For rosettes, think hair, hole, hose: Homer Wright rosettes circle a hair-like tangle of neuropil (medulloblastoma, neuroblastoma), Flexner-Wintersteiner rosettes a central lumen, the hole (retinoblastoma), and perivascular pseudorosettes a blood vessel (ependymoma). For Turcot syndrome, think polyps plus a brain tumor: APC (familial adenomatous polyposis) brings medulloblastoma; mismatch-repair (Lynch-type) genes bring glioblastoma.
Medulloblastoma itself fits one word, MEDULLO: Midline vermis, Embryonal (CNS WHO grade 4), Drop metastases, Unsteady trunk, Little blue cells, Linked syndromes, Obstructed fourth ventricle. It is the most common malignant brain tumor of childhood; 70% of cases are diagnosed before age 10, with a peak at 3 to 7 years.
Since 2016 the WHO has classified it by molecular group; its 2021 edition lists WNT-activated, SHH-activated (TP53-wildtype or TP53-mutant) and non-WNT/non-SHH (groups 3 and 4), and the group, more than the microscope, sets the prognosis.
Homer Wright rosettes mnemonic: hair, hole, hose
A rosette is a ring of tumor cells around a central hub; what fills the hub names it and hints at the tumor's line of differentiation.
| Hook | Rosette | Center | Classic tumors | Also seen in | Signals |
|---|---|---|---|---|---|
| Hair | Homer Wright | Hub packed with fibrillary neuropil, not an open lumen | Medulloblastoma, neuroblastoma | Pineoblastoma, retinoblastoma | Neuronal differentiation |
| Hole | Flexner-Wintersteiner | Central lumen holding small cytoplasmic extensions, no neuropil | Retinoblastoma | Pineoblastoma, medulloepithelioma | Photoreceptor differentiation |
| Hose | Perivascular pseudorosette | Blood vessel wrapped by tapered cell processes | Ependymoma (most, any grade) | Medulloblastoma, central neurocytoma, glioblastoma (less often) | "Pseudo": the hub is a vessel, not a true lumen |
- Hair → the neural "-blastomas." Homer Wright described his rosettes in the adrenal and sympathetic tumors now called neuroblastomas; medulloblastoma and pineoblastoma form them too.
- Hole → the eye. Flexner-Wintersteiner rosettes mimic photoreceptors around a lumen: think leukocoria and RB1.
- Hose → ependymoma. Cells aim tapered processes at a vessel. True ependymal rosettes, with an empty canal-like lumen, are rarer but pathognomonic.
- Rosettes suggest; they rarely prove. Age and location decide. A newer name: embryonal tumor with multilayered rosettes (ETMR, C19MC-altered), median age 2 to 3 years.
Are Homer Wright rosettes seen in medulloblastoma?
Yes, but only in a minority: about a third of cases (StatPearls: fewer than half), so their absence does not exclude the tumor. In a posterior fossa tumor they are nearly pathognomonic of medulloblastoma. The shared finding is densely packed small blue cells with scant cytoplasm and many mitoses. Medulloblastoma can also form perivascular pseudorosettes, so a pseudorosette alone does not prove ependymoma.
Neuroblastoma shares the blue cells and the rosettes, so let the site decide: it is the most common extracranial solid tumor of childhood (median age 17 months), arises in the adrenal medulla or sympathetic chain, and raises urine HVA and VMA in about 90%.
Turcot syndrome mnemonic: polyps plus a brain tumor
Turcot syndrome is a primary brain tumor plus multiple colorectal adenomas. A 1995 study of 14 families found two germline causes, and the brain tumor tells them apart.
