Atypical teratoid rhabdoid tumor, or ATRT, is a rare and aggressive brain cancer that primarily affects very young children. It accounts for about 6% of all childhood brain tumors, with a peak incidence in children from birth to age 2. The age-standardized incidence rate is roughly 1.4 per million children per year, making it uncommon but significant because of how quickly it grows and how young its patients tend to be.
How ATRT Differs From Other Brain Tumors
ATRT belongs to a category called embryonal tumors, meaning it arises from cells that are still in an early, undeveloped state. What sets it apart is its chaotic mix of cell types. Under a microscope, ATRT contains cells that resemble brain tissue, connective tissue, and even skin-like tissue all jumbled together. The tumor also contains distinctive “rhabdoid” cells with off-center nuclei that look like immature muscle cells. This multi-lineage appearance is unique to ATRT and helps pathologists distinguish it from other childhood brain tumors like medulloblastoma.
In children under 2, ATRT is roughly as common as medulloblastoma, the most well-known pediatric brain tumor. But it behaves differently: it tends to grow faster and responds less predictably to standard chemotherapy.
Where ATRT Grows
About two-thirds of ATRTs develop in the posterior fossa, the lower back portion of the skull that houses the cerebellum and brainstem. Common spots include the cerebellopontine angle (where the cerebellum meets the brainstem) and the cerebellar hemispheres. The remaining cases, roughly one-third to nearly half depending on the study, arise in the upper parts of the brain. This location split matters because it affects both the symptoms a child develops and the surgical options available.
Symptoms to Recognize
Because ATRT grows quickly, symptoms can appear and worsen over just days or weeks. In infants, one of the earliest signs is an unusual increase in head size, since the skull bones haven’t yet fused and can expand under pressure from the growing tumor. Older toddlers and young children are more likely to show headaches (especially in the morning), nausea and vomiting, unusual sleepiness, and changes in activity level.
Tumors in the posterior fossa often cause balance problems, lack of coordination, and difficulty walking. Some children develop facial pain, numbness, tingling, or even partial paralysis of the face due to pressure on cranial nerves. The rapid onset of these symptoms is a hallmark that distinguishes ATRT from slower-growing tumors.
The Genetic Root: A Single Missing Protein
ATRT has one of the simplest genetic profiles of any cancer. Nearly all cases are driven by the loss of a single gene called SMARCB1. This gene produces a protein (known as INI1) that serves as a critical component of a molecular machine responsible for remodeling chromatin, the tightly packed structure of DNA inside every cell. When this protein is missing, the cell loses its ability to properly control which genes get turned on and off.
The consequences are sweeping. Without INI1, cells can’t properly apply the brakes on growth. A key growth-promoting protein called Cyclin D1 runs unchecked, pushing cells to keep dividing. Several important signaling pathways that normally require upstream triggers become permanently activated, essentially running on autopilot. Remarkably, ATRT cells don’t accumulate the hundreds or thousands of additional mutations seen in most adult cancers. Loss of this one gene is essentially the sole recurring genetic event, and the cancer arises through disrupted gene regulation rather than widespread DNA damage.
In rare cases, a different gene called SMARCA4 (producing a protein called BRG1) is lost instead, but the result is functionally similar. Some children carry an inherited mutation in SMARCB1, which creates a predisposition syndrome that raises the risk of rhabdoid tumors in the brain, kidneys, or soft tissues.
How ATRT Is Diagnosed
Diagnosis starts with brain imaging, typically an MRI, but confirming ATRT requires a tissue sample. The definitive test is straightforward: pathologists stain tumor tissue for the INI1 protein. In ATRT, every tumor cell shows complete absence of this protein while the surrounding normal brain tissue retains it. This immunohistochemical stain is quick, reliable, and has transformed diagnosis since its introduction. In the rare SMARCA4-driven cases, loss of the BRG1 protein serves the same diagnostic purpose.
The current WHO classification (updated in 2021) requires molecular confirmation of either INI1 or BRG1 loss to formally diagnose ATRT. This requirement exists because some ATRTs lack the classic rhabdoid cell appearance and could be mistaken for other embryonal tumors without the protein test.
Treatment Approach
There is no single established standard treatment for ATRT. Instead, children typically receive a combination of surgery, chemotherapy, and radiation therapy, with the specific plan tailored to the child’s age and the tumor’s location and spread.
Surgery comes first. The goal is to remove as much tumor as possible, since data from the national AT/RT Registry suggests that children who achieve complete resection tend to survive longer. However, complete removal is often difficult because the tumor tends to invade surrounding brain tissue.
Chemotherapy for ATRT is intensive. Protocols tested in clinical trials have used combinations of multiple drugs delivered both into the bloodstream and directly into the spinal fluid. Some of the most promising regimens include high-dose chemotherapy followed by autologous stem cell rescue, a process where a child’s own stem cells are collected beforehand and then returned after treatment to help the bone marrow recover from the powerful drugs.
Radiation therapy plays an important role but presents a serious dilemma. Most ATRT patients are under 3 years old, and radiation to a developing brain carries significant risks for long-term cognitive and developmental harm. Current protocols from the Children’s Oncology Group set minimum age cutoffs: 6 months for tumors in the lower brain and 12 months for tumors in the upper brain. Proton beam therapy, which delivers radiation more precisely than traditional methods, has shown encouraging quality-of-life outcomes even in children treated before age 4, though concerns about neurodevelopmental effects remain.
Survival and Prognosis
ATRT outcomes have improved considerably over the past two decades but remain challenging. The five-year relative survival rate is about 48% for children up to age 14, 42% for teenagers and young adults ages 15 to 39, and 25% for adults over 40. Average survival times reported across studies range from 12 to 24 months, though many children live longer with aggressive multimodal treatment and enrollment in clinical trials.
Several factors influence prognosis. Younger age at diagnosis (particularly under 1 year) is generally associated with worse outcomes. The extent of surgical resection matters significantly. And whether the tumor has spread through the spinal fluid at diagnosis affects the treatment plan and outlook.
Targeted Therapies in Development
The simple genetic profile of ATRT has opened the door to targeted treatments. The most advanced is tazemetostat, a drug that blocks a protein called EZH2. When INI1 is missing, cancer cells become dependent on EZH2 to maintain their abnormal gene activity patterns. Tazemetostat is already FDA-approved for a related tumor in older patients (epithelioid sarcoma) and has been tested in a phase 1 trial specifically including children with relapsed or treatment-resistant ATRT. In that trial, 24% of ATRT patients (5 out of 21) showed a measurable response to the drug, a promising signal for a tumor with few established options at relapse.
Other pathways disrupted by INI1 loss, including cell growth signaling controlled by Cyclin D1, are also being explored as drug targets. Because ATRT is driven by gene regulation rather than accumulating DNA mutations, therapies that reset the cell’s epigenetic machinery represent a fundamentally different strategy from traditional chemotherapy.