What is the management of benign fibroxanthoma (benign tumor) to the knee?

Management of Benign Fibroxanthoma of the Knee

Complete surgical excision with clear margins is the definitive treatment for benign fibroxanthoma (atypical fibroxanthoma) of the knee, as this tumor behaves benignly despite its histologically alarming appearance. 1, 2

Understanding the Diagnosis

Benign fibroxanthoma, more commonly termed atypical fibroxanthoma (AFX), presents a diagnostic challenge because it appears histologically malignant but behaves clinically benign. 2, 3 The knee location is atypical, as AFX most commonly occurs on sun-exposed skin of the head and neck in elderly patients, or on the trunk and extremities in younger patients. 2, 3

Primary Treatment Approach

Surgical Excision

• Complete local excision is the treatment of choice and is considered adequate therapy. 2
• Mohs micrographic surgery has demonstrated excellent outcomes with minimal recurrence rates for AFX, with one series showing tumor-free status in all patients at 3-year follow-up (only one recurrence among 5 AFX cases). 1
• Standard wide excision with negative margins is appropriate when Mohs surgery is not available or practical. 1

Why Surgery is Sufficient

• Despite the histologically bizarre and malignant-appearing cells, the clinical behavior is benign in the vast majority of cases. 2, 3
• A large retrospective study of 57 patients showed benign clinical course in all but one patient over a median 9-year follow-up period. 4
• The tumor does not typically require the aggressive surgical margins needed for true sarcomas. 2

Critical Pitfalls to Avoid

Overtreatment Risk

• Do not perform unnecessarily radical surgery based solely on the alarming histological appearance. 3
• Many AFX cases (32 of 57 in one series) were initially misdiagnosed as soft tissue sarcomas (fibrosarcoma, dermatofibrosarcoma, etc.), leading to potential overtreatment. 4
• Correct diagnosis prevents unnecessary morbidity from overly aggressive resection. 3

Ensuring Accurate Diagnosis

• Confirm the diagnosis histologically before proceeding with treatment, as the distinction between AFX and true malignant tumors is critical. 3, 4
• Consider the patient's age and tumor location: AFX on the knee in a younger patient fits the less common presentation pattern. 2, 3

Surveillance After Treatment

• Monitor for local recurrence, though this is uncommon with complete excision. 1, 4
• One study noted that recurrence itself may have prognostic importance, as the single patient who developed lymph node metastases had a prior recurrence. 4
• Long-term follow-up is reasonable given rare reports of metastasizing behavior, though true AFX is considered benign. 5, 4

When to Suspect Malignant Behavior

• If local recurrence occurs after initial excision, consider more aggressive histological variants or misdiagnosis. 4
• Metastases are exceedingly rare but have been reported in isolated cases, typically to regional lymph nodes. 5, 4
• The histology of metastasizing cases does not reliably differ from benign AFX in terms of cellularity, atypia, or mitotic activity. 4

Practical Algorithm

1. Confirm diagnosis with adequate biopsy showing typical AFX features on sun-damaged, traumatized, or irradiated skin. 3
2. Perform complete surgical excision with clear margins using standard excision or Mohs micrographic surgery. 1, 2
3. Avoid radical resection that would cause unnecessary functional impairment at the knee. 3
4. Follow clinically for local recurrence, with heightened vigilance if recurrence develops. 4