Mnemonic: a turban (brain tumor) over the polyps, then P to P and M to M. Polyposis (APC) goes with the posterior fossa: medulloblastoma. Mismatch repair goes with malignant glioma (glioblastoma), not medulloblastoma.
| Feature | APC type | Mismatch-repair type |
|---|---|---|
| Catalog name | Brain tumor-polyposis syndrome 2; part of familial adenomatous polyposis (FAP) | Brain tumor-polyposis syndrome 1; filed under constitutional mismatch repair deficiency (CMMRD) |
| Gene | APC | MLH1 or PMS2 in 1995; MSH2, MSH6 also cause CMMRD |
| Inheritance | Autosomal dominant | Lynch syndrome dominant; CMMRD recessive |
| Colon | Hundreds to thousands of adenomas from about age 16 | Adenomas or cancer; Lynch was "hereditary nonpolyposis colorectal cancer" |
| Brain tumor | Medulloblastoma (11 of 14 patients), WNT group only | Glioblastoma; in CMMRD, childhood high-grade gliomas |
| Extra clue | Parent with a colectomy; osteomas, CHRPE, desmoids | Café-au-lait macules mimicking neurofibromatosis type 1 (CMMRD) |
How strong is the link? In the 1995 study, people with FAP had a 92-fold relative risk of cerebellar medulloblastoma, yet their absolute CNS tumor risk is about 1%, and GeneReviews advises a yearly neurologic exam. In Lynch syndrome, glioblastoma is the most frequent brain tumor.
The modern view. GeneReviews calls Turcot a historical label, now part of the FAP or Lynch syndrome spectrum. Most WNT medulloblastomas are sporadic, driven by somatic CTNNB1 (beta-catenin) mutations, so the National Cancer Institute (NCI) advises considering germline APC testing when a WNT tumor lacks one. Review the colon side in familial adenomatous polyposis and Lynch syndrome vs FAP, and the other brain tumor in glioblastoma.
The MEDULLO mnemonic: seven exam clues
| Letter | Hook | What to know |
|---|---|---|
| M | Midline; most common malignant childhood brain tumor | About 80% arise in the region of the fourth ventricle, at its roof in the vermis; peak age 3 to 7 |
| E | Embryonal | CNS WHO grade 4; older texts lumped it with PNETs, a term the WHO dropped in 2016 |
| D | Drop metastases | Seeds the spinal subarachnoid space through CSF; 10% to 40% have CNS spread at diagnosis |
| U | Unsteady trunk | Vermis → truncal ataxia, broad-based gait; a lateral (hemispheric) tumor → limb ataxia |
| L | Little blue cells | Sheets of small round blue cells; Homer Wright rosettes in about a third |
| L | Linked syndromes | FAP (APC, WNT), Gorlin (PTCH1, SUFU, SHH), Li-Fraumeni (TP53, SHH) |
| O | Obstructs the fourth ventricle | Obstructive hydrocephalus → morning headache, vomiting, papilledema, sixth nerve palsy |
One-liner: a young child with morning headaches, vomiting, truncal ataxia and a midline cerebellar mass that seeds the spine has medulloblastoma until proven otherwise.
WHO CNS5 (2021): molecular groups and histologic types
CNS5 lists four molecularly defined types, plus "medulloblastoma, histologically defined" when no molecular testing is done. Groups 3 and 4 are merged as non-WNT/non-SHH because separating them needs methylation profiling and changes little in treatment, though the names persist. Grades use Arabic numerals (CNS WHO grade 4).
