Figure 2. Flow Diagram of Inclusion Criteria and Exclusion Criteria.

The inclusion criteria are presented as follows: female sex, between 20–79 years at age of diagnosis, diagnosed as invasive breast cancer between 1990 and 2009, AJCC stages I to III and known radiotherapy status (599,548 patients). 93,954 patients diagnosed before 1990 were excluded as a result of unavailable hormone receptor data, while 39,179 patients diagnosed after 2009 were also excluded due to inadequate follow-up time. Besides, 68 patients with bilateral cases or unknown laterality were excluded. Regarding the primary tumor site, tumors located in the four quadrants and central portion were included, and tumors located on the nipple (2,398 patients), axillary tail (3,351 patients), overlapping region (97,205 patients) and not otherwise specified (57,950 patients) were excluded to avoid selection bias. So, 305,443 patients were included in this study eventually.