Although the EBCTCG meta-analysis showed that PMRT reduced locoregional recurrence and mortality in women with 1–3 positive lymph nodes, and increased patients’ radiotherapy session in recent years 15. However, there are many controversial issues to identify, such as ages, T stage, treatment modality, and molecular subtypes [16-18].
In this study, we identified patients from 2001–2011 because the outcomes of treatment were different before and after 2000 [6], with sufficient time for follow-ups. In the dataset before matching, we found that majority of the patients (73.5%) did not undergo PMRT, and the data was also unbalanced between the PMRT and No-PMRT groups, so we used propensity scores for balancing the characteristic factors between the 2 groups.
Previous reports have reported several factors that are related to the prognosis of breast cancer, such as clinical stage, pathologic grade, tumor size, absolute positive lymph nodes, and molecular subtypes [19-21]. Since human epidermal growth factor receptor 2 status was recorded in the SEER database after 2010, different molecular subtypes have been reported as prognostic factors for breast cancer. With the consideration of enough follow-up time, we did not include the status of human epidermal growth factor receptor 2 in this study. In our study, the multivariate Cox model showed that older age, larger tumor size, and more positive nodes were related to the poor OS, and PMRT, chemotherapy, and hormone receptor positivity were related to the better prognosis. Higher pathological grade, large tumor size (T2), and more positive nodes were related to worse BCSS; chemotherapy and hormone receptor positivity are protective factors for BCSS.
Several retrospective studies have shown that PMRT decreases LRR but has no effect on the OS and BCSS in women breast cancer patients with T1–2 and 1–3 positive nodes. In our study, we found that PMRT increased the OS of the patients but had no effect on the BCSS in the patients with T1–2 and 1–3 positive lymph nodes. In our study, we found that PMRT increased the OS of the patients but had no effect on the BCSS in the patients with T1–2 and 1–3 positive lymph nodes. The reason could be that the patients in the No-PMRT group might be associated with other diseases, which was not included in the SEER database, and this could have caused data bias.
Several studies have reported that not all the patients will benefit from PMRT [6-8, 10, 15,18, 22, 23]. Some researchers integrated the factors related to the benefit from PMRT and powered nomograms for predicting the LRR and OS, thereby assisting in the clinical decision of PMRT [24, 25]. In the present study, a study based on SEER database also showed that PMRT had no effect on breast cancer-specific survival (BCSS), and the median follow-up time was 76 months, which was not enough for evaluating the 10-year OS or BCSS [15]. In our study, we found that only patients with 3 positive nodes will benefit from PMRT for both the OS and BCSS. Histopathological grade is an important prognostic factor in N1 breast cancer patients; higher grade tumors had more LRR and worse OS than lower grade tumors and was independent of other risk factors [26, 27]. We found that the BCSS was worse when PMRT was administered to patients with histopathological grade IV breast cancer, which could be owing to the following reasons: first, the sample size was small (n = 167) compared with the sample size of patients having grades I, II, and III breast cancer (n = 706, 3762, and 5017, respectively). The data deviation was relatively large; second, systemic therapy including endocrine therapy, target therapy, and chemotherapy is unclear. Therefore, a further study is required; the result of an ongoing prospective randomized controlled trial SUPREMO is very much expected [28].
In conclusion, PMRT increased the OS and had no effect on the BCSS in the patients with T1–2 and 1–3 positive nodes. Subgroup analysis showed that only patients with 3 positive nodes could gain the benefit of PMRT for BCSS.