| CNS5 type | Share | Typical age | Usual site | Histology | Key genetics | 5-year outlook |
|---|---|---|---|---|---|---|
| WNT-activated | About 10% | Older children, teens (median 10), adults | Brainstem (rhombic lip) origin; midline or cerebellar peduncle/cerebellopontine angle | Classic | CTNNB1 (85% to 90%), nuclear beta-catenin, monosomy 6; germline APC | Best: near 100% in children |
| SHH, TP53-wildtype | 75% of SHH (SHH is 25% to 33% of all) | Infants under 3, adults | Cerebellar hemisphere | Desmoplastic/nodular, MBEN or classic | PTCH1, SUFU, SMO, GLI2; TERT in adults | Intermediate: over 80% if no high-risk features |
| SHH, TP53-mutant | 25% of SHH | Ages 5 to 18 | Cerebellar hemisphere | Often large cell/anaplastic | TP53, often germline; MYCN, GLI2 | Poor: under 30% to about 40% |
| Group 3 | About 25% | Infants, young children; boys 2:1 | Midline fourth ventricle | Classic or large cell/anaplastic | MYC amplification (about 15%), isochromosome 17q | Worst: 20% to 50% |
| Group 4 | 35% to 40%, the largest | Childhood, adolescence; boys 3:1 | Midline fourth ventricle; little or no enhancement | Classic | Isochromosome 17q, PRDM6, MYCN | Intermediate: 75% to 90% |
Histologic types are now mainly clues to the group:
| Histologic type | Looks like | Molecular home | Prognosis |
|---|---|---|---|
| Classic | Sheets of small round blue cells | Any group (most common) | Set by the group |
| Desmoplastic/nodular | Reticulin-free nodules in reticulin-rich zones | SHH only | Better at age 3 and younger |
| Extensive nodularity (MBEN) | Larger reticulin-free nodules | Infant SHH; Gorlin in 5 of 12 (one report) | Excellent in infants |
| Large cell/anaplastic | Large pleomorphic cells, prominent nucleoli, necrosis | Group 3, SHH-TP53-mutant | Poorest; diffuse anaplasia upstages to high risk |
Predisposition syndromes: Gorlin, Li-Fraumeni, FAP and others
About 5% of medulloblastomas arise from a germline variant, most often in SHH tumors (14% to 20% in a 1,022-patient study; up to 40% of pediatric SHH tumors once ELP1, the commonest predisposition gene, is counted). NCI advises genetic counseling for every patient with SHH medulloblastoma.
| Syndrome | Gene | Tumor group | Clues | Must-know point |
|---|---|---|---|---|
| Gorlin (nevoid basal cell carcinoma) syndrome | PTCH1 or SUFU; dominant | Infant SHH, usually desmoplastic or MBEN | Macrocephaly, jaw keratocysts, palmar pits, bifid ribs, falx calcification | Medulloblastoma in about 5% (33% with SUFU, under 2% with PTCH1), peak age 1 to 2; avoid radiation when possible, because basal cell carcinomas can erupt by the thousand in the field |
| Li-Fraumeni syndrome | TP53 | SHH-TP53-mutant, in children | Sarcomas, early breast cancer, adrenocortical carcinoma, brain tumors, leukemia in the family | Germline TP53 in 20% of children aged 5 to 16 with SHH tumors; poor prognosis |
| Familial adenomatous polyposis | APC; dominant | WNT only | A parent with a colectomy | Colonoscopy every 1 to 2 years from age 10 to 15 |
Gorlin syndrome also explains multiple early basal cell carcinoma in a family history.
Presentation: vermis, truncal ataxia and the fourth ventricle
- Raised intracranial pressure from obstructive hydrocephalus: fairly abrupt headaches, especially on waking, nausea and vomiting, lethargy and papilledema. Sixth nerve palsies usually reflect the hydrocephalus, not the tumor's site. Most children are diagnosed within 2 to 3 months of the first symptom.
- Cerebellar signs: vermis lesions cause truncal ataxia, a broad-based gait and titubation; hemisphere lesions cause limb ataxia, dysmetria and intention tremor (see vermis vs hemisphere signs). Nystagmus is common.
- No hydrocephalus in about 20%: often lateral tumors, with one-sided dysmetria and sometimes same-side sixth and seventh nerve weakness, until growth toward the midline blocks CSF.
- Infants: lethargy, feeding difficulties, lost milestones, a growing head, a bulging fontanel and the "sunsetting" sign (downward gaze from compression of the midbrain tectum).
Drop metastases, imaging and staging
Why the spine. The tumor spreads through CSF pathways; 10% to 40% of children have CNS dissemination at diagnosis, infants most often. Spread outside the CNS is very rare, so bone scans and marrow aspirates are done only for symptoms.
- CT, often the first scan: a hyperdense mass, dense because it is so cellular; under 10% calcify.
