title: Statistical interaction in the survival analysis of early breast cancer using registry data: role of breast conserving surgery and radiotherapy.
abstract: OBJECTIVE
  To identify subgroup effects that might influence the survival results of postoperative radiotherapy.
  
  
  METHODS
  Women selected from the Surveillance, Epidemiology, and End Results database, aged 40-69 years, with non-metastasized T1-T2 breast carcinoma, in whom axillary lymph node dissection was performed. Subgroup analyses were performed using proportional hazards models with interactions. Joint significance of subgroups was evaluated with the Wald test. Event was death from any cause.
  
  
  RESULTS
  Statistically significant interactions were found between type of surgery (breast-conserving [BCS] or mastectomy [ME]), radiotherapy [RT], T stage, and extent of nodal involvement, but not between treatments and nodal examination. For each treatment combination, ME-no RT, ME+RT, BCS-no RT, BCS+RT, the mortality hazard ratios were respectively: 1, 1.12, 1.11, 0.78 in T1, 0-3 positive nodes; 2.45, 2.77, 2.71, 1.92 in T2, 4+ nodes; 1.31, 1.38, 1.33, 1.19 in T2, 0-3+ nodes; and 3.41, 2.79, 3.44, 2.40 in T2, 4+ nodes. The corresponding joint tests showed: in the absence of radiotherapy, no significant survival disadvantage for breast-conserving surgery vs mastectomy; with radiotherapy, significant survival advantage for breast-conserving surgery irrespective of stage and for mastectomy in T2, 4+ nodes. For mastectomy in less advanced stages receiving radiotherapy, excess breast cancer deaths suggested undocumented adverse selection. The corresponding result was considered inconclusive.
  
  
  CONCLUSIONS
  The analyses found subgroup effects that should be taken into account to interpret treatment results in breast cancer.
authors.affiliation: Oncology Center, Academic Hospital, Vrije Universiteit Brussels, Jette, Belgium. conrvhgv@az.vub.ac.be
authors.forename: Vincent
authors.initials: V
authors.lastname: Vinh-Hung
authors.forename: Tomasz
authors.initials: T
authors.lastname: Burzykowski
authors.forename: Jan
authors.initials: J
authors.lastname: Van de Steene
authors.forename: Mia
authors.initials: M
authors.lastname: Voordeckers
authors.forename: Jan
authors.initials: J
authors.lastname: Lamote
authors.forename: Guy
authors.initials: G
authors.lastname: Storme
mesh_terms: D000328:Adult; D000368:Aged; D001530:Belgium / Q000453:epidemiology; D001943:Breast Neoplasms / Q000401:mortality* / Q000473:pathology / Q000532:radiotherapy / Q000601:surgery / Q000628:therapy*; D005260:Female; D006801:Humans; D008207:Lymphatic Metastasis; D015412:Mastectomy, Segmental; D008875:Middle Aged; D015999:Multivariate Analysis; D009367:Neoplasm Staging; D016016:Proportional Hazards Models; D018714:Radiotherapy, Adjuvant; D018426:SEER Program; D016019:Survival Analysis