The effect of lower mastectomy rates in screen-detected patients goes beyond the lower stage distribution in MSP-detected cancers. Patients with cancers detected through the MSP received consistently less likely a mastectomy and have overall half the odds as those not detected through the MSP.
Breast-conserving surgery (BCS) leads to a higher quality of life for many patients compared to mastectomy. Mastectomy patients usually reported a lower body image and sexual functioning (Montazeri 2008). In comparison with BCS, mastectomy is a more invasive procedure that sometimes results in complications such as infection, poor healing, and lymphedema and requires longer hospital stays (Andersen and Kehlet 2011). BCS results in less discomfort and pain, but requires (time-consuming) radiation and surveillance by mammography and might result in higher anxiety about recurrence.
There are, however, several reasons of personal, medical or preventive nature to choose a mastectomy in contrast to a BCS. These reasons include an increased risk of being diagnosed with second cancer due to BRCA mutations, larger tumours, multiple areas of the breast affected by cancer, and inflammatory breast cancer. An imbalance of these factors among the two groups may contribute to the observed difference in mastectomy rates.
Also, in most cases, BCS should be combined with radiotherapy to result in equivalent survival as mastectomies (Fisher et al. 2002; McLaughlin 2013). Mastectomies might therefore also be chosen when radiation therapy is medically contraindicated, frequently after previous BCS with radiation therapy, or on a personal level, if the patient prefers to avoid radiotherapy, e.g. living far from facilities offering radiation therapy (Mac Bride et al. 2013).
Patients with previous breast cancer are not permitted into the screening but are more likely to receive a mastectomy. However, the incidence of second breast cancer in Eastern Switzerland is low with 4.5% (Vogt 2017; Vogt et al. 2017) and can only explain part of the difference. As for distance to radiation therapy units, using urbanisation level as a proxy did not significantly influence mastectomy rates in Switzerland, however, using the surgeon and gynaecologist density did so (Herrmann et al. 2019). An imbalance of these factors may have contributed to the difference.
Furthermore, the mammography screening programme in St.Gallen follows strict quality assurance guidelines and may preferentially refer patients to specialized breast centres. It has been shown, that surgeons with higher caseloads and in multidisciplinary settings, such as in breast centres, are associated with decreased mastectomy rates for women with early BC (Gu et al. 2019). Specialized breast centres in Switzerland are certified and monitored by EUSOMA, a non-profit society that promotes evidence-based high quality care for breast cancer patients by multidisciplinary breast teams (EUSOMA 2020).
A strength of this study is the use of detailed information from the cantonal cancer registry. The cancer registry collected detailed information on the tumours including the reason for cancer detection and staging information. A limitation of the study is that only the TNM stage was available. In a follow-up study it is necessary to gather data on further possible influencing factors and analyse their influence on mastectomy rates.