- MRI, the test of choice: a large heterogeneous mass filling the fourth ventricle or, for SHH tumors, a hemisphere, with restricted diffusion. A taurine peak on MR spectroscopy is almost exclusive to medulloblastoma.
- Location hints at the group. In one MRI series, groups 3 and 4 sat in the midline fourth ventricle, SHH tumors in the hemispheres and WNT tumors at the cerebellar peduncle or cerebellopontine angle; group 4 enhanced little or not at all. StatPearls calls WNT tumors midline, so treat location as a hint.
- Image the whole brain and spine, ideally before surgery, because postoperative blood confuses the picture.
After surgery. Brain MRI within 24 to 72 hours measures residual tumor. Lumbar CSF cytology follows if safe, conventionally 14 to 21 days after surgery (StatPearls: at least 10 to 14), because earlier samples can carry cells shed by the operation. Up to 10% have CSF tumor cells with a clean MRI.
M stage: M0 no spread; M1 tumor cells in CSF only; M2 nodules in the brain's subarachnoid space or lateral or third ventricle; M3 spinal nodules (drop metastases); M4 spread outside the CNS.
Risk after age 3. Average risk is total or near-total resection (residual 1.5 cm² or less) without metastasis; high risk is metastasis, residual over 1.5 cm² or diffuse anaplasia. NCCN-based schemes add biology: resected, nonmetastatic WNT tumors are low risk; SHH TP53 mutation and group 3 MYC amplification are high risk.
Treatment outline: surgery, radiation by age, chemotherapy
- Maximal safe resection, aiming for gross-total or near-total removal (gross-total adds little over near-total). Some children need an endoscopic third ventriculostomy or a shunt.
- Cerebellar mutism (posterior fossa) syndrome appears 1 to 2 days after surgery in about 25% to 30%: loss of speech, ataxia, hypotonia and emotional lability. It is commoner after midline resections, and about half keep lasting deficits.
- Radiation depends on age and risk.
- Older than 3, average risk: craniospinal irradiation (CSI) to 23.4 Gy plus a tumor-bed boost to 54 Gy, ideally within 4 to 6 weeks of surgery. In the COG ACNS0331 trial (ages 3 to 7), 18 Gy CSI was inferior, while boosting the tumor bed instead of the whole posterior fossa was safe.
- Older than 3, high risk: CSI to 36 Gy, boosts to 54 to 55.8 Gy; carboplatin during radiation helped group 3 MYC-amplified tumors.
- Younger than 3 (some programs up to 4 or 5): CSI is omitted or delayed because it harms neurocognition, growth and endocrine function. Intensive chemotherapy replaces it, with high-dose intravenous and intraventricular methotrexate or high-dose chemotherapy with stem cell rescue; desmoplastic, MBEN and SHH tumors do well (5-year event-free survival over 90% in the German HIT 2000 trial).
- WNT tumors still get CSI: trials that left it out saw early relapses, and dose reduction is being tested instead.
- Chemotherapy runs during and after radiation: cisplatin and vincristine with lomustine or cyclophosphamide, plus etoposide for high risk. Chemotherapy before radiation did not improve survival in average-risk disease.
- Outcome and late effects. Overall 5-year survival is about 65% to 70%; 5-year survivors are considered cured. Late effects include growth hormone deficiency (nearly all after 36 Gy CSI, about half after 23.4 Gy), hypothyroidism (up to 75%), cisplatin hearing loss, neurocognitive decline (worse when younger) and second tumors in 5% to 7% by 10 years; protons are preferred when available.
