I learned something new from reading this research paper. The inside of the vitamin D3 cell has anti-tumor action. I knew vitamin D had protective properties, but it is the nucleus of the vitamin D cell that takes action.
The vitamin D receptor is present in normal mammary tissue (breast gland). What this does is act as a tumor suppressor gene. Vitamin D receptors regulate energy metabolism, reduce inflammation, and differentiate good cells from bad cells. The researchers suggest that breast cells produce receptors that maintain vitamin D levels in the breast tissue, and when these receptors stop working properly, deregulated cancer has a higher rate of development.
A similar article on colon cancer and vitamin D
Researchers found that vitamin D inhibits the genes of colon cancer-associated fibroblasts, giving it an anti-tumor effect.
A. Get some sun
B. Take your vitamin D supplements. The highest increase in vitamin D levels we have seen at the office after a summer of being in the sun only had a 10-point elevation in their levels.. All of us need supplementation.
C. Get your vitamin D tested. You need to know that you are taking the right amount. Too little is no good, and too much causes calcium to leave the bones.
Absorption of vitamin D depends on the health of the intestine. Some people can absorb vitamin D in 
many forms, and some cannot. We use the Perque D3 Cell Guard, which is in rosemary oil. This way you will absorb more than half of it in the mouth and the intestine will absorb the rest.
https://stan.store/drbrianmcguckin
https://www.sciencedirect.com/science/article/abs/pii/S030372071730237X
Function of the vitamin D endocrine system in mammary gland and breast cancerJoEllen Welsh PhD (Empire Innovations Professor)University at Albany Cancer Research Center, 1 Discovery Drive, Rensselaer, NY 12144, USAReceived 3 February 2017, Revised 12 April 2017, Accepted 12 April 2017, Available online 1 June 2017, Version of Record 22 July 2017.Molecular and Cellular EndocrinologyDate: 15 September 2017 Pages: 88-95 Volume: Volume 453
https://www.sciencedirect.com/science/article/abs/pii/S0960076018302954
Mechanisms of action of vitamin D in colon cancerReceived 22 May 2018, Revised 20 June 2018, Accepted 3 July 2018, Available online 4 July 2018, Version of Record 30 November 2018.The Journal of Steroid Biochemistry and Molecular BiologyVolume 185, January 2019, Pages 1-6