What is ICD-10 code C45.1?

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. It belongs to the ICD-10-CM chapter for neoplasms (c00-d49). C45.1 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C45.1 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. C45.1 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. C45.1 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for C45.1?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For C45.1, V28 maps to Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers at RAF 1.798 while V24 mapped to Lung and Other Severe Cancers at RAF 1.024. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting C45.1?

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 Because C45.1 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

2026 ICD-10-CM C45.1: Mesothelioma of peritoneum

ICD-10-CM Code View

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Code lookupC45.1Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of mesothelial and soft tissue (C45-C49)

C45.1

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Mesothelioma of peritoneum

A rare cancer that develops in the membrane lining the abdominal cavity (peritoneum), typically associated with asbestos exposure.

Buddy Insight

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.

CMS-HCC V28

Mapped

HCC 19

Myelodysplastic Syndromes, Mu...

RAF 1.798

CMS-HCC V24

Mapped

HCC 9

Lung and Other Severe Cancers

RAF 1.024

ACA/HHS

Mapped

HCC 9

HHS-HCC 9

Varies by metal level

ESRD/PACE

Mapped

HCC 9

Lung and Other Severe Cancers

RAF 0.181

RXHCC

Mapped

HCC 22

Cancer, Other Specified Sites

RAF 0.124

Code Book Path

Official

C45Mesothelioma

C45.1Mesothelioma of peritoneum

Inclusion Terms

Official

Mesothelioma of cul-de-sac
Mesothelioma of mesentery
Mesothelioma of mesocolon
Mesothelioma of omentum
Mesothelioma of peritoneum (parietal) (pelvic)

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C45.1 in this effective period.

Related Child Codes

Official

C45.0Mesothelioma of pleura

C45.2Mesothelioma of pericardium

C45.7Mesothelioma of other sites

C45.9Mesothelioma, unspecified

Includes

Official

ICD-10-CM does not list Includes notes for C45.1 in this effective period.

Excludes 1

Official

other malignant neoplasm of soft tissue of peritoneum (C48.-)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C45.1 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C45.1 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C45.1 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

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

MEAT Support

HCC Buddy guidance

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

Audit Caution

HCC Buddy guidance

Confusing peritoneal mesothelioma with peritoneal carcinomatosis from ovarian or gastrointestinal cancers — peritoneal carcinomatosis uses C78.6

Using the general peritoneal malignancy code (C48.1) instead of the specific mesothelioma code (C45.1)

Failing to confirm mesothelioma histologically — immunohistochemistry is essential for distinguishing mesothelioma from adenocarcinoma

Coding benign multicystic peritoneal mesothelioma as malignant mesothelioma

Common Mistakes

HCC Buddy guidance

C45.0 — Mesothelioma of pleura; different serosal surface (chest vs. abdomen)

C48.1 — Malignant neoplasm of specified parts of peritoneum; for non-mesothelioma peritoneal cancers

C78.6 — Secondary malignant neoplasm of retroperitoneum and peritoneum; for peritoneal carcinomatosis from ovarian, colon, or gastric cancer

C45.2 — Mesothelioma of pericardium; different serosal surface

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is C45.1 an HCC code?

Yes. C45.1 (Mesothelioma of peritoneum) maps to Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers under the CMS-HCC V28 risk adjustment model (and Lung and Other Severe Cancers under V24), with a community non-dual aged RAF of 1.798. It is billable for payment year 2026.

Coder answer: C45.1 is billable and maps to V28 HCC 19, Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: C45.1
Description: Mesothelioma of peritoneum
HCC (V28): HCC 19 — Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers
RAF: 1.798
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 19, Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers

1.798

V24HCC 9, Lung and Other Severe Cancers

1.024

ESRDHCC 9, Lung and Other Severe Cancers

0.181

RxHCCHCC 22, Cancer, Other Specified Sites

0.124

Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. 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.

Work C45.1 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

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.

Coder workflow notes

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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 Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers (HCC 19) with a community, non-dual, aged base RAF weight of 1.798. 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 1.024. 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.-)

Commonly Confused Codes

•C45.0: Mesothelioma of pleura; different serosal surface (chest vs. abdomen)
•C48.1: Malignant neoplasm of specified parts of peritoneum; for non-mesothelioma peritoneal cancers
•C78.6: Secondary malignant neoplasm of retroperitoneum and peritoneum; for peritoneal carcinomatosis from ovarian, colon, or gastric cancer
•C45.2: Mesothelioma of pericardium; different serosal surface