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C78.6 ICD-10-CM Code: Secondary malignant neoplasm of retroperitoneum and peritoneum

Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Coding at a Glance

Tabular directives
2 Excludes1 · 1 Excludes2
Risk adjustment
CMS-HCC V28: 1 category · CMS-HCC V22 category 8 · RxHCC V08 category 20

Inpatient Payment Groups (MS-DRG)

Potential MS-DRG participation — not a DRG assignment.

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v43, Appendix B.

  • MS-DRG 374 — DIGESTIVE MALIGNANCY WITH MCC (MDC 06)
  • MS-DRG 375 — DIGESTIVE MALIGNANCY WITH CC (MDC 06)
  • MS-DRG 376 — DIGESTIVE MALIGNANCY WITHOUT CC/MCC (MDC 06)

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Risk Adjustment (CMS-HCC)

Payment categories this diagnosis maps to under CMS-HCC V28, payment year 2026.

  • HCC 17 — Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic (supersedes HCC 18, HCC 19, HCC 20, HCC 21, HCC 22, HCC 23)

Other models: CMS-HCC V22 HCC 8 · RxHCC V08 HCC 20

Risk scores depend on the enrollee's full accepted diagnosis set and segment; a category mapping alone does not determine payment.

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for C78.6 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on C78.6 itself; “inherited from” names the category or block whose note applies here.

Excludes1 — Not Coded Here

Conditions generally not reported together with this code (Excludes1) -- an error unless the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Source: inherited from C78

Excludes2 — Not Included Here

Conditions not covered by this code, but which may be reported alongside it when both are present.

Source: inherited from C78

Coder workflow for C78.6

MedCoder structured workflow — derived from this code’s own official record

Before you code C78.6

  1. Confirm the documented site and behavior (this code’s block classifies malignant neoplasms), whether the site is primary or secondary, and whether the malignancy is current or a personal history: a previously excised malignancy with no further treatment and no evidence of disease is history, not current disease. Verify the site against the Table of Neoplasms. Neoplasm chapter guidelines (Guidelines I.C.2.a, I.C.2.d, I.C.2.m).

    See the Table of Neoplasms entries · Guide: Active cancer vs history of cancer →

  2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with C78.6. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

    See the official tabular notes

Choose the right path

  1. Does the documentation support a condition named in C78.6’s Excludes1 note?
    Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
    No → Continue.

    ReviewC7B.02, C7B.04

Consider C78.6. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement
Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).
Anatomical site
At the most specific level the record states; a site the classification separates cannot be assumed from a procedure or a measurement.
Behavior and current-versus-history status
Malignant, in situ, benign or uncertain; primary or secondary; current disease or personal history (Guidelines I.C.2.m).

Official instructions as workflow

  • Excludes1 — check before selecting C78.6(2 notes)

    Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with C78.6: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

    CompareC7B.02, C7B.04

    See the official tabular notes · Guidelines I.A.12.a

  • Excludes2 — not part of C78.6(1 note)

    Coding workflow: The conditions named in this note are not included in C78.6. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

    CompareC77.0

    See the official tabular notes · Guidelines I.A.12.b

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Documentation: Both the condition C78.6 describes and a condition named in its Excludes1 note are documented for the same encounter.

Coding question: Can both codes be reported?

Path: Review the Excludes1 note and the excluded code, and look for a provider statement on whether the two conditions are related.

Reason: Excludes1 means the two are not coded together; the exception is when the record shows the conditions are unrelated to each other (Guidelines I.A.12.a).

ReviewC7B.02, C7B.04

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Code Overview

Secondary malignant neoplasm of retroperitoneum and peritoneum is a billable ICD-10-CM diagnosis code (C78.6).

MedCoder summary Summary composed by MedCoder from this code's official ICD-10-CM record. The tabular instructional notes themselves appear verbatim below.

