Rhabdomyosarcoma or Fibrosarcoma? A Case of Large Ulcerated and Necrotic Lesion on the Forearm of a Newborn

Abstract

Congenital infantile fibrosarcoma (CIFS) is a rare malignant soft tissue tumor. The incidence of fibrosarcoma is estimated to be 0.3 per 100 000 population per year, and it accounts for less than 1% of all soft tissue sarcomas. We present a case of a 7-day-old newborn with a large ulcerated and necrotic lesion on the left forearm, which was initially misdiagnosed as rhabdomyosarcoma. Magnetic resonance imaging (MRI) revealed a soft tissue mass with cystic components affecting the forearm and distal humerus muscles. Fine-needle biopsy was performed and initially diagnosed as rhabdomyosarcoma but later confirmed as low-grade fibrosarcoma with positive immunostaining for vimentin. The patient underwent a transhumeral amputation with follow-up chemotherapy at a specialized oncology center. This case underscores the importance of interdisciplinary collaboration and specialized care in managing complex medical conditions in infants. Early detection and appropriate management of these tumors are essential for improving outcomes and reducing morbidity and mortality. Despite the rarity of this case, it serves as a reminder of the importance of considering neoplastic lesions in the differential diagnosis of soft tissue masses in newborns.

Introduction

Congenital infantile fibrosarcoma (CIFS) is a rare malignant soft tissue tumor that affects infants and newborns before the age of 2, with special properties for this age group, including a low risk of metastases and a good prognosis compared with adult fibrosarcoma, which has a high risk of metastasis. ¹ The tumor tends to be large and aggressively infiltrates nearby tissues. Its incidence is about 5 cases per million, and its prevalence is low. ² This tumor belongs to the soft tissue sarcoma group, which is a group of tumors that arise from mesenchymal cells and can affect any tissue, such as muscular or lipid tissue, as well as fibrous tissue and other tissues. Its diagnosis can be difficult and should be confirmed with pathological examinations. Fibrosarcoma accounts for one third of fetal soft tissue tumors. It is often misdiagnosed with other diseases, such as hemangioma, vascular malformation, and in our case, rhabdomyosarcoma. A biopsy should be taken to confirm the diagnosis. ³ We present a case of a 7-day-old newborn with a large ulcerated and necrotic lesion on the left forearm, which was initially misdiagnosed as rhabdomyosarcoma.

Case Report

Patient Information

A newborn infant presented with a swelling in the left forearm. After 12 hours of delivery, the swelling increased in size due to bleeding inside it, and it rapidly grew. Vitamin K was administered, and the infant’s vital signs returned to normal.

Clinical Findings

During physical examination, the lesion was found to be insert size 6 × 7 cm with definite borders located on the lateral side of the left forearm. It was ulcerated and necrotic, and blood vessels were visible through it (Figure 1). The pulse was palpable in both the radial and ulnar arteries.

Figure 1.

(A) The appearance of the lesion. (B) The appearance after the surgery.

Diagnostic assessment

Magnetic resonance imaging revealed a soft tissue mass (with cystic components) affecting the forearm and distal humerus muscles. The cystic components of the lesion were high signal on T1. In addition, there were findings suggesting an invasion of the vascular and neurological bundles of the ulna and radius (Figure 2). Enlargements were also observed in the axial lymph nodes, with the largest measures (6 × 10 mm).

Figure 2.

MRI revealed a soft tissue mass of the lateral aspect of the left forearm with predominantly high signal on T1.

Fine-needle biopsy was performed, and its histological examination revealed a primitive mesenchymal neoplasm most consistent with rhabdomyosarcoma (Figure 3). A trans humeral amputation (between the distal and middle thirds) of the humerus was performed, with all nerves cut as high as possible, without any serious complications (Figure 1).

Figure 3.

Histological examination of fine-needle biopsy demonstrating a primitive mesenchymal neoplasm.

Mass was sent for pathological studying, which revealed a spindle-cell neoplasm consistent with low-grade fibrosarcoma. Immunohistochemistry confirmed the diagnosis with vimentin (which was positive), S-100, desmin, and myogenin (which were negative) (Figure 4). The patient had an uneventful postoperative recovery and was followed up for chemotherapy at a specialized oncology center.

Figure 4.

