Ambiguous melanocytic lesions – those with histopathologic findings without a diagnostic consensus – are divided into thin and thick tumors. Thin lesions appear as dysplastic nevi and have a potential of being part of melanoma in situ or superficial spreading melanoma. We have explored diagnostic and therapeutic considerations of the thin lesions in a previous blog – “Atypical Nevus – Chasing Excision Margins”
This article will explore thick ambiguous melanocytic lesions: melanocytic tumors of uncertain malignant potential (MelTUMP) and spitzoid tumor of uncertain malignant potential (STUMP).
The designations MelTUMP and STUMP are provisional diagnoses representing heterogeneous groups of lesions whose behavior is yet to be determined. Similar terminology has included borderline melanoma, minimal deviation melanoma, ambiguous melanocytic lesions, Spitz nevus with atypical features (SNAF), blue nevus or deep penetrating melanocytic nevus.
Histology usually demonstrates a deep penetrating nevus with mitoses, dermal melanocytes with mitoses near the base and associated inflammatory infiltrate. These lesions lack certain features of true melanomas. Histologically diﬀerentiating severely atypical spitzoid neoplasms from spitzoid melanoma is particularly difficult in prepubertal children. These features in melanoma would correlate with metastatic capacity. In fact, a study of atypical spitzoid tumors (STUMP’s) found half of 67 cases contained tumor cells in lymph nodes. Despite that, only one patient died of progressive disease. (Ludgate MW, Fullen DR, Lee J, et al. The atypical Spitz tumor of uncertain biologic potential. A series of 67 patients from a single institution. Cancer 2009; 115: 631)
Several recent studies have looked at concurrent melanocyte deposits in lymph nodes of atypical melanocytic lesions finding that these lesions rarely progress to overt malignancy. If sentinel lymph node biopsies had not occurred, the intranodal melanocytes may have remained dormant or been eliminated. Several studies have documented that the presence of a positive sentinel lymph node in a child diagnosed with melanoma or MelTUMP does not convey the same negative prognostic information as it would for an adult patient. These lesions represent a biological entity of low-grade malignancy apart from conventional melanoma and melanocytic nevi. This suggests that atypical melanocytic neoplasms in children may be less aggressive in general and may be limited in their progression by host factors such as a more potent immune defense.
The utility of sentinel lymph node biopsy in evaluation of a MelTUMP or a STUMP remains controversial. The procedure has been found to be a staging and prognostic tool only in melanoma in adults. Extrapolating its utility with ambiguous tumors may result in potentially unnecessary medical procedures such as completion lymphadenectomy and adjuvant therapy. Sentinel lymph node biopsy in these scenarios remains controversial.
Additional information about these lesions is needed to help with decisions on further therapy. Molecular genetic testing represents the next frontier that may distinguish benign STUMP’s and MelTUMP’s from melanoma and guide treatment decisions. We will explore this technology in one of our next articles.