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Although we may think of bone as hard or "dead," it really is
a complex, living tissue. Like all other tissues of the body, it
contains living cells. There are several types of cells in our bones.
- Osteoblasts are responsible for forming
bone matrix (connective tissue and mineral that
gives bone its strength).
- Osteoclasts prevent too much bone
matrix from accumulating and help bones maintain their proper shape. By
depositing or removing minerals from the bones, osteoclasts help
control the amount of these minerals in the blood.
- Bone marrow (found in some bones)
contains fat cells and, most importantly, blood-forming cells. All
blood cells (red blood cells, white blood cells, and platelets) are
made here.
There are 2 main types of bones – flat bones and long bones.
The flat bones help to protect the brain and the organs of the chest,
abdomen, and pelvis. The skull bones and sternum
(breast bone), for example, are flat bones. The long bones support the
legs and arms. Muscles that move the arms and legs are attached to
these long bones.
Osteosarcoma (also called osteogenic
sarcoma) is the most common type of cancer that develops in bone. Like
osteoblasts of normal bone, the cells that form this cancer produce
bone matrix. However, the "malignant bone" tissue of an osteosarcoma is
not as strong as normal bones.
Like other cancers, osteosarcomas can spread beyond the bone
into nearby tissues (such as muscle, tendons, and fat). Cancer cells
from osteosarcoma can also sometimes break away from the main tumor and
spread through the bloodstream to other bones, or to the lungs or other
internal organs. This process is called metastasis.
Osteosarcoma develops most commonly near the ends of the long
bones, especially around the knees. In children and adolescents about
80% of osteosarcomas develop in the bones around the knee – the distal
femur (the part of the thigh bone next to the knee) and proximal
tibia (the part of the lower leg bone next to the knee). The proximal
humerus (the part of the upper arm bone close to the
shoulder) is the second most common location for osteosarcoma (about
10%). These sites tend to have a good outlook for cure. However, osteosarcoma
can develop in any bone, including the bones of the pelvis, shoulder, and jaw.
Pelvic osteosarcomas, and osteosarcomas arising in bones where surgery is
difficult tend to have a poorer outcome.
Not all malignant (cancerous) bone tumors
are osteosarcomas. Ewing tumor
is the second most common malignant bone tumor in children and is
described in a separate American
Cancer Society document. Most other types of bone cancers are
usually found in adults and are rare in children. These include
chondrosarcoma (cancer that develops from cartilage) and malignant
fibrous histiocytoma of bone.
Many bone tumors are not malignant. Cells that form benign
tumors do not have the ability to spread to other parts of the body and
are generally not dangerous. There are many types of benign bone
tumors. These tumors are formed by different types of cells. Benign
tumors formed by bone cells are osteomas. Benign
tumors formed by cartilage cells are chondromas.
Benign tumors with both bone and cartilage cells are osteochondromas.
Other benign bone tumors of children and young adults include
eosinophilic granuloma of bone, non-ossifying fibromas, xanthomas,
giant cell tumors, and lymphangiomas.
Some benign bone tumors can be diagnosed by physical
examination and how they look on x-rays. In other cases, a biopsy
(sample of tumor taken for examination under the microscope) is needed
to see whether the tumor is benign or malignant.
Subtypes of Osteosarcoma
Several subtypes of osteosarcoma can be identified by how
they look on x-rays and under the microscope. These subtypes are worth
mentioning because some have a much better prognosis
(outlook for survival) than others.
Based on how they look (how closely they resemble normal
bone) under the microscope, osteosarcomas can be classified as low
grade, intermediate, or high grade. If the tumor has few
dividing cells, it is a low-grade osteosarcoma. If the tumor has many
dividing cells it is a high-grade osteosarcoma. A high-grade
osteosarcoma also has a lot of dead cells in the tumor. This usually
means the cancer is growing so fast it outgrows its source of
nutrition.
High-grade Osteosarcomas
(These have different names based on their appearance under the
microscope.)
- Pagetoid – means coming from Paget disease.
- Extra skeletal – means starting in an non-bony area of the
body
- Post-irradiation – osteosarcomas in bone that had once
received radiotherapy
- osteoblastic
- chondroblastic
- fibroblastic
- mixed
- small cell
- telangiectatic
- high-grade surface
Intermediate-grade
Osteosarcomas
Low-grade Osteosarcomas
- parosteal
- intramedullary or intraosseous well differentiated
The overall grade of the tumor is more important than the
subtype. This tells doctors how likely it is that the cancer will
spread to other parts of the body. Low-grade tumors should be
completely removed, but the patients usually do not require
chemotherapy. These tumors have an excellent prognosis. All patients
with high-grade osteosarcoma require both chemotherapy and surgery. The
outlook and treatment of intermediate-grade osteosarcoma varies. Most
osteosarcomas that occur in children are high-grade. Last Revised: 03/19/2007
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