Case of the Month

Discussion

What are some key histologic features of atypical fibroxanthoma? What immunostains help for diagnosis of AFX?

Cellular proliferation of atypical spindle cells, epithelioid cells or multi-nucleated giant cells are seen in AFX.  Usually solar elastosis is present. Atypical mitosis, pleomorphism and hyperchromatism are seen.⁵ (Figure 2) As for immunostains: CD 10 is strongly positive, CD 68 is positive and S100, Desmin, CD34 and Pankeratin are all negative but there is no AFX specific immunstain.¹

What are the main entities in the differential diagnosis list for atypical fibroxanthoma?

Melanoma, spindle cell squamous cell carcinoma, poorly differentiated angiosarcoma, leiomyosarcoma, and, rarely, dermatofibrosarcoma protuberans, among others.

What is the prognosis of atypical fibroxanthoma and how often do they metastasize? What is the most common site for metastasis? What are some known factors contributing to increased risk of metastasis?

According to Helwig who first used the term “AFX” in 1961, this skin tumor shows high grade of nuclear pleomorphism but has a great prognosis and only exceptionally rare metastases.³ AFX is a diagnosis of exclusion. Metastases happen in less than 1% of AFX cases and less than 40 examples of metastatic AFX have been reported.  The most common location for metastasis is lymph node followed by visceral organs and parotid glands.^2,4 Since metastatic AFX is very rare, proving that a new tumor represents a genuine metastatic  AFX versus another primary tumor or metastasis may be very challenging. Moreover, AFX is a diagnosis of exclusion (similar to undifferentiated pleomorphic sarcoma). Vascular invasion, tumor necrosis and deep tissue invasion are known factors contributing to increased risk of AFX metastasis.¹ Interestingly in this case all of these contributing factors were absent. As for immunohistochemistry, there is no AFX-specific immunostain.¹ In addition to identical morphology, the neck mass was:

Positive:   CD10 (strong), Vimentin

Negative: S100, Melan A, multiple cytokeratins (AE1/AE3, 34BE12, MNF116), p63, and CD34.

The composite assessment of the histologic and immunohistochemical findings and clinical setting proved that this case is a genuine AFX metastasis and highlights a rare but important potential behavior of AFX.⁶

References

1. Lum, D J. Peritoneal metastases from an atypical fibroxanthoma. The American Journal of Surgical Pathology. 2006 August;30(8): 1041-1046
2. Satter  KE. Metastatic atypical fibroxanthoma. Dermatology Online Journal. 2012 18 (9): 3
3. Helwig EB. Atypical fibroxanthoma. In: Seminar Proceedings of 18th Annual Seminar of San Antonio Society of Pathologists, 1961. Tex State J Med. 1963; 59: 664-667.
4. Helwig EB, May D. Atypical fibroxanthoma of the skin with metastasis. Cancer. 1986 Jan 15;57(2):368-76.
5. Rapini R. Practical Dermatopathology. 2nd edition, 2012. 392-393.
6. Amini N, Fung MA. Atypical Fibroxanthoma Metastasis: a Mystery for One Thousand and One Nights. Presented at 2016 ASDP annual meeting. Chicago, IL.