C09.1
BillableMalignant neoplasm of tonsillar pillar (anterior) (posterior)
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C09.1 an HCC code?
Yes. C09.1 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors under the CMS-HCC V28 risk adjustment model (and Colorectal, Bladder, and Other Cancers under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for C09.1
For C09.1 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically — it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed C09.1 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C09.1 is the ICD-10-CM diagnosis code for malignant neoplasm of tonsillar pillar (anterior) (posterior). Cancer that starts in the tonsillar pillar, which is the fold of tissue on either side of the tonsil in the throat. C09.1 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of lip, oral cavity and pharynx (c00-c14).
Under the CMS-HCC V28 risk adjustment model, C09.1 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors (HCC 21) with a community, non-dual, aged base RAF weight of 0.545. Under the older CMS-HCC V24 model, C09.1 maps to Colorectal, Bladder, and Other Cancers (HCC 11) with a community, non-dual, aged base RAF weight of 0.306. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
Specify anterior or posterior pillar involvement when documented. Because C09.1 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for C09.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Specify anterior or posterior pillar involvement when documented
- •Distinguish from tonsillar fossa (C09.0) and other oropharyngeal sites
Clinical Significance
Malignant neoplasm of the tonsillar pillar involves cancer of the anterior or posterior faucial pillars, the mucosal folds that border the tonsillar fossa. Cancers of the tonsillar pillars may present differently from tonsillar fossa cancers and can extend to the soft palate or tongue base, requiring careful documentation of the extent of disease.
Documentation Requirements
- ✓Pathology-confirmed malignancy specifying tonsillar pillar origin
- ✓Specification of anterior vs posterior pillar if documented
- ✓HPV/p16 testing status
- ✓Laterality (left or right)
- ✓Assessment of extension to adjacent structures (soft palate, tongue base, tonsillar fossa)
Commonly Confused Codes
- •C09.0 — Tonsillar fossa; the fossa is the pocket itself, the pillars are the surrounding tissue folds
- •C05.1 — Soft palate; the anterior tonsillar pillar merges with the soft palate superiorly
- •C09.8 — Overlapping sites of tonsil; use when cancer spans fossa and pillar
- •C10.2 — Lateral wall of oropharynx; pillar cancers may extend to the lateral pharyngeal wall