Purpose: To examine the self-reported practice patterns of radiation oncologists in North America and Europe regarding radiotherapy to the internal mammary lymph node chain (IMC) in breast cancer patients. Methods and Materials: A survey questionnaire was sent in 2001 to physician members of the American Society for Therapeutic Radiology and Oncology and European Society for Therapeutic Radiology and Oncology regarding their management of breast cancer. Respondents were asked whether they would treat the IMC in several clinical scenarios. Results: A total of 435 responses were obtained from European and 702 responses from North American radiation oncologists. Respondents were increasingly likely to report IMC irradiation in scenarios with greater axillary involvement. Responses varied widely among different European regions, the United States, and Canada (p