Excerpt
The guidelines of sentinel lymph node biopsy (SLNB) in the management of thin melanoma (TM) are constantly debated [
1‐
4]. For this reason, we have read with great interest the paper of Joyce and colleagues, in which they have conducted a retrospective study on 318 patients affected by cutaneous malignant melanoma (MM), who underwent SLNB at their Institution [
1]. TM was diagnosed in 65 patients (35 with T1a and 30 with T1b); the SLNB was positive in one case (3.5%) belonging to the T1b subgroup [
1]. The patient was a 55-year-old female affected by 1-mm-thick superficial spreading melanoma of the back; lympho-vascular invasion was present, while ulceration, regression or perineural infiltration were absent [
1]. The overall false-negative rate was 20% with only one case of false-negative SLNB in the cohort of TM. The authors point to a possible limited role for SLNB in TM due to its low positivity rate, associated false-negative rate and related morbidity [
1]. Briefly, TM recapitulates the tumour progression of MM. Primary MM in fact evolves through two, scientifically accepted, phases: the radial growth phase (RGP) and the vertical growth phase (VGP) [
5,
6]. The term RGP refers to a clinical metaphor of MM growth as pigmented flat or plaque-like lesion, which expands along the radii of an imperfect circle. RGP may be confined into the epidermis (in situ) or it can migrate in the papillary dermis. Here, the cells may undergo apoptosis or may survive without to proliferate. In the latter case, the tumour cells persist in the dermis, without forming tumorigenic nodule. Hence, micro-invasive RGP in the absence of mitoses and regression is biologically indolent and lack of metastatic capacity [
7,
8]. However, if left untreated, RGP can move towards VGP over the time. In 1989, Clark and colleagues described an early (≤1 mm) VGP and distinguished it from RGP by exploiting a clear-cut criterion, based on the presence of a dominant nest within the papillary dermis [
5]. This expansive tumorigenic nest of about 50 cells have to be larger than any nest within the epidermis or surrounding dermis; mitoses are clearly identifiable inside it [
5]. The presence of an early VGP imputes to TM a risk for metastasis at roughly 8 years of follow-up [
5,
9]. Regression is the only attribute of RGP significantly correlated with survival [
10]. The presence of regression, especially if extensive (>75% of tumour volume), may in fact signal an underlying metastatic behaviour [
10‐
12]. We believe that micro-invasive RGP with regression >75% of tumour volume should be considered at uncertain tumorigenic potential and SLNB is prudentially indicated [
13‐
15]. On the basis of our experience, TM can be so subdivided in four subcategories: intra-epidermal RGP; non-tumorigenic micro-invasive RGP devoid of significant regression; micro-invasive RGP of uncertain tumorigenic potential at diagnosis, due to the presence of extensive regression (>75%) which could contain an early VGP clone; tumorigenic micro-invasive VGP [
16,
17]. Following a bio-histological approach, SLNB must be therefore performed in TM with early VGP and in TM with RGP of uncertain tumorigenic potential. In this way, a wise choice of those patients affected by TM to be submitted for SLNB can be achieved [
18]. All the retrospective studies concerning with TM should take into account the above-mentioned proliferative phases. …