Indexed Clinical Terms (2)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

Neoplasm Table Sites (21)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Anatomical sites from the official ICD-10-CM Table of Neoplasms that map to this code, with the matching behavior column.

  • Neoplasm, neoplastic, cavity, peritoneal — Malignant, Secondary
  • Neoplasm, neoplastic, connective tissue NEC, retroperitoneum — Malignant, Secondary
  • Neoplasm, neoplastic, cul-de-sac (Douglas') — Malignant, Secondary
  • Neoplasm, neoplastic, Douglas' cul-de-sac or pouch — Malignant, Secondary
  • Neoplasm, neoplastic, mesentery, mesenteric — Malignant, Secondary
  • Neoplasm, neoplastic, mesoappendix — Malignant, Secondary
  • Neoplasm, neoplastic, mesocolon — Malignant, Secondary
  • Neoplasm, neoplastic, omentum — Malignant, Secondary
  • Neoplasm, neoplastic, paranephric — Malignant, Secondary
  • Neoplasm, neoplastic, periadrenal (tissue) — Malignant, Secondary
  • Neoplasm, neoplastic, perinephric — Malignant, Secondary
  • Neoplasm, neoplastic, peripancreatic — Malignant, Secondary
  • Neoplasm, neoplastic, perirenal (tissue) — Malignant, Secondary
  • Neoplasm, neoplastic, peritoneum, peritoneal (cavity) — Malignant, Secondary
  • Neoplasm, neoplastic, peritoneum, peritoneal (cavity), parietal — Malignant, Secondary
  • Neoplasm, neoplastic, peritoneum, peritoneal (cavity), pelvic — Malignant, Secondary
  • Neoplasm, neoplastic, peritoneum, peritoneal (cavity), specified part NEC — Malignant, Secondary
  • Neoplasm, neoplastic, rectouterine pouch — Malignant, Secondary
  • Neoplasm, neoplastic, retrocecal — Malignant, Secondary
  • Neoplasm, neoplastic, retroperitoneal (space) (tissue) — Malignant, Secondary
  • Neoplasm, neoplastic, retroperitoneum — Malignant, Secondary

Verify Before Coding

  • CC as a secondary diagnosis (FY2026). Can raise the stay's MS-DRG severity tier.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name C78.6 or its code family, from the CMS ICD-10-CM tabular instructional notes and the CMS Table of Neoplasms. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Code First instruction: R18.0 — Malignant ascites.

Each of these codes carries a Code First note naming this condition — when that code is reported, THIS code is sequenced first, ahead of it.

Referenced by 8 Same-site neoplasm codes: C48.0 — Malignant neoplasm of retroperitoneumCompare C78.6 vs C48.0 →, C48.1 — Malignant neoplasm of specified parts of peritoneumCompare C78.6 vs C48.1 →, C48.2 — Malignant neoplasm of peritoneum, unspecifiedCompare C78.6 vs C48.2 →, D20.0 — Benign neoplasm of soft tissue of retroperitoneumCompare C78.6 vs D20.0 →, D20.1 — Benign neoplasm of soft tissue of peritoneumCompare C78.6 vs D20.1 →, D48.3 — Neoplasm of uncertain behavior of retroperitoneumCompare C78.6 vs D48.3 →, D48.4 — Neoplasm of uncertain behavior of peritoneumCompare C78.6 vs D48.4 →, D49.0 — Neoplasm of unspecified behavior of digestive systemCompare C78.6 vs D49.0 →.

These codes represent related neoplasm behaviors affecting the same anatomic site.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: CC — Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 22 clinically related codes on its CMS exclusion list.

