This document provides guidelines for delineating organs at risk (OARs) in breast radiation therapy planning for a 64-year-old postmenopausal woman who underwent lumpectomy and sentinel lymph node biopsy for early stage breast cancer. It describes how to contour the clinical target volume (CTV) of the breast and lumpectomy site, as well as OARs including the lungs, heart, esophagus, spinal cord, and brachial plexus based on the RTOG 1106 guidelines. Key steps include using external markers and wires placed at surgery to guide breast CTV delineation, and following anatomical landmarks to consistently contour the lungs, heart, esophagus and spinal cord from the cricoid to