I've already emailed the Specialist Nurses at PCUK and am waiting for a reply . I'll post the relevant info' when I get it.
From what I've gathered, although oestrogen is used in the treatment of cancers, it can also cause some prostate cancers to grow. I'm not aware of any tests which would give a patient that information.
I did come across 1 paper which stated that there have been no recorded cases of transgender HRT causing prostate cancer. I guess that only tells us that the research isn't there.
Looking at the endocrinology, not every transgender woman starts on HRT with the expertise of an endocrinologist specialising in transgender health. Transgender women may be managed with post-menopausal levels of hormones, some with pre-menopusal levels as the goal. Some women will argue that they require supra physiological levels of oestrogen to achieve their desired characteristics. Others still will do things entirely DIY , using online pharmacies and never seeing a specialist. To complicate matters further, whereas drugs such as spironalactone were once prescribed alongside oestrogen, increasingly , there are transgender women who are on cross gender hormones only.
Added to this is the experience that some individuals require much more oestrogen than others to achieve the same blood levels, and that any given level does not result in a standard level of feminisation.
And if you thought that wasn't complicated enough, there are the different methods od delivering oestrogen to the body-e.g. transdermal patches, i.m. and subcutaneous injection at various different body sites, gels, sublingual and oral pills. It is not surprising, given all the various pathways in the body that are governed by hormones, that doctors are risk averse and prescriptions are sometimes at the minimum level possible, rather than any feminisation, the level given is just designed to ward off osteoporosis.
I suppose given the relatively small population of transgender women, and all these variables, it is hardly surprising that there is a lack of good research data.