Section Editor: Allison Osmond MD FRCPC
June 10, 2026
Atypical fibroxanthoma (AFX) is a rare, low-grade skin tumor that usually affects older adults with sun-damaged skin. It most often develops in areas exposed to long-term ultraviolet (UV) light, such as the scalp, face, and ears. Although AFX can look alarming under the microscope, with cells that vary greatly in size and shape, it is low risk when strict diagnostic criteria are applied, and most people are cured with surgery alone.
AFX is closely related to a higher-risk tumor called pleomorphic dermal sarcoma (PDS). The two look very similar under the microscope, and the main difference is how far the tumor has grown: AFX is confined to the upper layer of the skin (the dermis) and does not invade deeper tissue, nerves, or blood vessels, while PDS does. This article explains what a diagnosis of AFX means, what the findings in your pathology report describe, and how those findings guide the decisions you and your care team make together.
Is atypical fibroxanthoma cancerous?
Although AFX can look worrisome under the microscope, when strict diagnostic criteria are used, it behaves much like a noncancerous (benign) growth. It does not usually spread to lymph nodes or other parts of the body, and most people are cured by surgery alone. Once the tumor has been completely removed, it usually does not come back. For this reason, AFX is best thought of as a low-risk tumor rather than a typical skin cancer, as long as the strict criteria that separate it from pleomorphic dermal sarcoma are met.
What causes atypical fibroxanthoma?
Long-term exposure to ultraviolet (UV) radiation is the leading cause of AFX. Over many years, UV light damages the DNA in skin cells, producing changes (mutations) in the genes that control how cells grow and repair themselves. The genes most often changed in AFX include TP53, the TERT promoter, and CDKN2A and CDKN2B. These same changes are seen in pleomorphic dermal sarcoma and other UV-related skin cancers, which is part of why these tumors are considered related.
Who gets atypical fibroxanthoma?
AFX mainly affects older adults with lighter skin, especially men, and people who live in regions with high sun exposure. It is uncommon in younger people. When it does occur in younger patients, it can be associated with inherited conditions that increase sensitivity to UV damage, including Li-Fraumeni syndrome (which involves changes in the TP53 gene) and xeroderma pigmentosum (a condition that makes the skin highly sensitive to UV light). AFX has also been reported in people with a history of radiation therapy, organ transplantation, or a weakened immune system.
What are the symptoms of atypical fibroxanthoma?
AFX usually appears as a single, quickly growing nodule on sun-damaged skin. The nodule may be pink, red, or flesh-colored, and it often develops a sore (ulcer) on its surface. It is generally painless and is usually no larger than about 2 cm across. Because it can grow quickly and ulcerate, it is sometimes mistaken for a more common skin cancer such as squamous cell carcinoma.
How is the diagnosis made?
AFX is diagnosed after a pathologist examines the tumor under the microscope. A small sample may first be taken by a biopsy, but a confident diagnosis usually requires removing the entire tumor, because the diagnosis depends partly on confirming that the tumor has not grown into deeper tissue.
Under the microscope, AFX is a cellular tumor confined to the dermis that does not invade the underlying fat, nerves, or blood vessels. The cells are pleomorphic, meaning they vary widely in size and shape, and often have large, irregular nuclei (the part of the cell that contains DNA). The cells may be spindle-shaped or rounded (epithelioid) and often divide rapidly, so frequent, sometimes abnormal, mitotic figures (dividing cells) are seen. Importantly, there is no tumor necrosis (areas of dead tumor) and no growth into deeper tissue, both of which would point to a different diagnosis. Several variants exist, including spindle cell, clear cell, granular cell, keloidal, myxoid, and sclerotic forms; these can make the diagnosis more difficult, especially on a small or partial biopsy.
Because AFX resembles several other skin cancers, the pathologist uses immunohistochemistry (special stains that detect specific proteins in cells) to rule them out. There is no single stain that proves a tumor is AFX, so the diagnosis is made by exclusion: the tumor is typically negative for melanocytic markers (such as S100, SOX10, and Melan-A), which helps rule out melanoma; negative for epithelial markers (such as cytokeratins, p63, and p40), which helps rule out squamous cell carcinoma; and negative for muscle markers (such as desmin and h-caldesmon). AFX usually shows strong staining for CD10, but because CD10 appears in many tumors this finding supports the diagnosis without proving it. A broad panel of stains is used, and if a marker that should be negative is strongly positive, the pathologist will reconsider the diagnosis.
