C37
BillableMalignant neoplasm of thymus
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C37 an HCC code?
Yes. C37 maps to Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model (and Colorectal, Bladder, and Other 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 C37
For C37to 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 C37 during that encounter — not just copy-forwarded from a problem list.
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 community, non-dual, aged base RAF weight of 0.671. Under the older CMS-HCC V24 model, C37 maps to Colorectal, Bladder, and Other Cancers (HCC 11) with a community, non-dual, aged base RAF weight of 0.307. 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.
Thymic cancers are relatively rare; ensure documentation clearly indicates thymus as primary site. Because C37 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 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