Named in the grouper logic of 3 MS-DRGs: DRG 374 (MDC 06), DRG 375 (MDC 06), DRG 376 (MDC 06).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):NEO070 — Secondary malignancies (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical process (MS-DRG)

Acts as CC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

C45.7 — Mesothelioma of other sites, C45.9 — Mesothelioma, unspecified, C76.8 — Malignant neoplasm of other specified ill-defined sites, C78.80 — Secondary malignant neoplasm of unspecified digestive organ, C78.89 — Secondary malignant neoplasm of other digestive organs, C79.89 — Secondary malignant neoplasm of other specified sites, C79.9 — Secondary malignant neoplasm of unspecified site, C7A.00 — Malignant carcinoid tumor of unspecified site, C7A.098 — Malignant carcinoid tumors of other sites, C7A.1 — Malignant poorly differentiated neuroendocrine tumors, C7A.8 — Other malignant neuroendocrine tumors, C7B.00 — Secondary carcinoid tumors, unspecified site, C7B.04 — Secondary carcinoid tumors of peritoneum, C7B.09 — Secondary carcinoid tumors of other sites, C7B.1 — Secondary Merkel cell carcinoma, C7B.8 — Other secondary neuroendocrine tumors, C80.0 — Disseminated malignant neoplasm, unspecified, C80.1 — Malignant (primary) neoplasm, unspecified, D49.81 — Neoplasm of unspecified behavior of retina and choroid, D49.89 — Neoplasm of unspecified behavior of other specified sites, +1 more

Same CMS-HCC risk category (V28)

CMS maps these diagnoses to the same Hierarchical Condition Category (Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic) for risk-adjusted payment.

C77.8 — Secondary and unspecified malignant neoplasm of lymph nodes of multiple regions, C78.00 — Secondary malignant neoplasm of unspecified lung, C78.01 — Secondary malignant neoplasm of right lung, C78.02 — Secondary malignant neoplasm of left lung, C78.1 — Secondary malignant neoplasm of mediastinum, C78.2 — Secondary malignant neoplasm of pleura, C78.30 — Secondary malignant neoplasm of unspecified respiratory organ, C78.39 — Secondary malignant neoplasm of other respiratory organs, C78.4 — Secondary malignant neoplasm of small intestine, C78.5 — Secondary malignant neoplasm of large intestine and rectum, C78.7 — Secondary malignant neoplasm of liver and intrahepatic bile duct, C78.80 — Secondary malignant neoplasm of unspecified digestive organ, C78.89 — Secondary malignant neoplasm of other digestive organs, C79.00 — Secondary malignant neoplasm of unspecified kidney and renal pelvis, C79.01 — Secondary malignant neoplasm of right kidney and renal pelvis, C79.02 — Secondary malignant neoplasm of left kidney and renal pelvis, C79.31 — Secondary malignant neoplasm of brain, C79.32 — Secondary malignant neoplasm of cerebral meninges, C79.40 — Secondary malignant neoplasm of unspecified part of nervous system, C79.49 — Secondary malignant neoplasm of other parts of nervous system, +57 more

Related risk categories

These categories interact through CMS's HCC hierarchy — one can suppress the other's risk-adjustment weight when both are present on a claim.

Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid, Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers, Lung and Other Severe Cancers, Lymphoma and Other Cancers, Bladder, Colorectal, and Other Cancers, Prostate, Breast, and Other Cancers and Tumors

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Secondary malignancies).

C77.9 — Secondary and unspecified malignant neoplasm of lymph node, unspecified, C78.00 — Secondary malignant neoplasm of unspecified lung, C78.01 — Secondary malignant neoplasm of right lung, C78.02 — Secondary malignant neoplasm of left lung, C78.1 — Secondary malignant neoplasm of mediastinum, C78.2 — Secondary malignant neoplasm of pleura, C78.30 — Secondary malignant neoplasm of unspecified respiratory organ, C78.39 — Secondary malignant neoplasm of other respiratory organs, C78.4 — Secondary malignant neoplasm of small intestine, C78.5 — Secondary malignant neoplasm of large intestine and rectum, C78.7 — Secondary malignant neoplasm of liver and intrahepatic bile duct, C78.80 — Secondary malignant neoplasm of unspecified digestive organ, C78.89 — Secondary malignant neoplasm of other digestive organs, C79.00 — Secondary malignant neoplasm of unspecified kidney and renal pelvis, C79.01 — Secondary malignant neoplasm of right kidney and renal pelvis, C79.02 — Secondary malignant neoplasm of left kidney and renal pelvis, C79.10 — Secondary malignant neoplasm of unspecified urinary organs, C79.11 — Secondary malignant neoplasm of bladder, C79.19 — Secondary malignant neoplasm of other urinary organs, C79.2 — Secondary malignant neoplasm of skin, +24 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main term “Carcinomatosis”; these codes share that main term but sit in a different category of the Tabular List.

