world cancer report - iarc
world cancer report - iarc
world cancer report - iarc
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after cessation of use. Interestingly, however,<br />
use of sequential oral contraceptives,<br />
containing progestogens only in the<br />
first five days of a cycle, is associated<br />
with an increased risk of endometrial <strong>cancer</strong>.<br />
For ovarian <strong>cancer</strong>, risk is reduced in<br />
women using combined oral contraceptives,<br />
the reduction being about 50% for<br />
women who have used the preparations<br />
for at least five years (Table 2.22). Again,<br />
this reduction in risk persists for at least<br />
10-15 years after cessation of use. It has<br />
also been suggested that long-term use of<br />
oral contraceptives (more than five years)<br />
could be a cofactor that increases risk of<br />
cervical carcinoma in women who are<br />
infected with human papillomavirus [9].<br />
Postmenopausal hormone replacement<br />
therapy<br />
Clinical use of estrogen to treat the symptoms<br />
of menopause (estrogen replacement<br />
therapy or hormone replacement therapy)<br />
began in the 1930s, and became widespread<br />
in the 1960s. Nowadays, up to 35%<br />
of menopausal women in the USA and<br />
many European countries have used<br />
replacement therapy at least for some period.<br />
The doses of oral estrogen prescribed<br />
decreased over the period 1975-83 and the<br />
use of injectable estrogens for estrogen<br />
replacement therapy has also diminished.<br />
On the other hand, the use of transdermally<br />
administered estrogens has increased<br />
progressively to about 15% of all estrogen<br />
replacement therapy prescriptions in some<br />
countries. In the 1960s, some clinicians,<br />
especially in Europe, started prescribing<br />
combined estrogen-progestogen therapy,<br />
primarily for better control of uterine bleeding.<br />
The tendency to prescribe combined<br />
estrogen-progestogen hormonal replacement<br />
therapy was strengthened when first<br />
epidemiological studies showed an<br />
increase in endometrial <strong>cancer</strong> risk in<br />
women using estrogens alone.<br />
A small increase in breast <strong>cancer</strong> risk is<br />
correlated with longer duration of estrogen<br />
replacement therapy use (five years<br />
or more) in current and recent users [8].<br />
The increase seems to cease several years<br />
after use has stopped. There appears to<br />
be no material difference in breast <strong>cancer</strong><br />
risk between long-term users of all hor-<br />
Fig. 2.71 Estimated risk of breast <strong>cancer</strong> by time since last use of combined oral contraceptives, relative<br />
to never-users. Data adjusted by age at diagnosis, parity, age at first birth and age at which risk of conception<br />
ceased. Vertical bars indicate 95% confidence interval.<br />
mone replacement therapy and users of<br />
estrogens alone. Nevertheless, one large<br />
cohort study and a large case-control<br />
study have provided strong evidence for a<br />
greater increase in breast <strong>cancer</strong> risk in<br />
women using hormone replacement therapy<br />
than in women using estrogens alone<br />
[10,11].<br />
For endometrial <strong>cancer</strong>, there is an<br />
increase in risk among women using estrogen<br />
replacement therapy, the risk increasing<br />
further with longer duration of use [8].<br />
In contrast, women using hormone<br />
replacement therapy have only a mild<br />
increase in risk compared to women who<br />
have never used any postmenopausal hormone<br />
replacement and this increase is<br />
much smaller than that of women who<br />
used estrogens alone. There seems to be<br />
no relationship between risk of ovarian<br />
<strong>cancer</strong> and postmenopausal estrogen use,<br />
while data on ovarian <strong>cancer</strong> risk in relation<br />
to hormone replacement therapy use<br />
are too scarce to evaluate.<br />
Prostate <strong>cancer</strong><br />
Normal growth and functioning of prostatic<br />
tissue is under the control of testosterone<br />
through conversion to dihydroxytestosterone<br />
[12]. Dihydroxytestosterone<br />
is bound to the androgen receptor, which<br />
translocates the hormone to the nucleus.<br />
There have been conflicting findings as to<br />
whether patients with prostate <strong>cancer</strong><br />
have higher levels of serum testosterone<br />
than disease-free controls. Diminution of<br />
testosterone production, either through<br />
estrogen administration, orchidectomy or<br />
treatment with luteinizing hormonereleasing<br />
hormone agonists, is used to<br />
manage disseminated prostate <strong>cancer</strong>.<br />
Mechanisms of tumorigenesis<br />
Breast <strong>cancer</strong><br />
The role of endogenous hormones in<br />
breast <strong>cancer</strong> development suggests the<br />
“estrogen excess” hypothesis, which stipulates<br />
that risk depends directly on<br />
breast tissue exposure to estrogens.<br />
Estrogens increase breast cell proliferation<br />
and inhibit apoptosis in vitro, and in<br />
experimental animals cause increased<br />
rates of tumour development when estrogens<br />
are administered. Furthermore, this<br />
theory is consistent with epidemiological<br />
studies [4,15] showing an increase in<br />
breast <strong>cancer</strong> risk in postmenopausal<br />
women who have low circulating sex hormone-binding<br />
globulin and elevated total<br />
and bioavailable estradiol.<br />
The “estrogen-plus-progestogen” hypothesis<br />
[15,16] postulates that, compared<br />
Reproductive factors and hormones<br />
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