It begins as melanoma in situ, confined to the epidermis and "biologically benign." In time, some cells penetrate the epidermis into the superficial dermis, a stage known as "melanoma without the competence for metastasis," or the "
radial growth phase."
Kashani-Sabet and his coworkers have shown that
radial growth phase melanoma can bypass the vertical growth phase and progress directly to metastasis in a small but clinically meaningful percentage of cases.
Suppression subtractive hybridization profiles of
radial growth phase and metastatic melanoma cell lines reveal novel potential targets.
Major Finding: Of 1,108 patients who underwent serial screening, there were no deaths due to melanoma or any other skin cancer, no metastases, and no sentinel node biopsies, since all melanomas were detected while in their
radial growth phase, when their Breslow depth was well under 0.75 mm.
For example, during a recent 5-year period in which 10 new cases of melanoma were detected through the screening program, all were in the
radial growth phase, the greatest Breslow depth was only 0.15 mm, and seven melanomas were in men over age 50--but only one cancer was detected by a patient.
This pattern of "superficial spreading" growth was recognized the most clearly by Clark, who referred to it as the "
radial growth phase" (RGP) of melanoma development.
The
radial growth phase comprises two steps: (a) the melanoma cells are entirely contained in the epidermis and the cancer is in situ (meaning in place), and (b) the invasive
radial growth phase occurs with the melanoma cells barely invading the dermis, but they do not flourish there.
These lesions do not have a
radial growth phase, as might be expected with something like melanoma.
The greatest Breslow depth was 0.15 mm; the lesions were all in
radial growth phase; 70% were in men aged older than 50 years; and only 10% of the lesions were detected by patients.
By definition, all in situ melanomas have only
radial growth phase. When melanoma invades into dermis, it may still have only
radial growth phase (single cells or small nests without mitotic figures).
(1) This type of melanoma is characterized by a prolonged
radial growth phase, termed lentigo maligna (in situ form), which grows slowly over 5 to 20 years before becoming invasive.
These lesions may represent unrecognized melanomas in the
radial growth phase. (16)
