Here is a rundown on some of the studies carried out on vitamin D and cancer.
Types of Cancer
A report which looked at research findings from 1966 to 2004 said that vitamin D can lower breast cancer risk by as much as 50%. 
Generally, vitamin D has been widely studied for its benefits on breast cancer prevention. However, more recent research suggests that vitamin D can help improve survival rates as well. Some research revealed that women with low vitamin D levels were more likely to develop more advanced breast cancers compared to those with higher vitamin D levels. 
Research carried out over an eight-year period in the 1980s on over 25,000 persons discovered that the colon cancer risk of those with normal levels of vitamin D was a massive 80% lower. 
Epidemiological studies have also revealed that higher vitamin D levels could lower colon cancer risk by about 60%. 
The Journal of the American Medical Association reported a study involving over 3,000 persons which showed that higher vitamin D and calcium intake was linked to lower rates of colon cancer. Other research studies affirmed this correlation. 
Colon cancer is more prevalent in areas of the United States with lower levels of sunshine. This trend is directly linked to vitamin D, as sunshine converts certain chemicals on human skin into the vitamin. 
Research found that vitamin D helped prevent the spread of kidney cancer cells by inhibiting their "ability to divide and replicate." It also "induces apoptosis" and "reduces invasiveness and angiogenesis." 
Non-small-cell lung cancers (NSCLCs) include squamous cell lung cancer, adenocarcinoma and large-cell carcinoma. NSCLCs make up four out of every five cases of lung cancer. A clinical study found that sufferers of early-stage NSCLC who had higher blood vitamin D levels had better survival rates than those with the least vitamin D in their blood. Further, this same trial raised "recurrence-free survival." 
Research found that, when activated vitamin D was applied to ovarian cancer cells, the growth of those cells was significantly suppressed. Ovarian cells have vitamin D receptors which, when occupied, contribute to the inhibition of "growth-signaling pathways" in ovarian cancer cells. 
Research has found that men who have more vitamin D in their blood have a 50% lower chance of developing forms of prostate cancer which are aggressive as compared to men with lower blood levels of vitamin D. 
Among the different types of skin cancer, melanoma is the most deadly. In vitro research conducted at Saarland University Hospital in Germany revealed that vitamin D lowered the spread of melanoma cells by up to 50%, and there have been other studies conducted elsewhere which have affirmed this finding. 
Other types of cancer which vitamin D has displayed preventive or protective benefits against include leukemia (blood cancer), pancreatic cancer, thyroid cancer, non-Hodgkin lymphoma and numerous others. 
Dr. Joseph Mercola believes so strongly in the role of vitamin D in fighting cancer that all the patients at his wellness center are tested for blood levels of this vitamin (it is worthwhile to note that, while vitamin D is usually mentioned as a vitamin, it would be more accurate to refer to it as a hormone, as its characteristics more closely resemble the latter).
Indeed, an aggregated review of 63 previous studies discovered that the "evidence suggests that efforts to improve vitamin D status... could reduce cancer incidence and mortality at low cost, with few or no adverse effects." 
When it comes to both cancer prevention and cancer treatment, it is clear that the humble vitamin D cannot be ignored.
One important thing to note is that aggressive oral supplementation of vitamin D can result in toxic overdose and should ideally be carried out under the guidance of a suitable health care practitioner. Undergoing tests on blood vitamin D levels would also be helpful.
Sources for this article include:
 Mercola, Joseph, Dr., and Pearsall, Kendra, Dr. Take Control of Your Health. Schaumburg, IL: Mercola.com, 2007. Print.
 Murray, Michael, ND, Birdsall, Tim, ND, Pizzorno, Joseph E, ND, and Reilly, Paul, ND. How to Prevent and Treat Cancer with Natural Medicine. New York, NY: Penguin Group, 2002. Print.
 Alschuler, Lise N, ND, FABNO, and Gazella, Karolyn A. The Definitive Guide to Cancer: An Integrative Approach to Prevention, Treatment, and Healing. NY, USA: Celestial Arts, 2010. Print.
About the author:
Reuben Chow has a keen interest in natural health and healing as well as personal growth. His main health websites Insights on Health and All 4 Natural Health focus on being healthy naturally, while his other health websites cover topics such as cancer research and information, holistic depression help, depression help and natural remedies, as well as omega 3 fatty acids. He also owns self improvement and inspirational websites like Inspiration 4 Living, allinspiration.com, Life Changing Quotes, and 101 Inspirational Ideas. Through his network of sites at The Journey of Life, he hopes to help improve people's lives.