C37 ICD-10-CM Code: Malignant neoplasm of thymus
C37 maps to CMS-HCC V28 21. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC Buddy coding tools
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FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of respiratory and intrathoracic organs (C30-C39)
C37
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMalignant neoplasm of thymus
Cancer that develops in the thymus, a small gland located behind the breastbone that is part of the immune system.
Buddy Insight
Thymic cancer is rare, with thymomas being the most common type.
CMS-HCC V28
MappedHCC 21
Coefficient HCC 21: 0.671 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 011
Code-level coefficient reference
ESRD/PACE
MappedHCC 11
Code-level coefficient reference
RXHCC
MappedHCC 22
Code-level coefficient reference
Code Book Path
Inclusion Terms
OfficialNo inclusion terms are included in this display for C37. Check the code and parent instructions in the Code Book.
Excludes 2
OfficialNo Excludes 2 notes are included in this display for C37. Check the code and parent instructions in the Code Book.
Related Codes
No related codes are included in this display for C37. Check the Code Book for the complete code path.
Includes
Official- malignant neoplasm of middle earInherited from C30-C39
Excludes 1
Official- malignant carcinoid tumor of the thymus (C7A.091)
- mesothelioma (C45.-)Inherited from C30-C39
Code First
OfficialNo Code First sequencing instructions are included in this display for C37. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for C37. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for C37. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
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 C37 an HCC code?
Yes. C37 (Malignant neoplasm of thymus) maps to HCC 21, Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.671. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.
Coder answer: C37 is billable and maps to V28 HCC 21, Lymphoma and Other Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- C37
- Description
- Malignant neoplasm of thymus
- HCC (V28)
- HCC 21 — Lymphoma and Other Cancers
- RAF reference coefficient
- 0.671
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. 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 context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.
Work C37 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for C37
For C37, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.
- 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
Coder workflow notes
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What This Code Means
C37 is the ICD-10-CM diagnosis code for malignant neoplasm of thymus. Cancer that develops in the thymus, a small gland located behind the breastbone that is part of the immune system. C37 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of respiratory and intrathoracic organs (c30-c39).
Under the CMS-HCC V28 risk adjustment model, C37 maps to Lymphoma and Other Cancers (HCC 21) with a source-labeled community, non-dual, aged reference coefficient of 0.671. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.
Thymic cancers are relatively rare; ensure documentation clearly indicates thymus as primary site. For C37, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, and MEAT documentation criteria for C37 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Thymic cancers are relatively rare; ensure documentation clearly indicates thymus as primary site
- •Do not confuse with thyroid cancer (C73)
Clinical Significance
Thymic cancer is rare, with thymomas being the most common type. These tumors are significant in risk adjustment because they are often associated with paraneoplastic syndromes, most notably myasthenia gravis, which adds complexity to patient management. Thymic carcinomas are more aggressive than thymomas and carry a worse prognosis.
Documentation Requirements
- ✓Pathology confirmation distinguishing thymoma (types A, AB, B1, B2, B3) from thymic carcinoma
- ✓WHO histological classification and Masaoka-Koga staging
- ✓Assessment for paraneoplastic syndromes (myasthenia gravis, pure red cell aplasia, hypogammaglobulinemia)
- ✓Imaging showing thymic origin in the anterior mediastinum
- ✓Resection status (complete vs. incomplete)
Excludes 1, Do NOT code together
- malignant carcinoid tumor of the thymus (C7A.091)
Commonly Confused Codes
- •C73: Thyroid cancer: 'Thymus' and 'thyroid' are easily confused in documentation: verify anatomical site
- •C38.1: Anterior mediastinum: Thymic tumors are in the anterior mediastinum; if documented as 'anterior mediastinal mass,' query for thymic origin
- •D15.0: Benign neoplasm of thymus: Some thymomas are benign; confirm malignancy
- •C85.1: Unspecified B-cell lymphoma: Mediastinal lymphomas may mimic thymic tumors