C09.0
BillableMalignant neoplasm of tonsillar fossa
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C09.0 an HCC code?
Yes. C09.0 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 C09.0
For C09.0 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 C09.0 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C09.0 is the ICD-10-CM diagnosis code for malignant neoplasm of tonsillar fossa. Cancer that develops in the tonsillar fossa, which is the pocket-like area where the tonsil sits in the throat. C09.0 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, C09.0 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, C09.0 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.
Confirm tonsillar fossa location is documented to distinguish from tonsillar pillar cancers. Because C09.0 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 C09.0 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Confirm tonsillar fossa location is documented to distinguish from tonsillar pillar cancers
- •Document laterality (left or right) when specified
Clinical Significance
Malignant neoplasm of the tonsillar fossa is one of the most common oropharyngeal cancers, with increasing incidence due to human papillomavirus (HPV) infection. HPV-positive tonsillar cancers have a markedly better prognosis than HPV-negative tumors, making HPV status documentation critical for accurate prognostication and treatment planning.
Documentation Requirements
- ✓Pathology-confirmed malignancy with histological type
- ✓HPV/p16 status documented (critical for prognosis and treatment decisions)
- ✓Specific documentation of tonsillar fossa as primary site
- ✓Laterality (right or left tonsillar fossa)
- ✓TNM staging using the appropriate staging system (AJCC 8th edition separates HPV-positive and HPV-negative oropharyngeal cancer staging)
Commonly Confused Codes
- •C09.1 — Tonsillar pillar; the fossa is the pocket where the tonsil sits, the pillars are the tissue folds flanking the tonsil
- •C09.9 — Tonsil, unspecified; avoid when fossa is specifically documented
- •C09.8 — Overlapping sites of tonsil; use when tumor spans multiple tonsillar subsites
- •C10.2 — Lateral wall of oropharynx; tonsillar fossa tumors may extend to the lateral oropharyngeal wall