Peer influence on physician use of shorter course external beam radiation therapy for patients with breast cancer. 
Social contagion among physicians may affect the dissemination of innovative and high value cancer care.  We applied social contagion theory to investigate the role of physician peer influence on the use of short courses of external beam radiation therapy (EBRT) for patients with breast cancer.  Using a cohort of Medicare beneficiaries with breast cancer, we constructed physician peer groups based on patient-sharing relationships.  Outcomes were a patient's receipt of 1) moderately hypofractionated adjuvant EBRT after breast conserving surgery and 2) short course palliative EBRT for bone metastases.  Using a longitudinal design, we used mixed effects logistic regression to examine the association between physician peer group rate of short course EBRT in 2011-2012 (T1) and patients' receipt of short course EBRT in 2013-2014 (T2).  During T2, a total of 17,248 patients received adjuvant therapy (32.3% moderately hypofractionated) from 3,235 physicians in 1,202 physician peer groups.  Compared to patients treated within peer groups in which no moderately hypofractionated adjuvant EBRT was used in T1, patients treated by a physician in a peer group with higher T1 use of moderately hypofractionated adjuvant EBRT were more likely to receive moderately hypofractionated adjuvant EBRT in T2 (adjusted odds ratio 2.03 [95% CI 1.62-2.54] and 2.61 [95% CI 2.04-3.35] for peer groups where 21-46% and 47-100% of radiation oncologists used moderately hypofractionated adjuvant EBRT in T1, respectively, compared to peer groups with no use of moderately hypofractionated adjuvant EBRT).  In contrast, there was no significant relationship between T1 peer group use and T2 receipt of short course palliative EBRT for bone metastases.  Physician peer groups significantly influenced use of short course EBRT in adjuvant therapy but not in palliative therapy for patients with breast cancer.