ARMS
Meaning of ARMS: The name points at a microscope pattern, not at the lungs: cell clusters walled off by fibrous bands.
Definition of ARMS
ARMS stands for alveolar rhabdomyosarcoma, a subtype of rhabdomyosarcoma. It is a soft tissue cancer that grows from immature cells meant to become skeletal muscle, the muscle that moves the body. It appears most often in older children and adolescents, commonly in the trunk, an arm or a leg, and it tends to behave more aggressively than the embryonal subtype.
Why is it called alveolar?
The name describes a pattern, not a place. Under the microscope the tumor cells sit in small clusters separated by fibrous walls, and the empty spaces between them resemble the alveoli, the tiny air sacs that fill the lung. The likeness is purely visual. Alveolar rhabdomyosarcoma has no connection to lung tissue and does not begin there. It begins in cells that were on their way to becoming skeletal muscle, and those precursor cells are scattered throughout the body. That is why the tumor turns up in the trunk, an arm, a leg, the head and neck region, or around the urinary and genital tract, including places with no obvious muscle bulk.
How does it differ from the embryonal subtype?
| Feature | Alveolar | Embryonal |
|---|---|---|
| Usual age | Older children, adolescents and young adults | Infants and younger children |
| Common sites | Trunk, arms and legs | Head and neck, urinary and genital tract |
| Microscope pattern | Clusters divided by fibrous bands | Loose sheets of primitive muscle cells |
| Usual behavior | Grows and spreads more readily | Less aggressive overall |
Both belong to the same disease family and both are soft tissue sarcomas of childhood, but the subtypes are separated at diagnosis because the alveolar pattern signals a greater chance that cells have already traveled to lymph nodes, lung or bone marrow. That difference feeds straight into how the disease is staged and how intensively it is followed.
What does the gene fusion add?
Most alveolar tumors carry a fusion gene, created when part of the PAX3 or PAX7 gene is joined to the FOXO1 gene by an exchange of material between two chromosomes. The fused gene builds an abnormal protein that keeps the cells dividing and blocks them from maturing into working muscle. Laboratories now test for the fusion as a matter of routine, because fusion status follows the behavior of the tumor more closely than the microscope pattern does on its own. A tumor that looks alveolar but carries no fusion tends to act like the embryonal subtype, so pathology reports increasingly describe rhabdomyosarcoma as fusion positive or fusion negative alongside the older appearance based label.
Also known as
Used in a sentence
Pathology review confirmed the alveolar subtype, consistent with a diagnosis of ARMS.
Good to know
ARMS is separated from embryonal rhabdomyosarcoma by its microscopic pattern and, increasingly, by testing for a PAX-FOXO1 fusion gene, a distinction tied to its more aggressive behavior and to how closely the disease is monitored.