RMS, or rhabdomyosarcoma, is a rare cancer that forms in skeletal muscle tissue. It’s far more common in children, but when it strikes adults, it tends to be more aggressive and harder to treat. Adults diagnosed with RMS generally face lower survival rates than children with the same disease, with five-year survival for some subtypes dropping to around 24%.
Because RMS is considered a childhood cancer, many adults have never heard of it. That unfamiliarity can make a diagnosis feel especially disorienting. Here’s what the disease looks like in adults, how it’s found, and what treatment involves.
How Adult RMS Differs From Childhood RMS
Rhabdomyosarcoma develops from cells that were supposed to become skeletal muscle but instead grew uncontrollably. In children, the most common form is called embryonal rhabdomyosarcoma, which accounts for the majority of pediatric cases. Adults can get embryonal RMS too (it makes up roughly 21 to 54% of adult cases), but they also develop subtypes that are rare or nonexistent in kids.
The four recognized subtypes are embryonal, alveolar, spindle cell/sclerosing, and pleomorphic. Pleomorphic rhabdomyosarcoma occurs almost exclusively in adults and is a high-grade, fast-growing form. Alveolar rhabdomyosarcoma, which appears in both children and adults, accounts for about 18 to 33% of adult cases and carries a particularly poor prognosis in people over 20.
The location of the tumor also shifts with age. In children, RMS most often appears in the head, neck, or genitourinary tract. In adults, the extremities (arms and legs) are the most common site, making up about 26% of cases. Tumors can also develop in the trunk, head and neck, or pelvic region.
Symptoms and Early Warning Signs
RMS in adults typically shows up as a painless, growing lump. Because the mass often develops in muscle tissue of the limbs or trunk, people may notice a firm swelling that increases in size over weeks or months. Pain isn’t always present early on, but it can develop when the tumor presses on nearby nerves.
Beyond a visible or palpable mass, symptoms depend entirely on where the tumor is located. A tumor near the bladder might cause urinary problems. One in the head or neck area could cause facial swelling, nasal congestion, or difficulty swallowing. In many cases, the lump itself is the only initial symptom, which is one reason diagnosis is often delayed.
What Drives the Cancer at a Genetic Level
The alveolar subtype of RMS is strongly linked to a specific genetic error: a swap of material between two chromosomes (chromosomes 2 and 13) that creates an abnormal fusion gene. This fusion gene essentially hijacks normal cell functions in multiple ways at once. It pushes cells to multiply faster, helps them resist the signals that would normally trigger cell death, and may block the cells from maturing into functional muscle tissue.
One of the key downstream effects is the overactivation of a protein involved in cell survival and invasion. Alveolar RMS cells appear to be dependent on this pathway, meaning the cancer essentially becomes “addicted” to it. There’s also evidence that this genetic change helps tumor cells spread to distant sites like bone marrow by activating a homing mechanism that guides cancer cells into new tissue environments. A smaller number of alveolar tumors carry a similar but distinct chromosomal rearrangement involving chromosomes 1 and 13.
How Adult RMS Is Diagnosed
When a suspicious mass is found, MRI is the preferred tool for evaluating the local tumor because of its ability to distinguish soft tissue detail. This helps doctors see exactly how large the tumor is, what structures it involves, and whether it’s invaded nearby tissue.
For checking whether the cancer has spread beyond the original site, a PET/CT scan is the most accurate option. Studies show PET/CT detects the full extent of disease with about 86% accuracy, compared to 54% for conventional imaging like CT or MRI alone. It’s especially good at identifying lymph node involvement, with sensitivity ranging from 80 to 100%.
A tissue biopsy confirms the diagnosis. For tumors close to the skin surface, an ultrasound-guided needle biopsy is often used to extract a tissue sample. The biopsy allows pathologists to identify the specific subtype and look for the genetic changes that influence prognosis and treatment planning.
Staging in Adults
Children with RMS are staged using a system developed by the Intergroup Rhabdomyosarcoma Study Group, which classifies disease into four groups based on how much tumor remains after initial surgery and whether cancer has spread. Adults, however, are typically staged using the American Joint Committee on Cancer system for soft tissue sarcomas. This system incorporates both clinical imaging and pathology results, factoring in tumor size, depth, lymph node status, and whether metastasis is present.
Treatment: Surgery, Chemotherapy, and Radiation
Treatment for adult RMS almost always involves a combination of surgery, chemotherapy, and radiation. The specific approach depends on the tumor’s location, size, subtype, and whether it has spread.
Surgery
When the tumor is in an arm or leg, the goal is to remove it completely while preserving as much limb function as possible. Surgeons aim for clear margins, meaning no cancer cells at the edge of the removed tissue. A margin of less than 1 cm is considered “close,” and when margins are tight, radiation therapy is typically added afterward to reduce the chance of the tumor returning locally. There’s no universally agreed-upon margin width that guarantees safety, so the decision often comes down to balancing complete removal against preserving the ability to use the limb.
Chemotherapy
Chemotherapy is a core part of treatment regardless of whether the tumor has been fully removed surgically. Drugs are given in cycles, typically once a week for the first several months, then less frequently. Total treatment duration usually ranges from six months to over a year.
The specific drug combination depends on risk level. Patients with low-risk disease may receive a two-drug regimen, while intermediate and high-risk patients generally receive a three-drug combination, sometimes alternating with a different pair of drugs. Even patients with metastatic disease (the highest risk group) receive aggressive multi-drug chemotherapy as the standard approach.
Radiation
Radiation therapy plays an important role in local control, especially when surgery can’t remove the entire tumor. The standard dose ranges from 36 to 54 Gy, delivered in small daily fractions over several weeks. For patients with only microscopic residual disease after surgery, doses on the lower end (36 to 41.4 Gy) are typical. Larger residual tumors call for higher doses, around 50.4 Gy. Research has tested whether delivering smaller, more frequent doses (twice daily) improves outcomes, but it did not show any benefit over the standard once-daily schedule.
Survival Rates and Prognosis
Adult RMS carries a significantly worse prognosis than the childhood form. For adults aged 20 and older with alveolar rhabdomyosarcoma, five-year cancer-specific survival is roughly 24%, dropping to about 21% at ten years. These numbers are notably lower than what’s seen in children and adolescents with the same subtype.
Several factors influence an individual’s outlook. Tumor location matters: cancers in the extremities or trunk tend to be harder to control than those in certain head and neck sites. The subtype plays a role as well. Pleomorphic RMS, found almost exclusively in adults, is high-grade by definition. Whether the cancer has already spread at the time of diagnosis is one of the strongest predictors of outcome. Adults are also more likely than children to experience treatment-related toxicity from the intensive chemotherapy protocols originally designed for younger patients, which can limit the doses they’re able to tolerate.
Despite these challenges, multimodal treatment combining surgery, chemotherapy, and radiation gives adults the best chance at long-term control. The rarity of adult RMS means that treatment decisions often benefit from evaluation at a specialized sarcoma center where oncologists have experience managing this uncommon diagnosis.