Pathophysiology
The earliest stage of melanoma starts when the melanocytes begin to grow out of control. Melanocytes are found between the outer layer of the skin (the epidermis) and the next layer (the dermis). This early stage of the disease is called the radial growth phase, and the tumor is less than 1mm thick. Because the cancer cells have not yet reached the blood vessels lower down in the skin, it is very unlikely that this early-stage cancer will spread to other parts of the body. If the melanoma is detected at this stage, then it can usually be completely removed with surgery.
When the tumour cells start to move in a different direction — vertically up into the epidermis and into the papillary dermis — the behaviour of the cells changes dramatically.
The next step in the evolution is the invasive radial growth phase, which is a confusing term; however, it explains the next step in the process of the radial growth, when individual cells start to acquire invasive potential. This step is important – from this point on the melanoma is capable of spreading. The Breslow's depth of the lesion is usually less than1 mm (0.04 in), the Clark level is usually 2.
The following step in the process is the invasive melanoma — the vertical growth phase (VGP). The tumour attains invasive potential, meaning it can grow into the surrounding tissue and can spread around the body through blood or lymph vessels. The tumour thickness is usually more than 1 mm (0.04 in), and the tumour involves the deeper parts of the dermis.
The host elicits an immunological reaction against the tumour (during the VGP), which is judged by the presence and activity of the tumour infiltrating lymphocytes (TILs). These cells sometimes completely destroy the primary tumour; this is called regression, which is the latest stage of the melanoma development. In certain cases, the primary tumour is completely destroyed and only the metastatic tumour is discovered.
Read more about this topic: Melanoma