Medulloblastoma vs pilocytic astrocytoma, ependymoma and hemangioblastoma
Quick rule: roof of the fourth ventricle (vermis) is medulloblastoma, floor is ependymoma; a cerebellar cyst with an enhancing mural nodule is pilocytic astrocytoma in a child, hemangioblastoma in an adult.
| Tumor | Typical patient | Location | Imaging | Histology | Clincher |
|---|---|---|---|---|---|
| Medulloblastoma | Child, peak 3 to 7; SHH also infants, adults | Vermis, roof of the fourth ventricle; SHH in a hemisphere | Solid, hyperdense on CT, restricted diffusion | Small round blue cells; Homer Wright rosettes in about a third | CSF drop metastases; CNS WHO grade 4 |
| Pilocytic astrocytoma | Child or young adult; most common childhood brain tumor overall | Cerebellum (42% to 60%); optic pathway in NF1 | Cyst with an avidly enhancing mural nodule in about two-thirds | Hair-like bipolar cells, Rosenthal fibers, eosinophilic granular bodies; GFAP-positive | Grade 1; KIAA1549-BRAF fusion; about 95% 10-year survival after complete resection |
| Ependymoma | Child (posterior fossa); adults mostly spinal | Floor of the fourth ventricle, out through the foramina of Luschka and Magendie | Calcification and cysts common | Perivascular pseudorosettes, true ependymal rosettes; dot-like EMA staining | Posterior fossa group A (infants, poor) vs B (older, better) |
| Hemangioblastoma | Adult, 30 to 65 | Cerebellum, spinal cord | Cyst with an enhancing mural nodule, flow voids, no calcification | Foamy stromal cells in dense capillaries; inhibin-positive | Von Hippel-Lindau in about a quarter; erythropoietin overproduction |
Compare stems in the ependymoma question walkthrough.
USMLE traps: classic teaching vs current evidence
| Trap | What is actually true |
|---|---|
| No Homer Wright rosettes, so not medulloblastoma | Only about a third show them; small blue cells make the diagnosis, and molecular testing sets the group |
| Perivascular pseudorosettes mean ependymoma | Most ependymomas have them, but so can medulloblastoma, central neurocytoma and glioblastoma |
| Turcot syndrome = FAP + medulloblastoma | Two types: APC gives medulloblastoma (WNT), mismatch repair gives glioblastoma; both are now filed under FAP and Lynch syndrome or CMMRD |
| Medulloblastoma is always midline | About 80% are near the fourth ventricle; SHH tumors favor a hemisphere and may cause limb ataxia without hydrocephalus |
| Group 3 means MYC amplification | MYC amplification marks the worst group 3 tumors but occurs in only about 15% of them |
| Every child gets craniospinal irradiation | CSI is omitted or delayed under age 3 and avoided in Gorlin syndrome when an alternative exists |
| Lumbar puncture for staging right after surgery | CSF cytology waits about 2 weeks after surgery; earlier positive results need confirmation |
| Medulloblastoma is the most common childhood brain tumor | Pilocytic astrocytoma is the most common overall; medulloblastoma is the most common malignant one |
More neurology walkthroughs are on the Neurology hub.
Worked USMLE-style vignettes
Vignette 1: the rosette. A 5-year-old boy has 6 weeks of morning headaches and vomiting, sways when sitting and has papilledema. MRI shows a solid, enhancing mass arising from the roof of the fourth ventricle, with restricted diffusion and hydrocephalus. Biopsy shows sheets of small cells with scant cytoplasm, some forming rings around a central tangle of pink fibrillary material. What is this structure?
Answer: A Homer Wright rosette, in a medulloblastoma.
Why not the others: a Flexner-Wintersteiner rosette surrounds a lumen (retinoblastoma); a perivascular pseudorosette surrounds a vessel (ependymoma, from the ventricular floor); Rosenthal fibers mean pilocytic astrocytoma; foamy stromal cells among capillaries mean hemangioblastoma, a tumor of adults.
Vignette 2: the family history. A 13-year-old girl has a resected medulloblastoma with nuclear beta-catenin and monosomy 6 but no CTNNB1 mutation. Her father had a colectomy at 30 for hundreds of adenomas. Which germline gene is most likely mutated, and what else does she need?
Answer: APC (familial adenomatous polyposis, the APC type of Turcot syndrome); colonoscopy surveillance, with testing offered to relatives.