Histologic grade
AFX is considered a low-grade tumor and is not given a numeric grade. The grading systems used for many soft tissue tumors are not applied here; instead, AFX sits at the low-risk end of a spectrum that includes pleomorphic dermal sarcoma at the higher-risk end. Your report will therefore not include a numeric grade, as is expected for this diagnosis.
Surgical margins
A margin is the rim of normal-looking tissue removed around the tumor during surgery. Because AFX is treated surgically, the margin status is one of the most important parts of the report: it tells you and your doctor whether the entire tumor was removed.
- Negative margin — No tumor cells are seen at the cut edge, which suggests the tumor was completely removed. AFX rarely returns after complete removal. The pathologist may also record the distance from the nearest tumor cells to the edge.
- Close margin — Tumor cells are near the cut edge but do not reach it. Depending on the distance, your doctor may discuss further surgery.
- Positive margin — Tumor cells are present at the cut edge, which means some tumor may remain. A positive margin is the main reason AFX comes back at the same site, and further surgery is usually considered.
What is the difference between atypical fibroxanthoma and pleomorphic dermal sarcoma?
AFX and pleomorphic dermal sarcoma (PDS) look very similar under the microscope but behave differently, and the distinction is based on how far the tumor has grown. AFX is limited to the dermis and does not invade fat, muscle, nerves, or blood vessels. PDS shows one or more higher-risk features: growth into deeper tissue, tumor necrosis, or invasion of nerves (perineural invasion) or blood and lymphatic vessels (lymphovascular invasion). If any of these features is present, the tumor is diagnosed as PDS rather than AFX. This matters because PDS carries a higher risk of coming back after treatment and, uncommonly, of spreading, so it is followed more closely. This is also why a confident diagnosis of AFX usually requires examining the entire removed tumor rather than a small biopsy.
What is the prognosis?
AFX has an excellent outlook when it is diagnosed correctly and completely removed. In most cases it does not come back. Local recurrence occurs in about 5% of cases, almost always when the tumor was not completely removed the first time. Spread to other parts of the body is very rare, and deaths from AFX are extremely uncommon. There is no formal staging system for AFX; what matters most for the outlook is that the tumor is confined to the dermis and has been completely removed. If a tumor shows features suggesting a higher risk of recurrence or spread, such as deep invasion, tumor necrosis, or invasion of nerves or vessels, it is reclassified as pleomorphic dermal sarcoma and is followed more closely.
What happens after this diagnosis?
The main treatment for AFX is complete surgical removal of the tumor. This is usually done either by wide local excision, which removes the tumor with a rim of normal skin, or by Mohs micrographic surgery, a technique in which the tumor is removed in thin layers that are checked under the microscope during the operation. Mohs surgery is especially useful on the face, scalp, and ears, where it helps remove the tumor completely while sparing as much healthy tissue as possible. Because AFX is a low-grade tumor, no additional treatment is usually needed once it has been completely removed.
After treatment, regular skin examinations are recommended, both to make sure the tumor has not returned and because people who have had AFX have sun-damaged skin and are at higher risk of developing other skin cancers. Protecting the skin from further sun exposure is an important part of long-term care. Care is usually coordinated by a dermatologist, often together with a surgeon experienced in skin cancer (such as a Mohs surgeon) and the pathologist who made the diagnosis.
Questions to ask your doctor
- Was the tumor completely removed, and were the margins clear?
- Was this definitely atypical fibroxanthoma, or were there any features of pleomorphic dermal sarcoma?
- Was the tumor confined to the dermis, or did it invade deeper tissue, nerves, or blood vessels?
- If the margins were positive, do I need more surgery?
- Would Mohs micrographic surgery be a good option, especially on my face or scalp?
- How likely is it that this tumor will come back?
- Do I need any additional treatment or imaging?
- How often should I have follow-up skin checks?
- Am I at higher risk of developing other skin cancers?
- What can I do to protect my skin from further sun damage?
- Given my age and history, should I be assessed for any inherited condition?
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