C45.1
BillableMesothelioma of peritoneum
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C45.1 an HCC code?
Yes. C45.1 maps to Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model (and Lung and Other Severe 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 C45.1
For C45.1 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 C45.1 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C45.1 is the ICD-10-CM diagnosis code for mesothelioma of peritoneum. A rare cancer that develops in the membrane lining the abdominal cavity (peritoneum), typically associated with asbestos exposure. C45.1 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of mesothelial and soft tissue (c45-c49).
Under the CMS-HCC V28 risk adjustment model, C45.1 maps to Lymphoma and Other Cancers (HCC 19) with a community, non-dual, aged base RAF weight of 0.105. Under the older CMS-HCC V24 model, C45.1 maps to Lung and Other Severe Cancers (HCC 9) with a community, non-dual, aged base RAF weight of 0.973. 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.
Peritoneal mesothelioma often presents later than pleural mesothelioma; document stage and extent of disease when available. Because C45.1 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 C45.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Peritoneal mesothelioma often presents later than pleural mesothelioma; document stage and extent of disease when available
- •Distinguish from other abdominal malignancies by confirming mesothelial origin in pathology documentation
Clinical Significance
Mesothelioma of the peritoneum is a rare but aggressive cancer of the abdominal lining, also associated with asbestos exposure although the association is weaker than with pleural mesothelioma. Peritoneal mesothelioma has a slightly better prognosis than pleural mesothelioma, especially when treated with cytoreductive surgery and hyperthermic intraperitoneal chemotherapy.
Documentation Requirements
- ✓Histologic confirmation of peritoneal mesothelioma (epithelioid subtype has better prognosis)
- ✓Asbestos exposure history if present
- ✓Extent of peritoneal involvement (peritoneal cancer index scoring)
- ✓Whether eligible for cytoreductive surgery with hyperthermic intraperitoneal chemotherapy
- ✓Distinction from pseudomyxoma peritonei or peritoneal carcinomatosis from other primaries
- ✓Treatment plan and disease status
Excludes 1 — Do NOT code together
- other malignant neoplasm of soft tissue of peritoneum (C48.-)