Mesenchymal Hamartoma of the Chest Wall in an Infant - A Benign Entity Masquerading as Malignancy.

Mesenchymal hamartoma of the chest wall is a rare tumor-like lesion encountered in neonates and infants. Although typically benign with no metastatic potential, it has alarming imaging and pathological features that mimic malignancy. We describe the imaging, surgical, and pathological features of this rare entity in a 1-month-old infant. Copyright:

INTRODUCTION

Mesenchymal hamartoma of the chest wall (MHCW) is a rare tumor-like congenital lesion encountered in neonates and infants. It can mimic malignancy both on imaging and histopathology. We report a case of an infant with this rare diagnosis who was treated by excision of the lesion and discuss the imaging and histopathological features encountered.

CASE REPORT

A 1-month-old male infant was brought to the outpatient department with a complaint of a palpable nodule in the chest wall noticed for 1 week. There was no record of any swelling at birth. There was no history of fever or fast breathing. Antenatal and birth history was unremarkable. Antenatal ultrasound examinations were within normal limits. On physical examination, the there was a small 2 cm × 2 cm sized hard nodular, nontender, immobile lesion palpable in the posterolateral chest wall on the right side. The lesion appeared to be in continuity with the underlying 8th rib. At this location, the rib could not be separately palpated from the lesion. The skin over the lesion was pinchable. The rest of the physical examination was within normal limits. Chest radiograph revealed a lytic destructive lesion in the posterior part of the right 8th rib. The lesion was expansile, had permeative nature with a wide zone of transition and areas of matrix mineralization. Adjacent ribs were unremarkable [Figure 1a]. Noncontrast computed tomography (CT) [Figure 1b-d] revealed a 5 cm × 3 cm × 3.5 cm sized expansile mass lesion arising from the posterior part of the right 8th rib with a large extra-osseous component projecting into the right thoracic cavity compressing the right lung. The inner cortex of the rib was destroyed. The outer cortex of the rib was relatively preserved with a small outward projection which likely corresponded to the clinically palpable hard nodule. The mass shows extensive matrix mineralization which was mixed osseous/cartilaginous in nature. There were no cystic areas in the lesion. The lower lobe of the right lung was displaced by the mass. No lung nodules or enlarged lymph nodes were seen. As the lesion was expansile, causing bony destruction and there was cosmetic deformity, surgical excision of the involved rib was planned. The option of close follow-up was discussed, but in the institutional tumor board discussion, the consensus opinion was that malignancy could not be completely excluded based on imaging. We did not perform a needle biopsy as it entails IV sedation and the results are not as accurate as excision biopsy. While rib excision does carry a risk of development of scoliosis at later age, it was considered small due to very young age of the infant and the fact that only one rib was being excised.

Figure 1

(a) Chest radiograph AP view showing a mass lesion in the right lower zone causing destruction of the posterior part of the right 8th rib. (b and c) Axial computed tomography sections (bone window) through the lesion show expansion of the posterior end of the right 8th rib with an adjacent large lobulated mass lesion projecting into the right hemithorax and displacing the lung. Note the mixed osseous/cartilaginous type of matrix calcification within the mass. (d) Volume rendered image of the computed tomography shows the mass lesion arising from the right 8th rib

Thoracotomy and en bloc resection of the mass was done under general anesthesia. Thoracotomy showed a mass arising from the eighth rib and compressing the lung [Figure 2]. The lesion was also stuck to the ninth rib. Complete excision with removal of the eighth rib was done using an extrapleural approach. Intercostal drainage (ICD) tube was inserted. The child was on elective ventilation for a day and extubated on day 2 and ICD was removed on day 3.

Figure 2

(a) Surface marking of the planned line of incision outlining the palpable part of the lesion. (b) Intra-operative photograph showing the dissected tumor attached to the rib. (c) Photograph of the excised tumor specimen

Gross examination showed a smooth-surfaced mass measuring 5.1 cm × 3 cm × 3.6 cm attached to a rib. On the cut section, the lesion appeared gray brown in color and was firm in consistency [Figure 3a]. Hematoxylin and eosin stain sections showed predominantly osteoid-forming lesion with focal areas of cartilaginous differentiation. Frequent mitoses (6 per 10 high-power field) were seen. There was no evidence of atypia or necrosis. No giant cells or aneurysmal bone cysts were seen. By immunohistochemistry, all the lesional cells were positive for SATB-2, indicating the osteoblastic origin of the lesion. Surprisingly, the Ki 67 index was high (40%) [Figure 3b-d]. Because of the high Ki 67 index, the possibility of well-differentiated osteosarcoma was considered. We sampled extensively and gave 12 blocks for wider examination. None of them showed malignant osteoid and a clear-cut zonation was seen. Moreover, p53 staining was only focally positive (maximum up to 5%) again arguing against a malignant tumor. Finally, osteosarcoma is extremely rare in this age group at this location. The age, radiological features, and presence of cartilaginous areas favored a diagnosis of mesenchymal hamartoma over well-differentiated osteosarcoma. The patient is doing well after 1 year of follow-up without any further treatment.

