Jacky Luiten

Patterns of treatment and outcome of ductal carcinoma in situ in the Netherlands | 99 7 Statistical analysis Patients were categorized by two ‐ year cohorts based on date of diagnosis. Trends in breast surgery were studied and expressed as proportion of all patients per two ‐ year cohort. The trends in use of adjuvant radiotherapy were expressed as proportions of all patients undergoing BCS per two ‐ year cohort. Trends in use of radiotherapy was also categorized by grade. Trends in axillary surgery were categorized per type of axillary treatment, within those categories, trends in treatment were expressed as proportion of all patients. Trends in axillary treatment was also categorized by type of breast surgery. When trends were compared, missing data was excluded for all subgroups. Statistical analyses were performed using SPSS, version 24.0 (SPSS, Inc., Chicago, USA). Chi ‐ square analyses were performed to compare proportional differences in categorical variables between groups. P ‐ values less than 0.05 were considered statistically significant. Kaplan ‐ Meier analyses, were performed to estimate the cumulative risk of invasive local recurrence, expressed as invasive relapse ‐ free survival (iRFS) [with 95% confidence interval] following BCS. Differences in the iRFS between periods of diagnosis and between patients with and without breast radiotherapy following BCS were compared by means of the two ‐ tailed log ‐ rank test. Results Between January 1989 and January 2019, 30,417 women aged 50–74 years were diagnosed with DCIS in the Netherlands. Baseline characteristics of these patients are shown in Table 7.1. Since 2011, 75.1% (10,444/13,913) of all patients had been detected by the national screening program. The number of new cases increased from 379 in 1989 ‐ 1990 to 3,573 in 2017 ‐ 2018. Of all patients 48.7% (11,238/23,065) was high grade, 34.5% (7,953/23,065) intermediate grade and 16.8% (3,874/23,065) low grade.