Why not the others: MLH1 or PMS2 (mismatch repair) points to glioblastoma; PTCH1, SUFU (Gorlin) and TP53 (Li-Fraumeni) give SHH tumors; VHL gives hemangioblastomas.
Vignette 3: the infant. A 20-month-old boy has irritability, vomiting and a head circumference above the 97th percentile. A right cerebellar hemisphere mass is nearly totally resected: medulloblastoma with extensive nodularity, SHH-activated, TP53-wildtype; spine MRI and CSF are clear. His mother had jaw cysts removed as a teenager and has had several basal cell carcinomas. What is the best next step?
Answer: Chemotherapy without craniospinal irradiation, plus germline testing for Gorlin syndrome (SUFU, PTCH1).
Why not the others: 23.4 Gy CSI with chemotherapy is the standard after age 3, and in Gorlin syndrome radiation can trigger basal cell carcinomas by the thousand; 36 Gy CSI is for high-risk older children; focal tumor-bed radiation added no survival benefit in localized disease in HIT 2000; APC testing fits WNT tumors.
Vignette 4: the best prognosis. A 10-year-old girl has a totally resected midline medulloblastoma: classic histology, nuclear beta-catenin, a CTNNB1 mutation, monosomy 6, no metastasis. Which statement is correct?
Answer: It is WNT-activated, the group with the best prognosis (survival approaching 100% in children), yet she still receives craniospinal irradiation and chemotherapy, because leaving out CSI led to early relapses.
Why not the others: MYC amplification and the worst outlook mean group 3; isochromosome 17q in the largest group means group 4; TP53-mutant SHH tumors of school-age children do poorly; a hemispheric desmoplastic tumor in an infant is SHH.
Key takeaways
- Rosettes are hair, hole, hose: Homer Wright (neuropil; medulloblastoma, neuroblastoma), Flexner-Wintersteiner (lumen; retinoblastoma), perivascular pseudorosette (vessel; ependymoma). Only about a third of medulloblastomas show Homer Wright rosettes.
- Turcot is polyps plus a brain tumor: APC (FAP) gives WNT medulloblastoma; mismatch repair (Lynch syndrome, CMMRD) gives glioblastoma.
- MEDULLO: midline vermis, embryonal grade 4, drop metastases, unsteady trunk, little blue cells, linked syndromes, obstructed fourth ventricle.
- CNS5 groups: WNT best; SHH TP53-wildtype (infants, adults) and group 4 (largest) intermediate; SHH TP53-mutant (school-age, Li-Fraumeni) poor; group 3 (infants, MYC) worst.
- Workup and treatment: whole-neuraxis MRI before surgery, CSF cytology about 2 weeks after; resection, then CSI (23.4 Gy average risk, 36 Gy high risk) with chemotherapy after age 3, chemotherapy without CSI under 3, and no radiation in Gorlin syndrome if avoidable.
Sources
- Childhood Medulloblastoma and Other Central Nervous System Embryonal Tumors Treatment (PDQ), Health Professional Version — National Cancer Institute (2025)
- Medulloblastoma — StatPearls, NCBI Bookshelf (2026)
- Louis DN, et al. The 2021 WHO Classification of Tumors of the Central Nervous System: A Summary — Neuro-Oncology (2021)
- Wippold FJ, Perry A. Neuropathology for the Neuroradiologist: Rosettes and Pseudorosettes — AJNR Am J Neuroradiol (2006)
- Hamilton SR, et al. The Molecular Basis of Turcot's Syndrome — N Engl J Med (1995)
- Waszak SM, et al. Spectrum and Prevalence of Genetic Predisposition in Medulloblastoma — Lancet Oncol (2018)
- Perreault S, et al. MRI Surrogates for Molecular Subgroups of Medulloblastoma — AJNR Am J Neuroradiol (2014)
- APC-Associated Polyposis Conditions — GeneReviews, NCBI Bookshelf (2022)
- Lynch Syndrome — GeneReviews, NCBI Bookshelf (2026)
- Nevoid Basal Cell Carcinoma Syndrome — GeneReviews, NCBI Bookshelf (2024)
- Li-Fraumeni Syndrome — GeneReviews, NCBI Bookshelf (2025)
- Constitutional Mismatch Repair Deficiency Syndrome — MedlinePlus Genetics, NLM (2025)
- Brain Tumor-Polyposis Syndrome 2 — MedGen, NCBI (2026)
- Mismatch Repair Cancer Syndrome 1 — MedGen, NCBI (2026)
- Neuroblastoma — StatPearls, NCBI Bookshelf (2026)
- Retinoblastoma — StatPearls, NCBI Bookshelf (2025)
- Ependymoma — StatPearls, NCBI Bookshelf (2025)
- Pilocytic Astrocytoma — StatPearls, NCBI Bookshelf (2024)
- Hemangioblastoma — StatPearls, NCBI Bookshelf (2025)
Frequently Asked Questions
What is the mnemonic for Turcot syndrome?
