**1. Introduction**

Primary bone tumors are rare in occurrence accounting for approximately 0.2% of all the tumors. Generally, primary tumors are localized, intra-compartmental, or extended, extra-compartmental [1]. Most benign tumors that have not spread to lymph nodes or other organs remain asymptomatic until their presence is indicated by a trivial insult [1, 2]. Therefore, it becomes important to categorize bone lesions in the early stages of their development for prognosis and diagnosis [2]. On the other hand, skeletal metastases are frequent in patients with breast, prostate, and lung cancer and also occur in other tumors such as myeloma, thyroid and renal cancer, lymphoma, and Ewing's sarcoma [3]. The bone becomes the most common site of metastases in humans due to its highly vascular nature, and this results in pain, pathologic fracture, and decreased quality of life [3].

Bone metastases are either osteolytic or osteoblastic, depending on the dominance of osteoclastic activity or osteoblasts, respectively [3]. As a result the radiological appearance of bone metastasis is lytic, sclerotic, or mixed [4]. General pathogenesis of bone tumors sequentially involves proliferation of primary neoplasm, local tissue invasion, intravasation into blood vessels, extravasation into bone marrow, tumor cell dormancy, proliferation in bone, and modification of bone microenvironment [3]. The site of metastases is governed by the "seed and soil" hypothesis by Paget which states that neoplastic cells grow or proliferate only in a suitable environment [3]. Tumor cells therefore migrate to the heavily vascularized areas of the skeleton, particularly the red bone marrow of the axial skeleton and the proximal ends of the long bones, the ribs, and the vertebral column [5]. Chemotactic factors such as CXCL10 (CXC motif ligand [CXCL]), CXCL12, and osteopontin have been reported to play a major role in tumor migration [6].

Tumors can be classified into low-grade (Grade I), intermediate-grade (Grade II), and high-grade (Grade III) tumors [1]. Grading of bone tumors is roughly based on the cellularity of the lesions compared to the extracellular matrix, nuclear features, the presence of mitotic figures, and necrosis [7]. Grade II tumors are more cellular, with a greater degree of nuclear atypia, hyperchromasia, and nuclear size. Grade III tumors have significant areas of marked pleomorphism, large cells with more hyperchromatic nuclei than Grade II tumors, occasional giant cells, and abundant necrosis [1]. High-grade bone tumors are the fastest growing and most aggressive group of classic osteoblastic subtype. On the basis of histological appearance, bone tumors are classified as parosteal and periosteal. Parosteal tumors belong to lowgrade subtype of osteosarcoma, whereas periosteal belongs to the intermediategrade subtype. Parosteal tumors have fibroblast appearance and are limited to bone surface, but periosteal tumors appear chondroblastic upon histology.
