INTRODUCTION W. L, 2012) Majority, patients with osteosarcoma

INTRODUCTION
Osteosarcoma, also known as osteogenic sarcoma, is one of the cancers that develop in the bone. Osteosarcoma arises mostly in the long bones and seldom in the soft tissues. Regardless of their rarity, osteosarcomas are the most common primary malignancy of bone in children and adolescents and the fifth most common malignancy among adolescents and young adults aged 15 to 19. Osteosarcoma is still incompletely understood. The age at appearance ranges from 10-25 years of age (Lisa, W. L, 2012) Majority, patients with osteosarcoma has localized pain for several months’ duration. The most significant finding on clinical presentation is soft tissue mass, which is commonly large and tender to palpation. At presentation, between 10 and 20 percent have demonstrable metastatic disease, most often involving the lung.
DEFINITION
Osteosarcoma is a primary malignant tumour of the bone characterised by the direct formation of immature bone or osteoid tissue by the tumour cells. It is known to arise from the metaphysis where the bone cells divide and grow. In histologic appearance, malignant production of osteoid may occur. Other cell populations may also be present, as these types of cells may also arise from pluripotential mesenchymal cells, but any area of malignant bone in the lesion establishes the diagnosis as osteosarcoma. This disease is also a deadly form of musculoskeletal cancer that it causes patients to die of pulmonary metastatic disease most frequently. Mainly osteosarcomas come up as solitary lesions within the fastest growing areas of the long bones of children. The most affected areas are the proximal humerus, proximal tibia and distal femur but practically any bone can be affected. But not all osteosarcomas happen in solitary manner. Multiple sites may become noticeable within about 6 months period (synchronous osteosarcoma), or over a period longer than that (metachronous osteosarcoma). Such osteosarcoma is particularly uncommon, but it tends to affect those younger than 10 years if happens (Mehlman, 2017).
ETIOLOGY
What really causes osteosarcoma is actually unknown. However, there are certain risk factors that can be recognized. For example, teenage growth spurt may contribute to the incidences of osteosarcoma as it is may be related to the rapid bone growth. Usually, people who are considered extraordinarily tall for their age are sometimes associated with osteosarcoma. Next, radiation that is more than 2000 rad to bones could also be the environmental factors to osteosarcoma. It is thought to occur due to pre-existing radiation exposure to treat other cancer condition. The exposure might contribute side effect of damaging the genetic material in the normal cells which then lead to the development of a “secondary” cancer called osteosarcoma. Other than that, people with rare, inherited cancer syndrome might also associate with osteosarcoma. For instance, children with hereditary retinoblasma, in which all the cells of the body have a mutation in the RB1 gene will probably affected by osteosarcoma. The risk of osteosarcoma becomes higher in the bones around the eyes if radiotherapy is used to treat the retinoblasma. The Li-Fraumeni syndrome is another example, in which it is caused by inherited mutations that turn off the p53 tumor-suppressor gene (Mehlman, 2017). These mutations put a person in higher risk of developing one or more types of cancer, including osteosarcoma, brain tumours, breast cancer and others.

PATHOPHYSIOLOGY
Generally, osteosarcoma can affect any types of bone most commonly on the extremities of the long bones nearby the metaphyseal growth plates. The common sites take many places such as the femur, tibia, humerus, skull, jaw and pelvis. Once there is mutation of tha DNA, it activates the oncogene which leads to a deactivation of the suppressor gene which causes a malignant osteoblast that leads to proliferation of abnormal osteoblasts. This causes formation of osteiod tissue. The osteiod tissue causes uncontrolled growth of the tumor in the bone and therefore overloading of the bone. There is then suppression of red bone marrow which leads to decreased production of blood cells. A decrease in RBC leads to anemia while a decrease in WBC leads to a lowered infection resistance. The overcrowding also causes pressure inside the bone which is a main cause of pain and fractures. The cancer then metastasizes spreading to other parts of the body especially the lungs, kidneys, CNS, liver and the spleen.
PATHOGENESIS
Osteosarcoma arises from the multipotent mesenchyma cells and frequently from the medullary cavity of the metaphysis of the growing long bones. The tumour starts out from the metaphysis, in which it is a site where the growth is more active. Then, the tumour will initiate beneath the periosteum and expand in two directions which are towards the medulla or towards the sub-periosteal area. In the medulla area, the tumour would destroy the bone trabeculae area and appear as irregular mass that permeate along the medullary canal. The periosteum of the bone at first provides impenetrable barriers and covers the bone as protection. But sometimes, on the diaphyseal part of the tumour, the periosteum is stripped for a short distance. Therefore, there might be deposition of new bone layers that is parallel to the midshaft of the bone. Due to these processes, Codman’s Triangle is formed, in which it describes the triangular area of the subperiosteal area that is created when there is lesion such as tumour lifts the periosteum away from the bone (Rathi, 2015).
IMAGING MODALITIES
Imaging modalities are the basis of providing diagnosis, treatment and predicting prognosis. There are several imaging techniques that are available to detect osteosarcoma, which include conventional radiography, magnetic resonance imaging (MRI), Computed Tomography (CT) scan and nuclear imaging. Conventional radiography is crucial to initially evaluate the bone lesions and the results may aid in determining diagnosis and suggest appropriate further examination. Meanwhile for CT, it is helpful for confusing radiographic appearance. CT suggests better indication of bone destruction and showing the soft tissue mass due to its cross-sectional images modality (Hide, 2017). Small amount of mineralized osseous matrix could also demonstrated by CT and flat bones, where periosteal changes might be harder to appreciate can also be seen (Hide, 2017). These features are hardly seen on plain x-ray. As for MRI, it is favoured in most cases due to its excellent evaluation of soft tissue contrast and bone marrow, plus its multiplanar capability (Hide, 2017).
CONCLUSION
As conclusion, osteosarcoma is rare in general but quite popular in the young generations. Great progresses have been made in treating osteosarcoma in the past few decades. Nevertheless, more research is needed to learn how best to handle osteosarcoma, for example those that have already spread when they are found. Many clinical trials are focusing on treating osteosarcoma using a diversity of strategies such as surgery, chemotherapy and radiation therapy.

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