C84.09
BillableMycosis fungoides, extranodal and solid organ sites
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C84.09 an HCC code?
Yes. C84.09 maps to Lung and Other Severe Cancers under the CMS-HCC V28 risk adjustment model (and Lymphoma 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 C84.09
For C84.09 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 C84.09 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C84.09 is the ICD-10-CM diagnosis code for mycosis fungoides, extranodal and solid organ sites. A type of skin cancer (mycosis fungoides) that has spread to organs or tissues outside the lymph node system. C84.09 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of lymphoid, hematopoietic and related tissue (c81-c96).
Under the CMS-HCC V28 risk adjustment model, C84.09 maps to Lung and Other Severe Cancers (HCC 20) with a community, non-dual, aged base RAF weight of 0.000. Under the older CMS-HCC V24 model, C84.09 maps to Lymphoma and Other Cancers (HCC 10) with a community, non-dual, aged base RAF weight of 0.675. 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.
This code captures extranodal involvement; document which specific organs or tissues are affected. Because C84.09 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 C84.09 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code captures extranodal involvement; document which specific organs or tissues are affected
- •Distinguish from nodal involvement codes to ensure accurate staging and prognosis assessment
Clinical Significance
Mycosis fungoides with extranodal and solid organ involvement represents the most advanced form of cutaneous T-cell lymphoma with spread to visceral organs beyond the skin and lymph nodes. This may include liver, lung, bone marrow, or gastrointestinal involvement. Extranodal mycosis fungoides carries a very poor prognosis and often indicates large cell transformation requiring aggressive systemic therapy.
Documentation Requirements
- ✓Skin biopsy confirming mycosis fungoides
- ✓Documentation of specific extranodal organs involved
- ✓Imaging or biopsy confirming organ involvement
- ✓Assessment for large cell transformation
- ✓TNMB staging reflecting visceral involvement and treatment plan