lymphoscintigraphy, wherein a low-activity radioactive substance is injected near the tumor. The injected substance, filtered sulfur colloid, is tagged with the radionuclide technetium-99m. The injection protocols differ by doctor but the most common is a 500 μCi dose divided among 5 tuberculin syringes with 1/2 inch, 24 gauge needles. In the UK 20 megabecquerels of nanocolloid is recommended. The sulphur colloid is slightly acidic and causes minor stinging. A gentle massage of the injection sites spreads the sulphur colloid, relieving the pain and speeding up the lymph uptake. Scintigraphic imaging is usually started within 5 minutes of injection and the node appears from 5 min to 1 hour. This is usually done several hours before the actual biopsy. About 15 minutes before the biopsy the physician injects a blue dye in the same manner. Then, during the biopsy, the physician visually inspects the lymph nodes for staining and uses a gamma probe or a Geiger counter to assess which lymph nodes have taken up the radionuclide. One or several nodes may take up the dye and radioactive tracer, and these nodes are designated the sentinel lymph nodes. The surgeon then removes these lymph nodes and sends them to a pathologist for rapid examination under a microscope to look for the presence of cancer. A frozen section procedure is commonly employed (which takes less than 20 minutes), so if neoplasia is detected in the lymph node a further lymph node dissection may be performed. With malignant melanoma, many pathologists eschew frozen sections for more accurate "permanent" specimen preparation due to the increased instances of false-negative with melanocytic staining. Clinical advantages There are various advantages to the sentinel node procedure. First and foremost, it decreases lymph node dissections where unnecessary, thereby reducing the risk of lymphedema, a common complication of this procedure. Increased attention on the node(s) identified to most likely contain metastasis is also more likely to detect micro-metastasis and result in staging and treatment changes. Its main uses are in breast cancer and malignant melanoma surgery, although it has been used in other tumor types (colon cancer) with a degree of success. Other cancers which have been investigated with this technique are penile cancer, urinary bladder cancer, prostate cancer, testicular cancer and renal cell cancer. Research advantages As a bridge to translational medicine, various aspects of cancer dissemination can be studied using sentinel node detection and ensuing sentinel node biopsy.