Figure 3

(a) Gross specimen of the resected tumor with the cut surface showing bony spicules and soft tissue. The inked surface of the tumor is seen on the right side of the picture. (b) Island of benign bone (arrows) surrounded by spindle cells and epithelioid cells. H and E, stain (×400). (c) Widespread positive staining for SAT B2 showing the osteoblastic origin of the lesion. Immunohistochemistry staining for SAT B2 (×100). (d) Ki67 positive staining showing the highly proliferative nature of the lesion. Immunohistochemistry staining for Ki 67 (×100)

DISCUSSION

MHCW is a rare congenital lesion encountered in neonates and infants. They arise from the ribs. They are not true neoplasms and are composed of proliferating and maturing skeletal elements. They do not exhibit invasive behavior and do not metastasize. There can be the erosion of adjacent ribs due to mass effect.[1] These lesions are usually solitary, although multifocal lesions have been reported.[2] Interestingly, these lesions are more often encountered on the right side than left. These lesions can present with respiratory distress or chest wall swelling. Often, the lesions are asymptomatic and are incidentally detected on chest radiographs. These lesions have variable growth potential. Majority of the lesions stop growing in the 1st year of life.[3] Large lesions causing respiratory distress and death have been reported. Rarely, these lesions can be associated with massive pleural effusion.[4] At the other end of the spectrum, there have been reports of spontaneous resolution.[5]

CT usually reveals a large partly calcified heterogeneous mass lesion arising from single or multiple ribs (more commonly from the posterior part) with a large extrapleural component. The calcification can be of osteoid or cartilaginous pattern. Hemorrhagic cavities with fluid–fluid levels (representing secondary aneurysmal bone cyst formation) can also be seen within the mass, although these are better demonstrated on magnetic resonance imaging.[1] These lesions show diffuse enhancement of the solid component on postcontrast imaging. Positron emission tomography-CT can show the accumulation of the FDG tracer within the mass.[6] Imaging differential diagnosis for these lesions includes common chest wall neoplasms in this age group – Ewing's sarcoma, Askin tumor, Langerhans cell histiocytosis, hemangioma, teratoma, and metastasis from neuroblastoma and lymphoma.

On gross examination, these tumors tend to be lobulated and well circumscribed. The tumors are composed predominantly of chondroid tissue with varying stages of ossification within the hyaline cartilage. A telangiectatic component with giant cells is another distinct feature. Immature spindle cells can be seen.[7]

In our case, there was diagnostic difficulty due to some atypical features. The lesion was completely solid. The cystic or fluid containing spaces within the lesion which are said to be characteristic were absent. On histopathology, the lesion was composed predominantly of islands of bone and immature spindle cells. The cartilaginous component which is usually predominant in these lesions was sparse. These features combined with the hypercellular proliferative nature (high Ki67 index) of the lesion initially led to suspicion of osteosarcoma. Malignant tumors are characterized by hyperchromasia and nuclear atypia. This feature was conspicuously absent. The lack of atypia, very young age of presentation, and cartilaginous component was admixed with disorderly arranged mesenchymal tissue with no atypia clinched the diagnosis of MHCW.

Ki 67 index in MHCW has not been previously reported. Ki 67 is a nuclear protein which plays an important role in cell proliferation. It has a short half-life (around 1 h) and is present during actively dividing phases of the cell cycle (G1, S, G2, and M phases). It is absent in the resting phase (G0 phase) of the cell cycle. Ki 67 expression is closely related to the proliferating fraction of cells in malignancies, and it is thus a reliable marker of tumor aggressiveness. A monoclonal antibody (MIB-1) is utilized to detect this nuclear protein. The proportion of tumor cell nuclei which are reactive for this antibody is expressed as a percentage value which is known as labeling index. Ki 67/MIB-1 labeling index or simply Ki 67 index is used as a prognostic and predictive marker in many malignancies including those of breast, cervix, prostate, lung, and central nervous system.[8] In MHCW, a high Ki67 index is likely due to the actively proliferating nature of the lesion and should not be mistaken as a marker for malignancy.

MHCW is cured by complete excision. There have been few reports of recurrence following incomplete excision.[9] Asymptomatic lesions can be kept under observation with close imaging follow-up.[3]

CONCLUSION

MHCW is a rare tumor of the ribs typically encountered in neonates and infants. It can mimic malignancy both on imaging and histopathology. Awareness of this unique entity is essential to avoid diagnostic errors and therapeutic misadventures.

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Conflicts of interest

There are no conflicts of interest.