Adherence to antiestrogen therapy
As many as 70% of breast cancer patients have tumours that have become dependent on oestrogens to live. Therefore, anti-estrogen therapy is a very effective and important adjuvant therapy to these women. This form of therapy consists of taking an estrogen-blocking tablet daily up to 10 years, and therefore takes place without the supervision of health professionals. As anti-estrogen therapy often generates a number of side effects with reduced quality of life, it increases the risk that patients themselves will discontinue treatment far too early. Unfortunately, this leads to an increased risk of recurrence of the breast cancer disease. Identification of patients at high risk of discontinuing anti-estrogen therapy may improve care and increase long-term survival in these women.
In order to identify risk factors for patients failing to take the anti-estrogen tablets (tamoxifen or aromatase inhibitors) as prescribed (= non-compliance), it is important to choose accurate research methods. In two studies recently published from PBCB, we compared the three most common methods for measuring compliance: 1. Patient-reported data, 2. Prescription database data, and 3. Drug analysis in blood. In the first study conducted by Dr. Kari Britt Hagen, evidence was found that patients underreported how well they complied with the prescription of the anti-estrogen medication. Only 7% of patients reported that they had stopped taking the anti-estrogen medication. During the same period, according to the Norwegian Prescription Database, 25% of the women had taken too little medication to be able to comply with the anti-estrogen therapy. When following patients over a 5-year period, 38% of patients stopped taking the tablets before they should. The dropout rate was highest after 2 years of treatment. The dropout pattern was similar for tamoxifen and aromatase inhibitors.
In a subsequent study by Dr. Thomas Helland, prescription database data were compared with drug analysis. Both methods indicated that approximately 18% of patients had discontinued tamoxifen treatment within 5 years. The results also indicate that prescription database data are a good yardstick for compliance with anti-estrogen therapy in Norwegian breast cancer populations. This is an important finding since many studies are based precisely on prescription database data.
In order to investigate the factors that contributed to breast cancer patients discontinuing endocrine treatment prematurely, questionnaire and clinical data were collected in both of the above two studies. In the first study by Dr. Hagen, she found evidence that high BMI was a risk factor for reduced adherence/early termination of anti-estrogen therapy. Dr. Helland found in his study that patients, who reported having experienced nausea and vaginal dryness, were more likely to quit tamoxifen early. Furthermore, it was found that patients who had not received chemotherapy also had a higher risk of early termination of tamoxifen treatment. Identifying risk factors for low compliance will be an important tool for both doctors and nurses who work with the follow-up of breast cancer patients.