Introduction
Hyperprolactinemia that causes complaints and leads to treatment is caused by a prolactinoma in the majority of cases [
1]. In the absence of visualization of an adenoma on MRI, and of secondary causes such as medication, the hyperprolactinemia is referred to as idiopathic [
2]. Low dose of a dopamine agonist is the treatment of choice in patients with a prolactinoma or with complaints caused by hyperprolactinemia. Dopamine agonists normalize prolactin levels and decrease tumor size in the majority of cases [
3]. However, due to a delay in diagnosis, these patients often have been exposed to increased levels of prolactin for months to years.
Prolactin has proliferative effects on mammary epithelial cells [
4]. Moreover, hyperprolactinemia is associated with oncogenesis of the mammary glands in rodents [
5]. However, the issue is still unresolved, whether hyperprolactinemia is associated with an excess risk for breast cancer in female subjects [
5,
6]. Nonetheless, case reports have suggested an association between prolactinomas and breast cancer [
7,
8]. In follow-up studies higher prolactin levels have been associated with an increased risk for breast cancer [
9,
10]. Unfortunately, studies on prolactinoma patients are often underpowered to assess the risk for breast cancer properly. Therefore, the aim of the present analysis was to assess the risk of breast cancer in a previously defined large cohort of patients treated for idiopathic hyperprolactinemia or prolactinomas.
Results
Patients and person-years
The original database consisted of 11,314 subjects with at least one prescription of dopamine agonists with a total of 37,380 prescriptions of dopamine agonists in the period from January 1, 1996 and December 31, 2006. A total of 1,607 patients with dopamine agonist—treated hyperprolactinemia were identified [
11]. For the present analysis only women were included (
n = 1,342).
Mean age of treated female patients was 36 ± 11 years. The initial treatment was bromocriptine in 545 patients (41%), cabergoline in 556 patients (41%) and quinagolide in 241 patients (18%). Pituitary surgery was performed in 31 patients. The mean initial treatment duration was 16.5 months. In 515 patients, dopamine agonist therapy was restarted.
Breast cancer
Based on hospital discharge codes, eight patients with breast cancer during follow-up were identified. The youngest patient was 38 years at the time of diagnosis, the oldest was 61 years. Breast cancer was diagnosed from 11 months to 8 years and 5 months after the first prescription of a dopamine agonist.
Standardized mortality ratio
The 1,342 patients accounted for a total 6,576 person years of follow up. Indirect standardization with incidence proportions from the general Dutch population revealed a total number of expected cases of 7.47. The calculated SMR for breast cancer risk in patients treated hyperprolactinemia was 1.07 (95% confidence interval 0.50–2.03).
Discussion
The present analysis was conducted to estimate the risk of breast cancer in a large cohort of patients treated for hyperprolactinemia. Compared to the general Dutch population, there was no clearly increased risk (SMR 1.07). However, because the number of patients with breast cancer was rather low, the confidence intervals of this analysis are rather broad, thereby prohibiting a definitive conclusion on the possible association between breast cancer risk and hyperprolactinemia.
This study has limitations. Although the cohort of included patients of dopamine agonist-treated hyperprolactinemia is relatively large, the number of breast cancer cases during follow-up was rather small (
n = 8). The low number of breast cancer cases is due to two facts: firstly, a relatively short follow-up period of about 5 years and secondly the relatively young age of patients at diagnosis. Because the confidence interval for a SMR is based on the number of events, i.e. the number of patients who develop breast cancer, and not on the number of patients in the cohort, the confidence intervals were wide. This limitation can only be overcome by including even larger cohorts, or by pooling existing data in a meta-analysis. Secondly, the diagnosis of hyperprolactinemia was based on prescriptions of dopamine agonists in a large pharmaco-epidemiological database. This has been shown to be a reliable approach for the identification of patients idiopathic hyperprolactinemia or prolactinomas [
11]. Another limitation of the present analysis is the fact that no prolactin values of individual patients were known. Therefore, analysis according to initial prolactin levels was not possible. Moreover, stratified analysis according to disease duration was not possible. Selection bias is a potential other limitation. The diagnoses in our cohort were based on hospital discharge codes, whereas the diagnoses from the Dutch cancer registry were based on either hospital discharge codes or on tissue diagnosis. Since breast cancer is almost invariable treated primarily by surgery, requiring hospital admission, the selection bias will not be large.
Exposure to overproduction of pituitary hormones can lead to an increased risk for cancer, as has been shown in acromegaly [
13].The suggested association between breast cancer and elevated levels of prolactin was based on case reports [
7,
8], basic research [
5] and epidemiological studies [
9,
10]. However, thus far, no association between breast cancer risk and idiopathic hyperprolactinemia/prolactinoma has been shown. This is mainly due to the fact that the majority of presented series with prolactinomas are underpowered to detect a possible association [
13].
In the present analysis there was no clear association between breast cancer and patients treated for hyperprolactinemia. The inclusion of both prolactinomas and idiopathic hyperprolactinemia enhances the generalizability of the results [
14]. However, the ultimate verdict on the association between prolactinomas and breast cancer would require a larger cohort. Despite this limitation our study is incompatible with a large breast cancer risk, i.e. a relative risk >2. Several considerations have to be taken into account when interpreting this result. Firstly, the association was assessed in female patients, whereas some case reports described a male patient with breast cancer [
7,
15]. In female patients the levels of prolactin at presentation are clearly lower than in men, mainly because men present more often with larger adenomas [
16]. In our database, breast cancer was not diagnosed in any of the male patients. Secondly, all patients in the cohort included for the present analysis were treated with dopamine agonists. This treatment is very effective in normalizing prolactin levels [
17]. This means that in the follow-up period after start of the medication prolactin levels can be assumed to be normalized in the vast majority of cases. However, in most patients the diagnosis is often delayed and the disease can therefore be accompanied by long-term overexposure to prolactin [
12]. Thirdly, estrogens are known to increase the risk of breast cancer [
4]. Elevation of prolactin leads to suppression of the estrogen-production by inhibition of the release of luteinizing hormone and the follicle stimulating hormone [
3]. The net effect of prolactin on breast cancer risk is therefore also dependent on the degree of suppression of the estrogen production. Theoretically, an increased risk for breast cancer due to hyperprolactinemia can be counterbalanced by the decreased production of estrogens.
In conclusion, this study showed no clear evidence for increased breast cancer risk in female patients treated for ether idiopathic hyperprolactinemia or prolactinomas. The uncertainty about the exact risk that is due to the relatively low number of breast cancer cases, should be overcome by pooling results in a future meta-analysis.