Maryam Kazemi Aghdam - Assistant Professor, Department of Pathology, Medical school, Shahid Beheshti University of Medical Sciences, Tehran, Iran.Pediatric Pathology Research Center, Research Institute for Children Health, Shahid Beheshti University of Medical Sciences, Tehran,
Maliheh Khoddami - Associate Professor, Department of Pathology, Medical school, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Pediatric Pathology Research Center, Research Institute for Children Health, Shahid Beheshti University of Medical Sciences, Tehran
Roxana Azma - Assistant Professor, Pediatric Radiology Department, Mofid Children’s Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran
Yalda Nilipour - Assistant Professor, Department of Pathology, Medical school, Shahid Beheshti University of Medical Sciences, Tehran, Iran.Pediatric Pathology Research Center, Research Institute for Children Health, Shahid Beheshti University of Medical Sciences, Tehran,

Case presentation: A 1.5-year-old boy was admitted to the hospital at the age of 3 days due to severe respiratory distress and a subcutaneous mass between the left 3rdand 5th ribs. He was operated and a 4x3x2.5 cm cartilagenous mass was resected with similar features to the recent tumor. At age 1.5 years, he was admitted for the secondtime due to left hemithorax mass. CT scan showed expansile lytic-sclerotic lesion between left 3rd and 5th ribs, growing into the mediastinum and a similar lesion attached to the lower rib. The lower rib lesion was a 5.5x4.5x1.5 cm tan elastic spherical mass with white elastic areas including hyaline cartilage at cut sections (the upper thorax mass was not removed). Histologically, it was a cartilaginous nodule surrounded by oval bland looking mesenchymal cells, revealing maturation of the cartilage into trabecular bone at the periphery, and foci of calcification. Introduction: Mesenchymal hamartoma of the chest wall (MHCW) or chondromatous hamartoma, first described in 1961, is a very rare benign tumor of bone. Hereby, a 1.5-year-old boy is presented with multiple left hemithorax chondromatous hamartoma due to its rarity and multiplicity. Discussion: MHCW occurs as intraosseous expansile mass typically involving the ribs. Eighty percent of cases occur prenatally or within first 6 months of life and are usually solitary, but bilaterality or multicentricity also rarely occur. Mesenchymal hamartoma is not a true neoplasm, but rather hamartomas with focal overgrowth of normal skeletal elements with no propensity for invasion or metastasis. The incidence is about 0.03% among primary bone tumors with male predominance. It appears as a 3 to 5 cm hard, immobile, subcutaneous, and extrapleural chest wall mass with deformed ribs and a cystic and solid appearance. Respiratory distress and ventilator dependence may occur.X-ray shows expansile mass with well-defined sclerotic margins involving multiple ribs with secondary aneurysmal bone cysts and speckled calcification. Radiologically, MHCW can be confused with malignant tumors like neuroblastoma, Ewing’s sarcoma, and malignant teratoma. Grossly, they are well circumscribed lobular masses with both cystic and solid areas. It is composed of hyalin cartilage, bone, small round oval or spindle mesenchymal cells, and osteoclastic giant cell. The differential diagnosis includes well- ifferentiated chondrosarcoma, aneurysmal bone cyst, chondroma, osteosarcoma, Langerhans cell histiocytosis, and osteochondromas. Usually, MHCW stops growing within the first year of life and some patients may have an excellent outcome with conservative treatment alone. Spontaneous regression has been observed in children. Prognosis is excellent and malignant transformation is very rare.

Mesenchymal hamartoma; chondromatous; bone tumor