Histological examination: A. spindle-cell neoplasm consistent with low-grade fibrosarcoma, B. positive staining for vimentin, C. negative staining for desmin, D. negative staining for myogin, and E. negative staining for S-100.

Discussion

Fibrosarcoma is a rare malignant tumor of fibroblasts that usually affects the deep soft tissues of the extremities, trunk, head, and neck.^4,5 Fibrosarcoma of the anterior abdominal wall is extremely rare and only a few cases have been reported in the literature.^4,5

In this case, we presented a 7-day-old newborn with a large fibrosarcoma of the left forearm that was present since birth and caused bleeding and bone destruction. This type of tumor is classified as infantile or congenital fibrosarcoma, which is the most common soft tissue sarcoma in children under 1 year of age.^6,7 Infantile fibrosarcoma differs from adult fibrosarcoma in its clinical presentation, histopathology, molecular features, and prognosis, as it usually presents as a rapidly growing mass at birth or shortly after, most commonly affecting the extremities.^6,7 The tumor may cause pain, ulceration, nerve compression, vascular compromise, and bone erosion.^5-7

The diagnosis of infantile fibrosarcoma usually depends on molecular testing, which can detect the specific gene fusion or rearrangement that is characteristic of this tumor, such as ETV6-NTRK3, LMNA-NTRK1, or TPR-NTRK1,^1,8 However, in the absence of molecular testing (which was not available in Syria due to the economic and military crisis), the diagnosis relies on the histology and immunohistochemistry of the biopsy specimen, which shows spindle-cell neoplasm with variable collagen production and mitotic activity.^6,8 Immunohistochemistry may help to differentiate infantile fibrosarcoma from other spindle-cell tumors by showing positive staining for vimentin and negative staining for S-100, desmin, smooth muscle actin and CD34.^6,8 In this case, the imaging studies and biopsy results were essential for the accurate diagnosis of fibrosarcoma, which showed spindle-cell neoplasm with variable collagen production and mitotic activity. The tumor cells were positive for vimentin and negative for S-100, desmin, smooth muscle actin, and CD34. The differential diagnosis of rhabdomyosarcoma and infantile fibrosarcoma is challenging, especially on fine-needle aspiration (FNA) biopsy, because both tumors are composed of small round blue cells with scant cytoplasm and high mitotic activity.

The treatment of infantile fibrosarcoma depends on the size, location, and stage of the tumor, as well as the age and general condition of the patient.^6,8 The mainstay of treatment is surgical resection with wide margins to achieve local control and prevent recurrence.^6,8 However, complete resection may not be possible in some cases due to the large size or involvement of vital structures. ⁶ In such cases, chemotherapy and/or radiotherapy may be used as neoadjuvant or adjuvant modalities to reduce the tumor size, facilitate surgery, or prevent metastasis.^6,8 Chemotherapy regimens usually include vincristine, actinomycin D, and cyclophosphamide (VAC) or vincristine, dactinomycin, and ifosfamide (VAI).^5,6 While radiotherapy may be indicated for unrespectable tumors, positive surgical margins, or metastatic disease.^6,8 In this case, the patient underwent a transhumeral amputation due to the extent of the tumor and the involvement of the vascular and neurological bundles of the ulna and radius.

The prognosis of infantile fibrosarcoma is generally better than that of adult fibrosarcoma, with 5-year survival rates ranging from 70% to 90%.^6,7 The main prognostic factors are tumor size, surgical margin status, and presence of metastasis at diagnosis.^6-8 In this case, the patient had a large tumor with bone destruction and bleeding that required amputation of the left upper limb. The histopathology confirmed the diagnosis of low-grade fibrosarcoma with positive immunostaining for vimentin.

In conclusion, we reported a rare case of infantile fibrosarcoma of the left forearm in a newborn, which presented as a large, bleeding mass with bone destruction. The diagnosis was based on histology and immunohistochemistry, as molecular testing was not available in our setting. The patient underwent a transhumeral amputation and was referred for chemotherapy. This case highlights the challenges and limitations of diagnosing and treating this rare tumor in a resource-poor setting, where molecular testing and limb-sparing surgery are not feasible. We recommend that further studies should be done to evaluate the efficacy and safety of different treatment modalities for infantile fibrosarcoma, especially in low-income and middle-income countries.