Diagnosis of an Atypical Spitz Tumor (AST) remains one of the most significant challenges in pediatric dermatopathology. These lesions occupy a difficult middle ground between benign Spitz nevi and malignant spitzoid melanoma. However, the integration of molecular diagnostics is increasingly allowing clinicians to classify these tumors with greater precision. Specifically, identifying specific kinase fusions can help determine the biological potential of a lesion and guide clinical management.

Diagnostic Profile of the Atypical Spitz Tumor

In a recently reported case, a 14-year-old girl presented with a changing pigmented lesion on her upper back. Initial histopathologic examination showed an asymmetric proliferation of epithelioid cells. While the lesion demonstrated dermal maturation, its overall architecture suggested an atypical profile. Notably, immunohistochemistry (IHC) played a vital role in the workup. The tumor showed diffuse positivity for SOX10 and Melan-A, while p16 expression remained retained. Furthermore, negative PRAME expression provided a crucial clue, as this marker is typically positive in conventional melanoma but absent in the Spitz spectrum.

To reach a definitive diagnosis, the medical team performed next-generation sequencing (NGS). This analysis identified a RET::MXT1 gene fusion, marking the first time this specific rearrangement has been documented in an Atypical Spitz Tumor. Interestingly, the absence of additional mutations or copy number alterations supported the classification of the lesion as a Spitz-spectrum tumor rather than a high-grade malignancy.

Clinical Implications and Management

Consequently, the identification of isolated RET fusions provides valuable data for risk stratification in children. Most kinase fusion-positive Spitzoid lesions follow an indolent clinical course, even when they exhibit concerning morphology. Therefore, molecular findings often prevent the over-treatment of pediatric patients. In this case, the integrated diagnostic approach allowed for a more confident classification, which is essential for determining the necessity of wide local excision or sentinel lymph node biopsy.

Moreover, clinicians should consider molecular testing for any spitzoid lesion that presents with diagnostic ambiguity. Thus, the discovery of novel partners like MXT1 expands our understanding of the genetic landscape of these rare tumors. In addition, these findings underscore the necessity of using morphology, IHC, and molecular data as a unified diagnostic pillar in modern dermatology.

Frequently Asked Questions

What is an Atypical Spitz Tumor?

An atypical Spitz tumor is a melanocytic lesion that has some, but not all, of the characteristics of a spitzoid melanoma. Because they have uncertain biological potential, they require careful histopathologic and molecular evaluation to differentiate them from benign nevi.

Why is RET fusion testing important?

Identifying a RET fusion helps confirm that a lesion belongs to the Spitz tumor spectrum. This information is vital for clinical management, as it typically indicates a lower risk of metastasis compared to conventional melanoma, allowing for more conservative treatment plans.

Disclaimer: This content is for informational and educational purposes only. It does not constitute medical advice or a professional relationship. Refer to the latest local and national guidelines for clinical practice.

References

1. Di Lorito A et al. Atypical Spitz Tumor With RET::MXT1 Gene Rearrangement in a 14-Year-Old Girl: An Integrated Histopathologic, Immunohistochemical, and Molecular Diagnostic Approach. J Cutan Pathol. 2026 May 07. doi: 10.1111/cup.70132. PMID: 42095305.

2. Kim D, et al. Clinical, Morphologic, and Genomic Findings in Spitz Tumors With RET Fusion: A Series of 31 Cases. J Am Acad Dermatol. 2025;92(2):345-352.

3. Wiesner T, et al. Kinase fusions are frequent in Spitz tumours and spitzoid melanomas. Nat Commun. 2014;5:3116.