Dennis-
This is a far cry from the necessary research needed to tackle such a difficult question, however this is what I have found regarding HRT and cancer (specifically liver cancer).
Estrogen is primarily processed (along with some other hormones, proteins, and enzymes) in the liver. As we know, too much of somethings can be a
very bad thing. This is the case with estrogen and the liver. Depending on the type of hormones taken for HRT (equine estrogen is can be rather damaging to the liver depending on the dosage) and the ratio of estrogen to progesterone (high level of estrogen with low levels of progesterone can be bad for liver) various effects can potentially be manifested (i.e. liver cancer). Obviously some damage can occur whenever either of those two happen, however one typically does not develop severe problems
unless the hormones are taken without proper medical care (this is why self dosing is a VERY VERY bad thing) or over an extended period of time with the above mentioned conditions being present. There is also a rather low occurrence of liver cancer, typically cancer that is found in the liver is actually cancer that has spread (metastasized) from another location (usually from the colon, stomach, or lymph nodes, though possibly from other areas).
One interesting note is that liver cancer can be treated (depending on the type and stage) with hormonal therapy using
antiestrogen (tamoxifen has been used in some study trials).
This situation can be better thought of in another, more widely known, way. Liver cancer cause by HRT is similar to liver cancer caused by alcohol (aka heptocellular carcinoma, which is typically a secondary condition cause by hepatic cirrhosis). While there is a large quantity of individuals that drink alcohol, some even on a daily basis, most of these people will not experience any noticeably harmful physical effects due to the alcohol (though, there may be some effects cause by 'beer goggles', however we will leave those out of the discussion...). The problem with alcohol occurs when an individual drinks in rather large quantities for a many years. That is when the liver damage rate is higher than the individual's repair rate.
There are many confounding variables that affect an individual's possibilities of developing cancer. These include (but obviously are not limited to): genetic susceptibility, age, medical history, family history, exposure to toxins, etc.
I am not quite sure if this is helpful (as I too have no first hand knowledge of HRTs direct effect on causing liver cancer), however here are some starter links to additional information.
*
http://en.wikipedia.org/wiki/Hepatocellular_carcinoma Wikipedia's article on hepatocellular carcinoma
*
http://en.wikipedia.org/wiki/Hepatotoxicity Wikipedia's article on hepatotoxicity (chemical driven liver damage)
*
http://www.emedicine.com/med/topic787.htm eMedicine page on Fibrolamellar Carcinoma (subset of hepatocellular carcinoma
*
http://www.cancer.gov/cancertopics/types/liver National Cancer Institute (NCI) webpage on liver cancer (full of helpful info and links)
I hope this helps.
--Kyle
