Background: Evidence-based guidelines for locoregional therapy of invasive breast cancer treated with mastectomy include adjuvant PMRT for: ≥4 positive axillary lymph nodes (LN); T3 or above; or a positive surgical margin. We assessed PMRT uptake using data from the Michigan Breast Oncology Quality Initiative (MiBOQI), a Blue Cross Blue Shield of Michigan/Blue Care Network-sponsored collaborative quality initiative, and identified factors influencing its use in Michigan.

Methods: We prospectively collected clinical data on all patients with stage I-III breast cancer in 25 health systems belonging to MiBOQI and identified patients who underwent mastectomy from 2008 to 2013. Patients with previous cancer, bilateral disease, or treated with neoadjuvant chemotherapy were excluded. Univariate and multivariate analyses were performed to identify independent factors associated with the use of PMRT in patients with 0, 1-3, and 4+ positive LNs. Covariates included age, hormone receptor status, HER2 status, surgical margin, T category, Charlson comorbidity index, and immediate reconstruction. Two-tailed p-values <0.05 were considered significant. Analyses were carried out using SAS software, version 9.4 (SAS Institute, Cary, NC).

Results: We identified 6,596 patients with stage I-III invasive breast cancer. Of these, 4,455 had no positive axillary LNs; 1,481, 1-3 positive LNs; and 660, ≥4 positive LNs. There was wide variation in PMRT use across MiBOQI sites, from 13% to 63.% in patients with 1-3 positive LNs (overall 42%) and from 35% to 91% in patients with 4+ positive LNs (overall 69%). In multivariate analyses stratified by nodal status (0, 1-3, 4+), age ≥ 70 yrs was negatively associated with PMRT. We also noted lower PMRT use in women aged 51-69 with 0 and 1-3 positive LNs (Table 1).

Table 1. Multivariate analysis: PMRT and age

  (+)LNs = 0 (+)LNs = 1-3 (+)LNs ≥ 4 
Age Odds ratio p-value Odds ratio p-value Odds ratio p-value 
≤50 y (reference) 1.0   1.0   1.0   
51-69 y 0.70 (0.49 - 0.98) 0.03 0.69 (0.54 - 0.88) <0.0001 1.03 (0.63 - 1.65) <0.0001 
≥70 y 0.60 (0.37 - 0.96)   0.26 (0.18 - 0.37)   0.31 (0.18 - 0.54)   
  (+)LNs = 0 (+)LNs = 1-3 (+)LNs ≥ 4 
Age Odds ratio p-value Odds ratio p-value Odds ratio p-value 
≤50 y (reference) 1.0   1.0   1.0   
51-69 y 0.70 (0.49 - 0.98) 0.03 0.69 (0.54 - 0.88) <0.0001 1.03 (0.63 - 1.65) <0.0001 
≥70 y 0.60 (0.37 - 0.96)   0.26 (0.18 - 0.37)   0.31 (0.18 - 0.54)   

In the 0 and 1-3 positive node groups, PMRT use was strongly associated with T category and close or positive margin status (Table 2).

Table 2: Surgical characteristics and PMRT uptake

  (+)LNs = 0 (+)LNs = 1-3 (+)LNs ≥4 
VARIABLE Odds ratio p-value Odds ratio p-value Odds ratio p-value 
Margin status             
Negative (reference) 1.0   1.0   1.0   
Positive 16.7 (10.5 - 26.7) <0.0001 2.27 (1.32 - 3.90) 0.001 0.69 (0.32 - 1.47) 0.90 
Close (<1 mm) 4.63 (3.03 - 7.10)   1.90 (1.29 - 2.80)   0.97 (0.55 - 1.69)   
              
T category             
T0 to T2 (reference) 1.0   1.0   1.0   
T3 to T4 26.2 (16.4 - 41.8) <0.0001 3.24 (2.19 - 4.78) <0.0001 0.86 (0.57 - 1.28) 0.45 
  (+)LNs = 0 (+)LNs = 1-3 (+)LNs ≥4 
VARIABLE Odds ratio p-value Odds ratio p-value Odds ratio p-value 
Margin status             
Negative (reference) 1.0   1.0   1.0   
Positive 16.7 (10.5 - 26.7) <0.0001 2.27 (1.32 - 3.90) 0.001 0.69 (0.32 - 1.47) 0.90 
Close (<1 mm) 4.63 (3.03 - 7.10)   1.90 (1.29 - 2.80)   0.97 (0.55 - 1.69)   
              
T category             
T0 to T2 (reference) 1.0   1.0   1.0   
T3 to T4 26.2 (16.4 - 41.8) <0.0001 3.24 (2.19 - 4.78) <0.0001 0.86 (0.57 - 1.28) 0.45 

Finally, there was no association between PMRT use and hormone receptor status, HER2 status, Charlson comorbidity index, or reconstructive surgery at the time of mastectomy.

Conclusions: PMRT use across Michigan was lower than the American College of Surgeons Commission on Cancer target of 90% in patients with 4 or more positive LNs. Contrary to common belief, immediate reconstructive surgery was not independently associated with decreased PMRT uptake. Understanding reasons for nonuse of PMRT may lead to interventions to increase its use by MiBOQI member institutions in patients for whom it is indicated.

Citation Format: Gorski DH, Braun T, Munir K, Griggs JJ, Breslin TM, Henry NL. Factors affecting the administration of post-mastectomy radiation therapy (PMRT) in Michigan [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-10-08.