INRG Staging System (INRGSS) & Image-Defined Risk Factors (IDRFs)
Pre-treatment, imaging-based staging for neuroblastoma, designed so patients can be staged before any surgery or biopsy.
It pairs with IDRFs to predict surgical risk and guide whether upfront resection is safe.
Although ultrasound is generally the first-line imaging modality for paediatric patients presenting with a palpable mass, if neuroblastoma is suspected and needs detailed evaluation, CT or MRI is the recommended imaging modality for complete assessment of the primary lesion at diagnosis.
INRGSS stages
| Stage | Definition (simplified) |
|---|---|
| L1 | Localised tumour, confined to one body compartment (neck / chest / abdomen / pelvis), no IDRFs. |
| L2 | Locoregional tumour with ≥1 IDRF and/or extension into ipsilateral contiguous compartments (e.g. chest–abdomen), but no distant mets. |
| M | Distant metastatic disease (bone, marrow, non-regional nodes, lung, etc.), except MS pattern. |
| MS | Metastatic disease in child <18 months (~<547 days), with mets confined to skin, liver, and/or bone marrow (<10% marrow cells, no cortical bone lesions). |
No special love for the midline in INRGSS (unlike INSS stage 3). What matters is:
compartments + IDRFs + metastatic pattern.
What are IDRFs?
Image-Defined Risk Factors = CT/MRI features that predict difficult / high-risk surgery (involvement of vital structure which dissection cannot preserve function, e.g. vascular encasement, intraspinal canal compromise, airway compression, porta hepatis invasion).
- Any IDRF present → L2 (if otherwise locoregional).
- More IDRFs → higher risk of complications, more likely to need induction chemo before resection.
- IDRFs can disappear after chemo, allowing safer delayed surgery → post-induction/pre-operative imaging re-evaluation is recommended.
Major IDRF categories (conceptual groupings)
Don’t memorise all 20; know the patterns:
multi-compartment, vessels, airway, spinal canal, porta/mesentery/renal pedicle, pelvic notch.
1. Multi-compartment extension
- Tumour extending into two contiguous ipsilateral compartments
(neck–chest, chest–abdomen, abdomen–pelvis) = IDRF by itself.- Non-contiguous disease in a different body compartment or non-regional nodes = metastatic (M/MS)
- 1R/1L (Virchow nodes) are considered regional nodes in context of cervicothoracic primary.
- Superior mediastinum nodes are considered regional nodes in context of thoracoabdominal primary.
- Inguinal nodes are considered regional nodes re considered regional nodes in context of pelvic primary.
- Non-contiguous disease in a different body compartment or non-regional nodes = metastatic (M/MS)
2. Vascular encasement
- Neck / cervicothoracic: encasement of carotid, vertebral, subclavian vessels, internal jugular vein.
- Thorax / thoracoabdominal: encasing aorta and major branches.
- Abdomen / pelvis (high-yield list):
- Encasement of coeliac axis and/or SMA origin.
- Involvement of SMA branches at mesenteric root.
- Renal pedicle(s) involved (artery and/or vein).
- Encasement of aorta, IVC, iliac vessels.
Encasement = >180° arterial contact or non-visualised vein lumen → IDRF.
Mere contact (<180° artery, flattened but patent vein) is not an IDRF (except renal arteries are flagged more aggressively in practice).
Special case for renal pedicle(s): Any degree of contact (no need encasement) is considered IDRFs as this finding is specially increased surgical difficulty.
3. Airway compression
- Compression/narrowing of trachea or main bronchi (as seen in short axis) = IDRF.
- Just displacement without luminal narrowing = not IDRF.
4. Infiltration of adjacent organs / porta / mesentery
- Infiltration of:
- Porta hepatis / hepatoduodenal ligament.
- Duodeno-pancreatic block or mesenteric root.
- Pericardium, diaphragm, kidney, liver, mesentery (true invasion = blurring of tumor/organ interfaces or loss of demonstrable separate fat plane, not just displacement).
5. Intraspinal extension
Not every dumbbell tumour = IDRF.
IDRF only if any of:
- Tumour occupies >1/3 of canal in axial plane, or
- Perimedullary CSF space obliterated, or
- Cord signal abnormal (oedema/myelopathy).
If intraspinal component is small with preserved CSF rim and normal cord signal → no IDRF.
6. Pelvis
- Tumour encasing iliac vessels, or
- Crossing the sciatic notch (cross the oblique line from ischial spine to lateral sacral body, risk to plexus/sciatic nerve).
7. Thoracoabdominal
Lower mediastinal tumour infiltrating the costovertebral junction between T9 and T12 → IDRF because of theoretical risk of injuring the artery of Adamkiewicz / anterior spinal artery and causing cord ischaemia.
Reporting terminology (INRG imaging committee flavour)
Use these words in your report – they map directly to L1 vs L2.
- Separation: fat plane preserved between tumour and vital structure → L1.
- Contact: fat plane lost, but
- Artery contact <180°
- vein flattened but lumen visible → generally L1.
- Encasement: artery contact >180° and/or vein lumen not seen → IDRF → L2.
- Compression (airway only) – reduced airway calibre → IDRF → L2.
- Invasion
- Renal pedicle(s): any degree of contact
- Intraspinal extension
- Multi-compartment: contiguous ipsilateral compartments involved → IDRF → L2.
- Multifocal disease: separate primaries; not an IDRF but stage using worst lesion.
- Can be synchronous or metachronous.
- Rare, usually seen in familial or syndromic context.
