sentinel node imaging
What is a lymphoscintigraphy scan?
This Nuclear Medicine scan is the best way to image the lymphatic system of the body. The lymphatics are a complex network of small vessels, similar in many ways to blood vessels, only instead of being transporting conduits for blood they transport a clear fluid called lymph. Lymph nodes are found at various points along the lymphatic channels. Lymphoscintigraphy demonstrates the lymphatics in various parts of the body, and not only shows the anatomical location and network pattern but also demonstrates the function of the lymphatics.
What is a sentinel lymph node?
Patients with certain types of cancer, most commonly breast cancer and melanoma, may undergo surgery to remove the cancer and also nearby lymph nodes. The initial spread of these types of cancer is often via the lymphatic system. Sentinel node imaging with lymphoscintigraphy shows the direction of lymph flow from the site of the cancer, and also the closest lymph nodes to the cancer. These lymph nodes are called the sentinel lymph nodes. When the sentinel node is located, the surgeon can remove them with a small operation to check whether there has been spread or not. Before lymphoscintigraphy was available, a larger operation was required to remove all the lymph nodes near the cancer. This could result in many side-effects, particularly limb swelling, known as lymphoedema. The likelihood of this occurring is dramatically reduced if the surgeon can biopsy the sentinel lymph node and avoid lymph node clearance.
How is the scan performed?
For patients with breast cancer a small injection is given into the skin around the nipple, or areola. For patients with melanoma an injection is given superficially into the skin near the melanoma or around the site of the scar if the melanoma has already been removed. This injection contains the radiopharmaceutical Tc-99m colloid. These tiny particles travel freely in the lymphatics. The patient lies down comfortably with the gamma camera over the area in question. Images are taken showing the flow of particles within the lymphatic system and lymph nodes. This may take between one and four hours.
The injection and scan can be done on the morning of the surgery or the day before.
How is the scan interpreted?
The lymphatics will flow toward the sentinel lymph nodes where the radiotracer will accumulate. The nuclear medicine technologist can mark the skin overlying the sentinel lymph node with an indelible marker pen, and estimate the depth of the lymph nodes for the surgeon. Often another imaging technique called SPECT (single photon emission computed tomography) will also be performed. This can be combined with a low-dose computed tomography (CT) scan to further help localise the lymph nodes.