Jacky Luiten

106 | Chapter 7 Discussion This large population ‐ based study among patients aged 50–75 years shows a tenfold increase in the numbers of patients with DCIS in the Netherlands the last three ‐ decades. The use of BCS increased from 47.7% in 1995 ‐ 1996 to 72.7% in 2017 ‐ 2018. Among patients undergoing BCS a sharp rise in the use of adjuvant radiotherapy was observed from 28.9% in 1995 ‐ 1996 to almost 90% in 2011 ‐ 2012, followed by a drop to 74.9% in the most recent years. ALND has been replaced by SLNB over the years, which itself is increasingly replaced by no axillary surgery (55.8%) in patients undergoing BCS in 2017 ‐ 2018. Patients diagnosed with DCIS have become significantly more likely to receive BCS, which is in accordance with the findings in previously published studies. 19,20 BCS has become the preferred surgical treatment for invasive breast cancer, after several randomized clinical trials had shown that BCS with radiotherapy offers the same overall survival rate as mastectomy alone. 21,22 These findings may have had a positive effect on the use of BCS for DCIS as well. Another likely explanation might be that digital mammography screening detects lesions with smaller tumor sizes, which therefore may be more suitable for BCS. 23,24 Unfortunately, our data did not include information on the extent of DCIS lesions. Even though adjuvant radiotherapy following BCS is still considered as a standard therapeutic option in most treatment guidelines, the guidelines also state that radiotherapy can be omitted in cases of DCIS <10mm, low ‐ and intermediate grade and excised with adequate margins. 10 We observed a decrease in the proportion of patients receiving additional radiotherapy the last five years. iRFS analyses showed that the risk of invasive local recurrence was dependent on the use adjuvant radiotherapy and that this association was much stronger for the patients diagnosed in the older period (1989 ‐ 2010). The absolute difference in the 5 ‐ year risk of local recurrence between patients with and without adjuvant radiotherapy was only 0.5% (0.7% vs. 1.2%, respectively) for the patients treated since 2011. A possible explanation for the low risk of invasive local recurrence in the recent period might be increased consensus on the relevance to obtain tumor ‐ negative resection margins after BCS. 25 Another explanation might be the higher sensitivity of digital mammography, resulting in the detection of smaller lesions. This drives the discussion about omitting radiotherapy in a larger proportion of the patients undergoing BCS. For the survival analysis 2011 was chosen as cut ‐ off point, because digital mammography was fully implemented in the Netherlands since then.