2026 ICD-10-CM Diagnosis Code C45.1Mesothelioma of peritoneum
ICD-10-CM Codes›C00–D49›C45-C49›C45
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 19
- Chronic Condition
C45.1 is a billable ICD-10-CM diagnosis code for mesothelioma of peritoneum. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 374 through 376. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 6 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Mesothelioma.
For Medicare Advantage risk adjustment, C45.1 maps to CMS-HCC Category 19 (Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers) under the V28 model, adding a risk factor of about 1.798 for a community, non-dual, aged beneficiary in payment year 2026.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
C45.1 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2026.
Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Malignant mesothelioma of mesentery
- Malignant mesothelioma of mesocolon
- Malignant mesothelioma of omentum
- Malignant mesothelioma of parietal peritoneum
- Malignant mesothelioma of pelvic peritoneum
- Malignant mesothelioma of peritoneum
- Malignant neoplasm of mesocolon
- Malignant neoplasm of omentum
- Malignant neoplasm of peritoneum and retroperitoneum
- Neoplasm of omentum
- Neoplasm of peritoneum and retroperitoneum
- Neoplasm of the mesocolon
- Overlapping malignant neoplasm of retroperitoneum and peritoneum
- Primary malignant mesothelioma of mesentery
- Primary malignant mesothelioma of omentum
- Primary malignant mesothelioma of parietal peritoneum
- Primary malignant mesothelioma of pelvic peritoneum
- Primary malignant mesothelioma of peritoneum
- Primary malignant neoplasm of omentum
- Primary malignant neoplasm of parietal peritoneum
- Primary malignant neoplasm of pelvic peritoneum
- Primary malignant neoplasm of the mesentery
- Primary mesothelioma of overlapping sites of retroperitoneum, peritoneum and omentum
Tabular List NotesGuidance
Coding notes and annotation back-references applicable to this code.
Inclusion Terms
- Mesothelioma of cul-de-sac
- Mesothelioma of mesentery
- Mesothelioma of mesocolon
- Mesothelioma of omentum
- Mesothelioma of peritoneum (parietal) (pelvic)
Type 1 Excludes
- other malignant neoplasm of soft tissue of peritoneum C48
These terms are the conditions for which that code is to be used. The terms may be synonyms of the code title, or, in the case of "other specified" codes, the terms are a list of the various conditions assigned to that code. The inclusion terms are not necessarily exhaustive. Additional terms found only in the Alphabetic Index may also be assigned to a code.
A type 1 excludes note is a pure excludes note. It means "NOT CODED HERE!" An Excludes1 note indicates that the code excluded should never be used at the same time as the code above the Excludes1 note. An Excludes1 is used when two conditions cannot occur together, such as a congenital form versus an acquired form of the same condition.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Mesothelioma (malignant) C45.9
mesentery C45.1
mesocolon C45.1
omentum C45.1
peritoneum C45.1
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Mesothelioma
a tumor derived from mesothelial tissue (peritoneum, pleura, pericardium). it appears as broad sheets of cells, with some regions containing spindle-shaped, sarcoma-like cells and other regions showing adenomatous patterns. pleural mesotheliomas have been linked to exposure to asbestos. (dorland, 27th ed)Mesothelioma, Cystic
a peritoneal mesothelioma affecting mainly young females and producing cysts of variable size and number lined by a single layer of benign mesothelial cells. the disease follows a benign course and is compatible with a normal life expectancy, requiring occasionally partial excision or decompression for relief of pain or other symptoms. malignant potential is exceptional. (from holland et al., cancer medicine, 3d ed, p1345)Mesothelioma, Malignant
a type of mesothelioma with a tendency to metastasize. most tumors originate from either the pleura or peritoneum, tumors may also originate in the pericardium or testicular tissue. it is associated with asbestos exposure. somatic mutations identified in wt1, bcl10, cdkn2a, nf2, and bap1 genes are associated with the malignancy. omim: 156240.Solitary Fibrous Tumor, Pleural
a rare neoplasm, usually benign, derived from mesenchymal fibroblasts located in the submesothelial lining of the pleura. it spite of its various synonyms, it has no features of mesothelial cells and is not related to malignant mesothelioma or asbestos exposure.
Patient EducationClinical
Mesothelioma
The tissue that lines your lungs, stomach, heart, and other organs is called mesothelium. Mesothelioma, also known as malignant mesothelioma, is cancer of that tissue. It usually starts in the lungs, but can also start in the abdomen or other organs.
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert C45.1 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About C45.1Overview
What is the ICD-10 code for mesothelioma of peritoneum?
The ICD-10-CM code for mesothelioma of peritoneum is C45.1 (sometimes written as C451). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
Is C45.1 (Mesothelioma) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report mesothelioma of peritoneum on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What MS-DRG does C45.1 group to?
When mesothelioma of peritoneum is the principal diagnosis on an inpatient stay, it groups to MS-DRG 374, 375, 376, with relative weights from 0.9243 to 2.1387 depending on complications. Higher weights mean higher Medicare reimbursement.
Is C45.1 a CC or MCC?
CMS lists C45.1 as a CC (complication or comorbidity) for FY 2026. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 6 closely related codes in its exclusion list.
What is the ICD-9 equivalent of C45.1?
Under the General Equivalence Mappings, mesothelioma of peritoneum converts to ICD-9-CM 158.8 (mal neo peritoneum NEC). The mapping is approximate, so confirm the match fits the documentation.
What HCC is C45.1?
C45.1 (mesothelioma of peritoneum) maps to CMS-HCC Category 19 (Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers), commonly written as HCC 19, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2026. It mapped to HCC 9 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 22.
Does C45.1 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, C45.1 adds a risk adjustment factor of about 1.798 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.957 to 1.989 depending on the payment segment). A more severe related category (HCC 17 and HCC 18) supersedes it when both are reported. See the full factor table on the HCC 19 category page.
