Lymph nodes are small immune organs located throughout the body. Cancerous tumour cells that escape a tumour often travel to lymph nodes before spreading to other parts of the body. A sentinel lymph node is defined as one of the first lymph nodes a tumour cell will travel to after it leaves the primary tumour. Patients diagnosed with breast cancer or melanoma may undergo a procedure called a sentinel lymph node biopsy to look for tumour cells in these lymph nodes. In order to find the sentinel lymph nodes, a doctor will inject a dye or radioactive tracer close to the primary tumour. The doctor will then examine nearby lymph nodes for the colourful dye or use a special machine to find the radioactive tracer. Once found the lymph nodes will be removed and sent to a pathologist for examination under a microscope. For some types of cancer, pathologists perform a special test called immunohistochemistry to look for small groups of tumour cells in sentinel lymph nodes. The chance of finding tumour cells in other lymph nodes is lower and patients have a better prognosis if no tumour cells are found in any of the sentinel lymph nodes examined. Pathologists will describe a lymph node as ‘negative’ if no tumour cells are found and ‘positive’ if tumour cells are found in the lymph node.