Asymptomatic Mesenchymal Hamartoma of the Chest Wall in Child With Fluorodeoxyglucose Uptake on PET/CT—Report of a Case

Pathological Findings

The resection specimen was 25 × 17 × 20 mm in size. It was encapsulated by pleura with smooth surface (Fig. 2B). On bisection, the mass was solid and white uniformly with areas of hemorrhage. The mass was on the ribs, but ribs were intact without destruction and invasion.

Histologically, the mass consist of mature hyaline cartilage and loose fibrous tissue. There are immature fibroblast-like cells (Fig. 2C). In the ribs, there was unnatural mature hyaline cartilage. It was considered related to the mass (Fig. 2D).

Discussion

Mesenchymal hamartoma of the chest wall (MHCW) in infants is a rare abnormality that is usually a benign condition. Other names have included benign mesenchymoma, infantile osteochondroma, osteochondrosarcoma, and infantile cartilaginous hamartoma.^2–6 The currently accepted name of mesenchymal or chest wall hamartoma, as initially proposed by McLeod and Dahlin in 1979,¹ is now considered the most appropriate, as it best reflects the benign nature and multiple histologic components of this lesion.¹ Its incidence is estimated to be 1 in 3000 among primary bone tumors, and <1 in million in the general population.^16,17 Mesenchymal hamartomas are not considered true neoplasms and are composed of maturing, proliferating normal skeletal elements, with no propensity for invasion or metastasis.⁵ Approximately 70 cases have been described to date.^6,7

The incidence is the ratio with a male predominance of 2:1 to 4:1. Mesenchymal hamartoma of the chest wall accounts for 7 to 14% of all solitary lung nodules.¹³ The most common modes of clinical presentation of MHCW are as a chest wall mass in infancy, with or without respiratory distress. Less commonly, MHCW might be an incidental finding on a chest x-ray done for other reasons, such as respiratory tract infection.

Clinically, it usually presents as a unique lesion involving more frequently the right hemithorax, although bilateral and multiple ipsilateral cases have been described previously.^4,6–8 Typically, these lesions arise from the central portions of one or several ribs, and can range in size from several centimeters to very large expansile lesions involving most of the thoracic cage. Radiological findings on chest x-ray, CT, and MRI are usually quite typical.⁷ Cross-sectional imaging (CT and MRI) clearly depicted the rib origin, osseous expansion, and associated extrapleural soft-tissue masses. The intrinsic characteristics corresponded with the pathologic composition of the lesion. Computed tomography best demonstrated the matrix mineralization, seen in 100% of lesions, with the mineralization appearing chondroid alone in 79%.⁷ Hemorrhagic cystic regions (secondary aneurismal bone cyst areas) also were seen with CT (fluid levels in 64% of lesions), although T2-weighted MR images were superior in demonstrating this feature in 80% of cases.⁷

Histologically, the lesion is composed of cartilage, smooth muscle, and respiratory epithelium forming a disorderly mass. The stroma consists of oval or spindle mesenchymal cells with no atypia or abnormal mitotic activity. Osteoclast-like giant cells in the vicinity of blood-filled cysts resemble aneurysmal bone cysts.

In this case, chest x-ray, CT, and MRI all are useful in diagnosis, with CT being particularly useful because of the calcification and bony origin.⁴ Mesenchymal hamartoma of the chest wall is usually considered a benign expansive lesion with a favorable outcome. However, for approximately 70 cases of mesenchymal hamartoma reported, to our knowledge there are 4 reports of malignant transformation.^9–12 And it may be considered radiographically suspicious of malignancy due to the presence of rib destruction and rapid growth. We suspected malignant MHCW of our case, so PET/CT was done. A percutaneous needle biopsy tends to cause massive bleeding, thus making it difficult to achieve a definitive diagnosis.^18,19 Considering the possibility of malignancy at the time of the operation, we believed the partial resection of the ribs was acceptable.

It is known that PET/CT imaging can facilitate diagnosis in adults with malignant diseases by enabling the differentiation between benign and malignant tumors. Positron electron tomography/CT may be useful for pediatric neoplastic lesions, the nature of which is often difficult to determine preoperatively.

Furthermore, glucose transporter-1 protein (Glut-1) is the major glucose transporter protein in tumor cells. Fluorodexyglucose is now widely used to evaluate regional glucose metabolism in a variety of tumors, and high correlations were found between FDG uptake and Glut-1 expression in most malignant tumors.^20–22 Uptake of FDG includes many complicated factors, and it would be difficult to make a differential diagnosis in mesenchymal tumors using FDG uptake alone. Zhao et al²³ reported that the benign tumor patients who had high FDG uptake had negative scores in Glut-1. Even in our case, Glut-1 might be useful for preoperative diagnosis.²³

In summary, MHCW is a non-neoplastic benign lesion in the majority of cases. However, PET/CT might be a useful tool to distinguish malignant MHCW in children. Its early and complete excision is the adequate therapy to avoid lethal respiratory complications and more aggressive treatments, although there is recommendation for conservative management in asymptomatic cases.