Breast Fibromatosis in a Patient With a History of Treated Breast Cancer: A Case Report.

1
Introduction
Desmoid‐type fibromatosis (DTF) of the breast is rare, accounting for 0.2% of breast tumors, and its rising prevalence has prompted new management considerations [1]. Few cases appear in the literature. Breast fibromatosis should be included in the differential diagnosis of local recurrence after lumpectomy when clinical and imaging findings diverge [2]. Mammary DTF etiology remains debated. Associations include Gardner syndrome, silicone breast implants, and surgical trauma [3, 4]. Because DTF often mimics carcinoma clinically, awareness of this entity is vital for clinicians managing breast cancer. Diagnostic imaging frequently yields ambiguous results, and the differential diagnosis encompasses both benign and malignant lesions. Patients with DTF may experience chronic pain, edema, and functional impairment [5]. A case is reported of a patient with prior breast‐conserving surgery, chemotherapy, and radiotherapy who developed a right breast mass while receiving tamoxifen therapy with an unusual mammography pattern.

2
Case Presentation
In 2022, a 41‐year‐old woman with a history of two full‐term pregnancies and 18 months of lactation for each‐her last childbirth having occurred 10 years earlier‐was diagnosed with right‐sided invasive ductal carcinoma (IDC). The tumor measured 3.5 cm at its greatest dimension and was classified as grade 2. Immunohistochemical analysis demonstrated positivity for estrogen receptor (ER) and progesterone receptor (PR), negativity for HER2, and a Ki‐67 proliferation index of 25%. Histopathologic evaluation revealed a cribriform growth pattern, without ductal carcinoma in situ (DCIS) or lymphovascular invasion (LVI), and identified one metastatic lymph node among the nine sampled lymph nodes. Breast‐conserving surgery (BCS) and axillary lymph node dissection (ALND) were performed via a periareolar incision at the 3 o'clock position. Adjuvant chemotherapy included four cycles of doxorubicin (120 mg) and cyclophosphamide (1200 mg), followed by four cycles of docetaxel (140 mg). Radiotherapy delivered 5000 cGy in 25 fractions to the whole breast, with an additional 2000 cGy boost to the tumor bed in five fractions. Tamoxifen (20 mg daily) was initiated. In 2023, a total abdominal hysterectomy and bilateral salpingo‐oophorectomy were performed for ovarian suppression.
During routine follow‐up in 2024, mammography and ultrasonography revealed a 1.0 cm × 0.7 cm × 0.8 cm mass with peripheral vascularity at the 2o'clock position in the right upper inner quadrant (UIQ). The lesion was categorized as BI‐RADS 3 due to features suggestive of fat necrosis and benign appearance on both ultrasound and mammography, prompting a conservative follow‐up plan. Six months later, an ultrasound demonstrated an irregular, hypoechoic, vascularized mass measuring 1.7 cm × 2.5 cm × 2.0 cm in the same quadrant. Core‐needle biopsy (CNB) confirmed the presence of fibromatosis. Given the presence of radiotherapy‐induced lymphedema, conservative management was initially pursued.
Three months later, repeat imaging, including mammography and ultrasonography, was performed. Mammography (Figure 1) showed a well circumscribed, round mass, while sonography indicated interval growth to 3.2 cm × 3.2 cm × 1.7 cm, increased vascularity, and a BI‐RADS 4A classification. Physical examination revealed a firm, moderately mobile nodule with overlying skin dimpling (Figure 2). A second CNB again confirmed fibromatosis. Immunohistochemical staining showed positivity for CK5/6, AE1/AE3, cyclin D1, CD34, SMA, desmin, p63, BCL2, β‐catenin, and vimentin, with rare Ki‐67 expression.
Rapid lesion progression and worsening pain necessitated wide local excision. Postradiotherapy lymphedema and the previous periareolar incision limited surgical options; therefore, an elliptical incision with a 1 cm margin was made directly over the lesion (Figures 3 and 4). Intraoperative frozen section revealed a 3 cm nodule without malignant features. A surgical drain was not placed to minimize complications within the irradiated field. Final histopathologic analysis (Figure 5) confirmed fibromatosis with negative surgical margins. At 1‐month follow‐up, the surgical site showed satisfactory healing despite radiation‐associated tissue changes. All procedures adhered to institutional and national ethical standards as well as the principles of the Declaration of Helsinki (2013 revision). Written informed consent for publication was obtained from the patient and is available for review by the editor. Table 1 summarizes the clinical case.

3
Conclusion and Results
Breast fibromatosis is a rare, locally aggressive, benign tumor that clinically and radiologically mimics carcinoma on physical examination, mammography, and ultrasonography. Histopathological analysis remains essential for definitive diagnosis. The lesion typically presents as a unilateral, solitary mass. Medical therapy with tamoxifen may be ineffective, and treatment response may depend on the tumor's pathological characteristics, which should be individually assessed. For resectable, symptomatic, and enlarging tumors, wide local excision remains the treatment of choice. Although mammographic findings are frequently described as spiculated, a round mass may also be observed, as demonstrated in this case.