Picture a turban, for the brain tumor, sitting over colon polyps, then match P to P and M to M. Polyposis goes with the posterior fossa: germline APC mutations (familial adenomatous polyposis) cause medulloblastoma of the WNT-activated group. Mismatch repair goes with malignant glioma: mismatch-repair mutations such as MLH1 or PMS2 (Lynch syndrome or constitutional mismatch repair deficiency) cause glioblastoma. Genetics references now treat Turcot as a historical name for these conditions.
What is a mnemonic for Homer Wright vs Flexner-Wintersteiner rosettes?
Think hair, hole, hose. Homer Wright rosettes ring a hair-like hub of fibrillary neuropil and are seen in medulloblastoma, neuroblastoma and pineoblastoma. Flexner-Wintersteiner rosettes ring a central lumen, the hole, that mimics photoreceptors and are classic for retinoblastoma. Perivascular pseudorosettes wrap tapered cell processes around a blood vessel, the hose, and are typical of ependymoma, although medulloblastoma, central neurocytoma and glioblastoma can show them too.
Are Homer Wright rosettes seen in medulloblastoma?
Yes, but only in a minority, about a third of tumors (StatPearls says fewer than half), so their absence does not rule medulloblastoma out. When they appear in a posterior fossa tumor, they are nearly pathognomonic of it. The shared finding is densely packed small blue cells with scant cytoplasm and many mitoses. Neuroblastoma forms the same rosettes, so the tumor's site and urine catecholamine metabolites separate the two.
What does the MEDULLO mnemonic stand for?
M is midline: the cerebellar vermis at the roof of the fourth ventricle, in the most common malignant brain tumor of childhood. E is embryonal, CNS WHO grade 4. D is drop metastases through the cerebrospinal fluid to the spine. U is unsteady trunk, meaning truncal ataxia. L is little blue cells, sometimes in Homer Wright rosettes. L is linked syndromes. O is obstruction of the fourth ventricle, causing hydrocephalus.
What are the WHO 2021 molecular groups of medulloblastoma?
The 2021 WHO classification lists four types: WNT-activated; SHH-activated and TP53-wildtype; SHH-activated and TP53-mutant; and non-WNT/non-SHH, which contains groups 3 and 4. WNT tumors, about 10% of cases and mostly in older children, have the best prognosis, approaching 100% survival in children. TP53-mutant SHH tumors and MYC-amplified group 3 tumors do worst, while group 4, the largest group, has an intermediate outlook.
Which genetic syndromes cause medulloblastoma?
About 5% of medulloblastomas arise from a germline variant. Gorlin syndrome (PTCH1 or SUFU) causes SHH tumors in infants, often desmoplastic, and radiation is avoided when possible because basal cell carcinomas erupt in the field. Li-Fraumeni syndrome (TP53) causes TP53-mutant SHH tumors in school-age children. Familial adenomatous polyposis (APC), which includes the APC type of Turcot syndrome, causes WNT tumors. Newer genes include ELP1, the most common one in childhood SHH tumors, plus BRCA2 and PALB2.