Melanoma : Radial Growth Phase
The concepts of radial (horizontal) growth phase and vertical (invasive) growth phase are important to understand with regards to the histological classification of melanomas.
Radial growth phase includes melanoma-in-situ as well as microinvasion into the papillary dermis (microinvasive radial growth phase) which consists of single cells or small clusters histologically similar to their intraepidermal counterpart. Features of regression may be present. There are no mitoses. Microinvasive radial growth phase lacks metastatic potential and has excellent prognosis.
This image shows melanoma-in-situ. Atypical melanocytes in clusters or singly are seen at all levels of epidermis with a pagetoid spread.