Unusual Subcutaneous Metastasis of a Malignant Granular Cell Tumor Mimicking Desmoid Fibromatosis: A Case Report

Abstract

Background: Malignant granular cell tumor of the larynx is an exceptionally rare Schwann cell neoplasm with high metastatic potential, and metastatic soft-tissue lesions may mimic benign entities such as desmoid-type fibromatosis on imaging, posing a significant diagnostic challenges. Aims: To report an unusual case of subcutaneous abdominal wall metastasis from a malignant laryngeal granular cell tumor (Abrikossoff tumor) mimicking a desmoid-type fibromatosis on magnetic resonance imaging (MRI), and to highlight the contribution of diffusion-weighted imaging (DWI) and of a multidisciplinary approach in this diagnostic challenge. Presentation of Case: A 54-year-old woman with a 9 cm laryngeal granular cell tumor, treated by total laryngectomy with focally positive margins (R1 resection) and adjuvant radiotherapy, developed locoregional and pulmonary recurrence approximately 24 months postoperatively, both confirmed as the same histological type. Thirteen months later, a subcutaneous mass of the right anterolateral abdominal wall was detected. Contrast-enhanced computed tomography (CT) showed a well-circumscribed soft-tissue mass with mild post-contrast enhancement and regular contours. MRI demonstrated a lobulated lesion with intermediate T2 signal, marked diffusion restriction with a low apparent diffusion coefficient (ADC) value of 0.9 × 10⁻³ mm²/s, and intense homogeneous post-gadolinium enhancement, initially suggestive of a desmoid-type fibromatosis. Percutaneous biopsy confirmed a granular cell tumor metastasis identical to the laryngeal primary. Discussion: Subcutaneous metastases from a malignant granular cell tumor may share several morphological MRI features with abdominal wall desmoid-type fibromatosis. In this case, the lesion showed marked diffusion restriction with a low ADC value, a finding not typical of desmoid-type fibromatosis, which usually shows mild to moderate diffusion restriction with intermediate to high ADC values. The laryngeal primary had been initially labeled benign, yet its subsequent metastatic behavior — together with clinicopathological features such as large size and focally positive margins — indicated an underestimated malignant granular cell tumor. Conclusion: In a patient with a previously treated granular cell tumor, a new soft-tissue mass should raise the suspicion of metastasis. Multiparametric MRI, particularly DWI, combined with a multidisciplinary correlation of clinical, radiological and pathological data, is valuable in differentiating this entity from desmoid-type fibromatosis and in guiding histological confirmation.