplementation_in_men_with_prostate_cancer/

Also compare the effects of 200mcg and 400mcg Se. You should also consider the form(s) used.
http://www.wellnessresources.com/studies/the_nutritional_prevention_of_cancer_400_mcg_per_day_selenium_treatment/

This long-time editor of the New England Journal of Medicine wrote a book some years ago very critical of how Pharma corrupts studies. Remember she saw both the studies that they accepted for publication and those that were rejected. She teaches at Harvard. She's mainstream, not natural.
http://www.nejm.org/doi/full/10.1056/NEJM200410073511522#t=article

Thankfully people today have alternative ways to learn. They don't have to rely on what the doctor feels is best--for profits.