Alveolar Soft Part Sarcoma (ASPS) is a very rare, slow-growing tumor of an unknown origin, that arises mainly in children and young adults. ASPS is highly angiogenic: it involves intensive growth of new blood vessels, that connect the tumor to the blood system, and enable dissemination of tumor cells into the blood stream. Those tumor cells can then easily migrate into other parts of the body – typically the lungs, and the brain.
ASPS is a sarcoma, and that indicates that this cancer initially arises in tissues that connect, support, or surround other structures and organs of the body. The term “soft tissue” indicates that this sarcoma does not originate in bone, but in soft connective tissues. ASPS arises in muscles and deep soft tissue of the thigh or leg (low extremities), but many times will appear in the upper extremities: hands, the neck and the head. While ASPS is soft tissue sarcoma, it may spread and grow inside bones.
The term “Alveolar” in the name comes from the microscopic pattern (histopathology) that one sees upon analysis of ASPS slides under the microscope. Tumor cells seem to be arranged in the same pattern as the cells of the small air sacks (alveoli) are organized in the lung. However, this is just a structural similarity. ASPS was first described and characterized in 1952 by a pathologist named Christopherson.
ASPS is a rare cancer. While sarcomas constitute about 1% of all cancers (15% of all childhood cancers), ASPS is less than 1% of all the sarcoma cases. According to the American Cancer Society, about 9530 new cases of soft tissue sarcoma will be diagnosed in the USA in 2006. That predicts only around 100 new cases of ASPS.
ASPS may exist in the patient’s body for a long time before being diagnosed. The primary tumor may grow large, gradually pushing aside surrounding tissues, before causing discomfort. Therefore, ASPS symptoms may either be a painless swelling or a soreness caused by compressed nerves or muscles affecting the range of motion in the area.
What causes ASPS
Chromosomal analysis of ASPS shows the breaking and joining of two chromosomes (chromosomal translocation) in the tumor cells. A piece of chromosome X breaks and is joined to chromosome 17. This translocation creates a fusion between two genes named ASPL and TFE3 (ASPL-TFE3 fusion transcript), which results in the formation of an aberrant protein (termed fusion protein). This fusion protein is not found in normal cells. Two forms of ASPL-TFE3 fusion proteins are detected in ASPS patients, defining two ASPS types: type I and type II.Currently it is not known if the prognosis of ASPS patients with type I is different from those with type II.
Dr. Ladanyi, at the Memorial Sloan-Kettering Cancer Center, in New York, has pioneered this work.