Sentinel Lymph Node in Thin and Thick Melanoma

The recommendation for sentinel lymph node biopsy (SLNB) is a controversial problem in the management of cutaneous malignant melanoma (CMM). Rovere et al. performed a study with the objective to assess the epidemiologic profile of patients with CMM, who underwent SLNB in Blumenau-Santa Caterina region in Brazil [1]. According to ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up of CMM, the authors wrote that SLNB is recommended for CMM with thick- ness > 1 mm. Rovere et al. concluded that Breslow thickness, ulceration, nodular subtype, Clark‘s level IV are associated with SLNB status [1]. We believe that recommendations for SLNB should reflect the biological concept of CMM evolution. The histogenetic theory considers two steps of CMM progression – the radial growth phase (RGP) and vertical growth phase (VGP), with the exception of nodular CMM lacking RGP [2–4]. Two sub-categories of RGP are recognized – the intra-epidermal RGP (in situ melanoma) and the micro-invasive RGP. The former is characterized by proliferation of malignant, transformed melanocytes above the basement membrane (Clark’s level I), with pagetoid or lentiginous morphologic pattern. The latter shows invasion into the papillary dermis (Clark‘s level II or III), with presence of single cells or small nests, in absence of tumor nodule or papule. The dermal nests are invariably smaller than the junctional ones, while the cytological appearance of the junctional and dermal component is overlapping. The absence of dermal mitoses is an absolute criterion; in fact, micro-invasive RGP lacks metastatic potential [2–4]. However, according to our observations, metastases may be found in 1–2% of micro-invasive RGPs, when they are associated with significant regression (> 75%; > 0.75 mm in depth). Therefore, it is likely that this regression may incorporate melanoma cells able to metastasize, with metastases prior the occurrence of regression [5]. VGP designates the point at which CMM becomes biologically capable of producing metastatic events. The tumor infiltrates as nodules of malignant melanocytes, filling the superficial papillary dermis with tendency for deep invasion into reticular dermis (Clark’s le-vel IV) and subcutaneous tissue (Clark’s level V) [2–4]. For definition, VGP includ- es the properties of ‚tumorigenicity‘ and ‚mitogenicity‘ [2–4]. Because of the increasing frequency of thin me-lanomas, there is a great need to develop more refined predictors of SLNB. Today, the scientific community is focusing on the clinical significance of different histological subtypes of thin melanoma [6]. In fact, there is a subset of patients affected by thin melanoma, who develop nodal or distant metastases with worse prognosis [7]. From our experience, as reported in Tab. 1, thin melanoma can be categorized in the following four histological subtypes: I. the intra-epidermal RGP; II. the non- -tumorigenic micro-invasive RGP without regression; III. the micro-invasive RGP with regression of uncertain tumorigenic potential; IV. the tumorigenic early invasive VGP [8]. In micro-invasive RGP with regression and in all cases of VGP, including the early invasive VGP of thin (≤ 1 mm) melanoma (pT1) and the invasive VGP of thick (> 1 mm) melanoma (pT2, pT3, pT4), the SNLB should be performed, independently from other risk factors, because the tumor has the potential to metastasize.

Dr. Luca Roncati, MD, PhD

Department of Diagnostic and Clinical Medicine and of Public Health Section of Pathology

University of Modena and Reggio Emilia

Policlinico Hospital I-41124 Modena (MO)

Italy

e-mail: emailmedical@gmail.com

Submitted: 26. 9. 2016