Teaching Case

A 14-year-old girl presented with left shoulder pain mainly during gym lessons. She consulted a pediatrician after having pain for 6 weeks. Clinical examination suggested a thoracic mass for which a chest X-ray was performed.

The chest X-ray showed a large calcified mass of the left upper hemithorax with underlying rib destruction. The subsequent enhanced CT revealed a large calcified heterogeneous contrast-enhancing mass surrounding the region of the 3rd left rib, which seemed osteolytic, associated with an osteolytic lesion of the third dorsal vertebrae and invasion of the spinal canal. The mass seemed to spare the axillary and subclavian vessels and was supplied by an enlarged left mammary artery. A few suspected lung metastases were also detected.
MRI showed abnormal signal intensity of the left side of the 3rd dorsal vertebral body and pedicle with intraspinal extension of the mass and displacement of the spinal cord to the right. The spinal cord showed no signal abnormality.

Biopsy of the mass revealed rib osteosarcoma. Chemotherapy was then started, according to AOST-0031 protocol (high-dose methotrexate, cisplatin and doxorubicin), followed by surgery.

Osteosarcoma is defined as a primary aggressive malignant neoplasm arising from primitive transformed cells of mesenchymal origin that exhibit osteoblastic differentiation and produce malignant osteoid[1, 2, 3].

Primary osteosarcoma (as opposed to the secondary type presenting in old age) is the most common histological form of primary malignancy in adolescents. It typically originates in the metaphyseal areas of the long bones (femur, tibia and humerus). Only 3% of primary osteosarcomas involve the chest wall[4] and 34% of these rare cases occur in the ribs[5]. The first differential diagnosis for the paediatric population in this location is Ewing sarcoma, which presents no osteoid matrix as opposed to osteosarcoma.

Pathological fractures and metastatic dissemination, usually in the lungs, are the most frequent complications of osteosarcoma.

Characteristic X-ray features of osteosarcoma are lytic and sclerotic lesions, a wide zone of transition, a permeative or moth-eaten appearance, sun-burst reaction, periosteal lifting with formation of Codman's triangles in the long bones, and new bone formation in the soft tissues.

CT delineates the extraosseous and intraosseous extent of the tumour and is used to detect distant metastatic disease. The main advantage of CT is its ability to show small areas of osteoid matrix, in predominantly lytic lesions, that cannot be detected with conventional radiography or MR imaging, allowing to make a radiological differential diagnosis with tumours that most commonly affect the chest wall, such as Ewing sarcoma[6].

MRI is the most accurate tool for determining the limits of tumour within and outside the bone and it is used for local staging prior to treatment. MRI is superior to CT in the assessment of tumour extension into the spinal canal and epidural space and is crucial for surgical planning[7, 8]. Given the ability to detect necrotic areas, enhanced sequences are used in association with diffusion to judge chemotherapy response correlating with the degree of histological necrosis.
Whole body MRI as well as PET-CT are increasingly used for staging and detecting systemic dissemination of the tumour, local recurrence, and metastases after treatment.

Chemotherapy usually precedes surgery that consists of en bloc resection of the lesion, often with amputation.

The most important predictors of the outcome and survival rates are metastatic spread at the time of diagnosis, margins of resection, and the histological degree of necrosis following induction chemotherapy; a rate of more than 90% being considered as a good response to chemotherapy[9].