Nuclear medicine aspects of melanoma and breast lymphatic mapping

A successful sentinel lymph node biopsy (SLNB) procedure in melanoma or breast cancer patients requires an accurate map of the pattern of lymphatic drainage from the primary site. Lymphoscintigraphy (LS) can provide such a map in each patient. This requires an understanding of lymphatic physiology, an appropriate small particle radiocolloid, high resolution collimators, and imaging protocols that detect all sentinel nodes (SNs) in every patient regardless of their location. The SN is not always found in the nearest node field and is best defined as "any lymph node receiving direct lymphatic drainage from a primary tumor site." Patterns of lymphatic drainage from the skin are not clinically predictable and unexpected drainage has been found from the skin of the back to SNs in the triangular intermuscular space or through the posterior body wall to SNs in the paraaortic, paravertebral, and retroperitoneal areas. Drainage from the head and neck is to unexpected nodes in 30% of patients. Upper limb drainage can be to SNs above the axilla. Interval nodes are not uncommon as SNs, especially on the trunk. Lymphatic drainage may involve SNs in multiple node fields and drainage across the midline of the body is quite common. In the breast, although 94% of patients have a SN in the ipsilateral axilla, 46% also have SNs outside the axilla, especially in the internal mammary chain (40%). Failure to biopsy all SNs in each patient thus has the potential to understage a significant number of patients with breast cancer. Micrometastatic disease can be present in any SN regardless of its location, and for the SLNB technique to be accurate a biopsy most be performed on all true SNs in every patient. LS is an important first step to ensure this goal is achieved.