Doctors typically recommend a technique called sentinel lymph node biopsy (SLNB) to Merkel cell carcinoma patients, regardless of tumor size, to determine whether the disease has spread (metastasized) to the lymph nodes. Minimally invasive, with a low risk of side effects, the technique is very important in determining a patient’s prognosis, since once cancerous cells reach the lymph nodes, they can easily spread through the lymphatic vessels, pass into the bloodstream and travel throughout the body. These nodes are small swellings in the lymphatic system where the lymphocytes (white blood cells) form, and where lymph, a colorless fluid containing the white blood cells, is filtered. Lymph bathes the tissues, draining through the lymphatic vessels into the bloodstream.
What Happens During a Sentinel Node Biopsy
• To perform a sentinel node biopsy, the medical team begins by identifying (locating) the sentinel nodes, using a technique known as lymphoscintigraphy, or lymphatic mapping. They inject a radioactive tracer and/or blue dye at the tumor site, then the tracer and dye travel in lymphatic fluids draining through the lymphatic vessels from the tumor to the local lymph node basin. An instrument that detects the tracer maps the path from the tumor to the sentinel lymph nodes, the first one to three nodes in the basin. Any cancer cells spreading from the tumor would most likely follow along this same path.
• The surgeon removes the sentinel nodes. Then, in a lab, a physician examines them under a microscope, looking for cancer cells.
• If the biopsy reveals any cancer cells, the surgeon removes all the other local nodes from the basin, or, in certain cases, uses radiation therapy to destroy them.
Benefits of Sentinel Lymph Node Biopsy
It is still controversial whether sentinel lymph node biopsy, followed by excision of all the lymph nodes in the local nodal basin if the biopsy reveals cancer, actually extends survival. However, doctors still routinely recommend the technique, since cancerous cells reach the lymph nodes in about one in three MCC patients, and often aren’t detectable just on physical examination. Physicians believe that lymphatic mapping and SLNB serve some important purposes for MCC patients:
• Lymphatic mapping identifies the mostly likely region where cancer cells may have spread.
• A sentinel lymph node biopsy reveals “micrometastases,” metastatic cells that are not visible to the naked eye and cannot be detected through a standard physical examination.
• The results of SLNB help the doctor determine the prognosis: If the sentinel nodes are negative, patients have about an 80 percent five-year survival rate. If they are positive, patients have about a 50 percent five-year survival rate.
• The results also help doctors assess what stage of advance the cancer has reached, and whether further treatment is needed. They help guide the choice of treatment.
• Removing the cancerous nodes and/or instituting a course of radiation therapy helps prevent local recurrence.
Stages of Merkel Cell Carcinoma
Once the original tumor biopsy, the sentinel node biopsy and any other tests performed (including imaging tests and blood tests) are completed, the oncologist will determine the stage of the disease. The stage represents how advanced and widespread the cancer is. It takes into consideration factors such as:
• tumor thickness, ulceration and growth rate (mitotic rate).
• whether the cancer has reached the nearby lymph nodes and how many nodes are involved.
• whether cancer cells have spread beyond the lymph nodes to distant locations.
• what body areas and organs are involved.
Choice of treatments depends directly on what stage the cancer has reached. There is no universal staging system for MCC, but today, the system most often used is the American Joint Committee on Cancer (AJCC) TNM system, where T stands for the primary (original) tumor (its size, growth rate and other factors), N indicates spread to the local lymph nodes (and extent of nodal involvement) and M indicates distant metastases (spread to body areas, lymph nodes and organs beyond the local lymph nodes). The AJCC system includes five stages:
- Stage 0: In situ tumors (superficial tumors that have not penetrated beyond the epidermis, the skin’s outermost layer).
- Stage I and stage II: Tumors not known to have advanced to the local lymph nodes, with stage I including relatively smaller, lower-risk tumors and stage II including larger and/or higher-risk tumors.
- Stage III: Tumors known to have reached the local lymph nodes but not beyond.
- Stage IV: Distant metastases, tumors whose cancer cells have spread to distant body areas, lymph nodes or organs beyond the local lymph nodes.
Each of these stages, furthermore, is broken down more precisely, from lowest to highest risk, depending on different characteristics of the primary tumor, nodal metastases and/or distant metastases.
Deborah S. Sarnoff, MD