Atypical fibroxanthomas (AFX) are rare cutaneous tumors, which typically present as a solitary ulcerated papule or nodule on sun-damaged skin. Despite malignant-appearing features on histology, AFX typically pursue a benign clinical course. In rare instances, AFX can form collision tumors with other lesions. However, to the best of our knowledge, AFX in collision with a nevus has never been previously reported. In this study, we describe such a lesion for its novelty and challenge in diagnosis, as this case was originally considered to be melanoma arising in a nevus. On histologic examination, there were 2 distinct populations of cells; one composed of markedly atypical and pleomorphic epithelioid and oval to spindled cells, consistent with AFX, and the other, a bland-appearing intradermal nevus with congenital features. The AFX population stained positive with smooth muscle actin, CD10, and CD68 and was negative for S100, SOX10, Melan-A, desmin, pancytokeratin, CK5/6, and p63. Deep to this was a second population of small, bland-appearing melanocytes in a broad, band-like distribution. This unusual collision tumor between AFX and an intradermal nevus highlights the important role immunohistochemistry plays in avoiding the misdiagnosis and potential overtreatment of benign or low-grade lesions, and in identifying potential mimickers.
*Department of Dermatology, University of Colorado, Denver, CO; and
†Deparment of Pathology, Southern California Permanente Medical Group, Los Angeles, CA
Correspondence: David Cassarino, MD, PhD, Department of Pathology, Southern California Kaiser Permanente Medical Group, Sunset Medical Center, 4867 Sunset Boulevard, 2nd floor, Los Angeles, CA 90027 (e-mail: David.S.Cassarino@kp.org).
D. Cassarino had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study concepts and design: D. Cassarino and A. Steel. Acquisition, analysis, and interpretation of data: D. Cassarino, A. Steel, and M. Debbaneh. Drafting of the manuscript: D. Cassarino, A. Steel, and M. Debbaneh. Critical revision of the manuscript for important intellectual content: D. Cassarino, A. Steel, and M. Debbaneh. Statistical analysis: none. Administrative, technical, or material support: none. Study supervision: D. Cassarino.
The authors declare no conflicts of interest.