In neuroimaging, “extra-axial” doesn’t mean outside the skull, it means outside the brain parenchyma (the brain tissue itself) but still inside the skull.
Intra-axial = within the brain tissue (e.g., gliomas, abscesses).
Extra-axial = outside the brain tissue but within the cranial cavity (e.g., meningiomas, metastases on the dura, arachnoid cysts, subdural hematomas).
So: Outside the skull would be extracranial. Extra-axial means the lesion is intracranial but not in the brain substance.
A simple way to remember:
Axial = brain substance
Extra-axial = outside the brain substance, but inside the head
Looking further into a Transcalvarial mass – A transcalvarial mass is a lesion that extends through the calvarium (skull bones), connecting the intracranial (inside the skull) compartment with the extracranial (outside the skull) space.
Key points:
The calvarium = the dome-like skull bones that encase the brain.
"Transcalvarial" = crossing through the calvarium.
Seen with aggressive tumors, metastases, or sometimes infections that erode bone.
Imaging will often show a continuous mass that breaches both the inner and outer tables of the skull. So, compared to extra-axial (inside skull, outside brain), a transcalvarial mass goes a step further — it escapes the skull.
The question is – what is this type of biopsy considered to be?
This is one of those “it depends” situations in coding, because a transcalvarial mass spans both intracranial and extracranial compartments.
General coding logic:
Intracranial procedures (craniotomy/craniectomy approaches, brain or meningeal biopsies) are coded under intracranial biopsy codes.
Extracranial procedures (scalp, subcutaneous, or skull-only masses) are coded with extracranial biopsy codes.
For a transcalvarial mass, the deciding factor is where the biopsy specimen is obtained:
If the surgeon biopsies the intracranial portion (inside the dura or cranial cavity) → code as intracranial.
Some Examples:
00B00ZX – Excision of brain, open approach, diagnostic.
00B73ZX – Excision of cerebral hemisphere, percutaneous approach, diagnostic.
00B24ZX – Excision of dura mater, percutaneous endoscopic approach, diagnostic.
If the biopsy is from the extracranial portion (outside the skull or superficial component) → code as extracranial.
Some Examples:
0JB00ZX – Excision of scalp subcutaneous tissue and fascia, open approach, diagnostic.
0HB1XZX – Excision of facial skin, external approach, diagnostic.
0QB00ZX – Excision of skull (cranial bone), open approach, diagnostic.
If documentation doesn’t specify, query the provider, because the coding pathway hinges on the biopsy site.
References:
- Centers for Medicare & Medicaid Services, & National Center for Health Statistics. (2025). ICD-10-PCS official guidelines for coding and reporting, FY 2025. U.S. Department of Health & Human Services. https://www.cms.gov/medicare/icd-10/2025-icd-10-pcs
- American Hospital Association. (2017). Biopsy coding when lesions cross boundaries. Coding Clinic for ICD-10-CM/PCS, 4th Quarter, 28–29.
- American Hospital Association. (Ongoing). Coding Clinic for ICD-10-CM/PCS. American Hospital Association.
- Greenberg, M. S. (2020). Handbook of neurosurgery (9th ed.). Thieme.