embryonal rhabdomyosarcoma
Meaning of embryonal rhabdomyosarcoma: Named for the tissue it imitates rather than the tissue it came from, which is why it appears in places holding no muscle.
Definition of embryonal rhabdomyosarcoma
Embryonal rhabdomyosarcoma is a cancer of soft tissue whose cells resemble the immature muscle cells of a developing embryo. It is the commonest soft tissue cancer of childhood and appears most often in the head and neck, around the eye, and in the bladder, vagina or other parts of the urinary and genital tract. It can arise where no skeletal muscle exists.
Where does it actually come from?
Not from mature muscle. The cells arise from primitive tissue that has begun to commit to becoming skeletal muscle but never completed the journey, and such tissue is scattered widely during development. That is why the tumor turns up in the bladder, the bile ducts and behind the eye, sites that contain no skeletal muscle at all. Under the microscope the cells look like the muscle precursors of an embryo, and some show the internal striped pattern of a muscle fiber caught partway through forming.
How does it differ from the alveolar type?
| Feature | Embryonal | Alveolar |
|---|---|---|
| Usual age | Infants and young children | Older children and adolescents |
| Common sites | Head, neck, urinary and genital tract | Arms, legs, trunk |
| Genetic signature | No fusion gene; loss of material from chromosome eleven | A fusion joining the PAX and FOXO1 genes |
| Outlook | Generally more favorable | Generally less favorable |
The fusion gene is the dividing line that counts. Its presence defines alveolar rhabdomyosarcoma, and classification now rests on whether the fusion is there rather than on appearance alone, because it predicts behavior more reliably than the microscope does.
What forms does it take?
Two named variants sit inside the embryonal group. The botryoid form grows as soft grape-like clusters bulging into a hollow space such as the vagina or the bladder, and it carries a comparatively good outlook. The spindle cell form is built from elongated cells and is seen most often beside the testis. Both count as rhabdomyosarcoma and both lack the fusion gene.
How does it behave?
It grows locally and pushes into surrounding structures, and it can spread through the bloodstream to the lungs, bone and marrow, or along lymph channels. Symptoms usually come from the site rather than from the illness as a whole: a painless lump, a blocked nostril, a bulging eye or difficulty passing urine. Treatment of this soft tissue cancer combines chemotherapy with surgery or radiation, and the site strongly influences what is possible.
Also known as
Used in a sentence
Embryonal rhabdomyosarcoma was confirmed on biopsy of the bladder mass.
Good to know
The tumor is named for the immature muscle cells it resembles rather than for an origin in muscle, and classification now depends on whether a PAX-FOXO1 fusion gene is present rather than on appearance alone.
Word origin
Where “embryonal rhabdomyosarcoma” comes from
Rhabdomyosarcoma joins the Greek rhabdos, meaning rod, with mys, meaning muscle, and sarx, meaning flesh, plus the tumor ending -oma; embryonal comes from the Greek embryon, an unborn young.