4
Discussion
DTF is a clonal proliferation of fibroblasts and myofibroblasts arising from muscle fasciae and aponeuroses. Breast involvement is rare, accounting for 0.2% of all breast tumors [6]. Surgical trauma has emerged as a key contributing factor. Mammography remains the primary modality for breast screening. DTF typically appears as a spiculated mass resembling carcinoma [7]. Diagnostic accuracy appears to be lower than with other modalities [8]. In this patient, mammography revealed a round mass with smooth borders—an atypical finding diverging from the more common spiculated presentation. A Mayo Clinic review of 125 suspected cases, including eight with imaging, identified spiculated masses as the predominant finding [9].
Ultrasonography offers greater diagnostic accuracy for DTF [10]. In this case, ultrasound showed a lobulated, hypoechoic, vascularized mass in the upper inner quadrant, categorized as BI‐RADS 4. Magnetic resonance imaging (MRI) can delineate the extent of tumors in large lesions, often revealing a poorly defined, spiculated mass with iso‐ to T2‐hyperintense signals and internal heterogeneity [11]. MRI was omitted due to confirmation via core‐needle biopsy and conclusive sonographic findings; MRI may not be necessary when other modalities provide sufficient diagnostic clarity. DTF arises from mesenchymal tissue and often occurs in individuals with prior breast trauma, surgical intervention, or pregnancy [12]. Clinically and radiologically, the tumor frequently mimics carcinoma. The differential diagnosis includes a wide range of benign and malignant entities. Patients may experience persistent pain, edema, and functional impairment [5].
Typical clinical features include skin dimpling or retraction over a firm, mobile mass. Lesions near the nipple may induce nipple retraction [13]. In this patient, severe pain and skin retraction prompted further evaluation. Antiestrogen therapies and NSAIDs have shown efficacy in stabilizing or regressing sporadic extra‐abdominal DTF [14]. However, tamoxifen therapy failed to produce clinical improvement after 6 months in this case, suggesting that assessment of estrogen receptor expression or other tumor‐specific pathological features may be essential to evaluate tamoxifen efficacy in fibromatosis. The 2018 NCCN guidelines provide limited guidance for selecting a watch‐and‐wait strategy in DTF, recommending surgical resection upon disease progression or based on symptoms, comorbidities, and tumor biology–despite high recurrence rates and challenges in achieving R0 resection [15, 16, 17]. In this case, initial conservative management was followed by wide local excision due to lesion growth and worsening symptoms at 6 months.
Chronic radiation dermatitis typically emerges ≥ 90 days after radiotherapy and is often permanent. Common features include xerosis, hyperkeratosis, dyspigmentation, telangiectasia, and anhidrosis [18, 19, 20]. Radiation‐induced injury to sweat and sebaceous glands, along with epidermal thickening, keratinization, and loss of subcutaneous collagen fibers, can impair wound healing and complicate re‐excision. In this patient, a 2‐week follow‐up excluded wound‐related complications. Aggressive breast DTF may invade the pectoralis muscle or fibroaponeurotic fascia or remain distinct from adjacent musculature. All reported cases have clinically and radiologically mimicked carcinoma [21].
Immunohistochemically, the tumor exhibited nuclear positivity for β‐catenin and diffuse positivity for vimentin. Nuclear β‐catenin expression is a hallmark of DTF and is commonly attributed to mutations in the CTNNB1 gene [14]. Vimentin expression supports the mesenchymal origin of the tumor. The absence of SMA and desmin, which may exhibit variable positivity in fibromatosis, suggests a less myofibroblastic phenotype; however, their absence does not exclude the diagnosis, as these markers are not uniformly expressed. Negative CD34 staining effectively rules out solitary fibrous tumor, which typically exhibits strong CD34 positivity. A major diagnostic challenge in mammary fibromatosis is distinguishing it from metaplastic carcinoma, as spindle cell tumors with myoepithelial features may be misdiagnosed, even when cytokeratin expression is weak or absent. In this case, both epithelial markers (AE1/AE3) and myoepithelial markers (CD14 and p63) were negative [22]. Similarly, negativity for CK5/6, AE1/AE3, and p63 excluded epithelial‐origin tumors such as metaplastic carcinoma, which may histologically mimic fibromatosis. The absence of BCL2 and cyclin D1 expression further helped differentiate this tumor from low‐grade sarcomas, which may express these markers. The low Ki‐67 index reflected the indolent and non‐metastatic nature of fibromatosis, though the risk of local aggression and recurrence remains a clinical concern. Negative expression of cytokeratins, SMA, p63, and CD34 further supported the diagnosis of fibromatosis [23, 24].

Author Contributions

Saba Ebrahimian: conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration, resources, software, supervision, validation, visualization, writing – original draft, writing – review and editing. Sakineh Soleimani Varaki: supervision, writing – review and editing. Faezeh Zabihi: supervision, writing – review and editing.

Funding
The authors have nothing to report.

Disclosure
Permission to Reproduce Material From Other Sources: All reproduced images included in the manuscript were approved by the patient, the attending physician, and the hospital.

Ethics Statement
Written informed consent was obtained from the patient prior to submission to the journal.

Consent
Written informed consent was obtained from the patient for the publication of this case report and any accompanying images. A copy of the signed consent is available for review by the editorial office.

Conflicts of Interest
The authors declare no conflicts of interest.