2026 ICD-10-CM Diagnosis Code C37Malignant neoplasm of thymus
ICD-10-CM Codes›C00–D49›C30-C39›C37
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 21
- Chronic Condition
C37 is a billable ICD-10-CM diagnosis code for malignant neoplasm of thymus. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 826 through 830, 843 through 845. 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 2 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Endocrine system cancers - thymus.
For Medicare Advantage risk adjustment, C37 maps to CMS-HCC Category 21 (Lymphoma and Other Cancers) under the V28 model, adding a risk factor of about 0.671 for a community, non-dual, aged beneficiary in payment year 2026.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
C37 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 neoplasm of thymus gland
- Malignant thymoma
- Neuroendocrine carcinoma of thymus
- Primary malignant neoplasm of thymus
- Primary malignant thymoma
- Primary small cell neuroendocrine carcinoma of thymus
- Primary thymic carcinoma
- Stage I: Grossly and microscopically encapsulated
- Stage IIa: Microscopic transcapsular invasion
- Stage IIb: Macroscopic capsular invasion into thymic or surrounding fat, or grossly adherent to, but not breaking through, mediastinal pleura or pericardium
- Stage III: Macroscopic invasion of neighboring organs
- Stage IVa: Pleural or pericardial dissemination
- Stage IVb: Hematogenous or lymphatic dissemination
- Thymic epithelial neoplasm stage finding
- Thymoma
- Thymoma type A
- Thymoma type AB
- Thymoma type B
- Type C thymoma
Tabular List NotesGuidance
Coding notes and annotation back-references applicable to this code.
Type 1 Excludes
- malignant carcinoid tumor of the thymus C7A.091
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.
Carcinoma (malignant) See Also: Neoplasm, by site, malignant;
thymic C37
Thymoma See Also: Neoplasm, thymus, by type;
malignant C37
metaplastic C37
sclerosing C37
type A C37
type AB C37
type B1 C37
type B2 C37
type B3 C37
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Thymoma
a neoplasm originating from thymic tissue, usually benign, and frequently encapsulated. although it is occasionally invasive, metastases are extremely rare. it consists of any type of thymic epithelial cell as well as lymphocytes that are usually abundant. malignant lymphomas that involve the thymus, e.g., lymphosarcoma, hodgkin's disease (previously termed granulomatous thymoma), should not be regarded as thymoma. (from stedman, 25th ed)
Table of NeoplasmsClinical
Patient EducationClinical
Thymus Cancer
The thymus is a small organ in your upper chest, under your breastbone. Before birth and during childhood, the thymus helps the body make a type of white blood cell. These cells help protect you from infections.
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert C37 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About C37Overview
What is the ICD-10 code for malignant neoplasm of thymus?
The ICD-10-CM code for malignant neoplasm of thymus is C37. It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
Is C37 (Malignant neoplasm of thymus) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report malignant neoplasm of thymus on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What MS-DRG does C37 group to?
When malignant neoplasm of thymus is the principal diagnosis on an inpatient stay, it groups to MS-DRG 826, 827, 828, 829, 830, 843, 844, 845, with relative weights from 0.8516 to 4.6778 depending on complications. Higher weights mean higher Medicare reimbursement.
Is C37 a CC or MCC?
CMS lists C37 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 2 closely related codes in its exclusion list.
What is the ICD-9 equivalent of C37?
Under the General Equivalence Mappings, malignant neoplasm of thymus converts to ICD-9-CM 164.0 (malignant neopl thymus). The mapping is a direct match.
What HCC is C37?
C37 (malignant neoplasm of thymus) maps to CMS-HCC Category 21 (Lymphoma and Other Cancers), commonly written as HCC 21, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2026. It mapped to HCC 11 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 C37 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, C37 adds a risk adjustment factor of about 0.671 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.493 to 0.739 depending on the payment segment). A more severe related category (HCC 17, HCC 18, HCC 19, and HCC 20) supersedes it when both are reported. See the full factor table on the HCC 21 category page.
