Jacky Luiten

118 | Chapter 8 If DCIS is diagnosed and surgical treatment is required, accurate pre ‐ operative localization of the lesion is of utmost importance in order to facilitate adequate resection when breast conserving surgery (BCS) is performed. In chapter 6 two methods for the localization of non ‐ palpable DCIS lesions were compared: wire ‐ guided localization (WGL), which is currently the most frequently used localization technique, and radioactive iodine ‐ 125 seed guided localization (I ‐ 125 GL). Patients in the I ‐ 125 GL group had a significantly lower risk of extensively involved resection margins compared to patients in the WGL group (4.4% vs. 12.8%; p =0.048). This resulted in fewer patients in the I ‐ 125 GL group who needed an additional surgical treatment. I ‐ 125 GL proved to be an attractive alternative to WGL for the resection of non ‐ palpable DCIS in BCS. In chapter 7 we evaluated the patterns of care in treatment of DCIS since the introduction of the national screening program, using the national data of the Netherlands Cancer Registry. The proportion of DCIS patients undergoing of BCS has increased over the years (47.7% in 1995 ‐ 1996 to 72.7% in 2017 ‐ 2018; p <0.001) with a decline in the use of adjuvant radiotherapy in more recent years (89.6% in 2011 ‐ 2012 to 74.9% in 2017 ‐ 2018; p <0.001) especially for low grade DCIS. Survival analyses showed that the risk of invasive local recurrence was strongly dependent on the use adjuvant radiotherapy, especially for the patients diagnosed in the period 1989 ‐ 2010 (1.3% [95% CI 1.0% ‐ 1.6%] for patients with vs. 5.8% [95% CI 4.8% ‐ 6.7%] for patients without radiotherapy; p <0.001). The absolute difference in the risk of local recurrence between patients with and without adjuvant radiotherapy was much smaller for the patients treated in more recent years (0.7% [95% CI 0.5% ‐ 0.9%] vs. 1.2% [95% CI 0.6% ‐ 1.8%]; p =0.01). We also observed a de ‐ escalating trend in axillary treatment in DCIS patients, with the replacement of axillary lymph node dissection by sentinel lymph node biopsy. In the most recent years we observed an increasing trend of patients not receiving any axillary staging 36.9% in 2005 ‐ 2006 to 55.8% in 2017 ‐ 2018 ( p <0.001). In order to avoid overtreatment in patients with DCIS, tailoring treatment to the probability of progression is the next step in preventing overtreatment. The main aim for future research should be improving our ability to refrain from invasive treatment, when possible. Finally, in chapter 8 the results of this thesis are summarized and discussed in chapter 9 .