C80.0 — Disseminated malignant neoplasm, unspecified

Lab tests where this diagnosis supports Medicare coverage (NCD)

Medicare's National Coverage Determination (NCD) program lists this diagnosis as medical justification for these lab tests.

Iron Panel, Pregnancy Test

Contextual Map

Every relationship of C78.6 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run C78.6 with this related code in Claim Check

Hierarchy

Same site

Referenced by Code First instructions

  • R18.0 — Malignant ascites[Code First]: “secondary malignant neoplasm of retroperitoneum and peritoneum (C78.6)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Referenced by Same-site neoplasm codes

Clinical classification (CCSR)

Risk adjustment (CMS-HCC)

  • HCC 17 — Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic [CMS-HCC]: “Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic — supersedes HCC 18 (Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid), HCC 19 (Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers), HCC 20 (Lung and Other Severe Cancers), HCC 21 (Lymphoma and Other Cancers), HCC 22 (Bladder, Colorectal, and Other Cancers), HCC 23 (Prostate, Breast, and Other Cancers and Tumors)”— CMS-HCC V28 · 2026

MS-DRG Grouper

  • CC — Complication or Comorbidity [MS-DRG severity]: “As a secondary diagnosis this code can raise the stay's MS-DRG severity tier (CC).”— CMS MS-DRG Definitions Manual (Appendix C) · FY2026
  • DRG 374 — DIGESTIVE MALIGNANCY WITH MCC[MS-DRG]: “DIGESTIVE MALIGNANCY WITH MCC (MDC 06)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
  • DRG 375 — DIGESTIVE MALIGNANCY WITH CC[MS-DRG]: “DIGESTIVE MALIGNANCY WITH CC (MDC 06)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
  • DRG 376 — DIGESTIVE MALIGNANCY WITHOUT CC/MCC[MS-DRG]: “DIGESTIVE MALIGNANCY WITHOUT CC/MCC (MDC 06)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026

MDC crossing

  • MDC 06 — Diseases and Disorders of the Digestive System[MDC crossing]: “Diseases and Disorders of the Digestive System — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 5,744 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Neoplasm sites (21)

  • Neoplasm, neoplastic, cavity, peritoneal — Neoplasm, neoplastic, cavity, peritoneal — Malignant, Secondary [Neoplasm table]: “Neoplasm, neoplastic, cavity, peritoneal (Malignant, Secondary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • Neoplasm, neoplastic, connective tissue NEC, retroperitoneum — Neoplasm, neoplastic, connective tissue NEC, retroperitoneum — Malignant, Secondary [Neoplasm table]: “Neoplasm, neoplastic, connective tissue NEC, retroperitoneum (Malignant, Secondary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • Neoplasm, neoplastic, cul-de-sac (Douglas') — Neoplasm, neoplastic, cul-de-sac (Douglas') — Malignant, Secondary [Neoplasm table]: “Neoplasm, neoplastic, cul-de-sac (Douglas') (Malignant, Secondary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • Neoplasm, neoplastic, Douglas' cul-de-sac or pouch — Neoplasm, neoplastic, Douglas' cul-de-sac or pouch — Malignant, Secondary [Neoplasm table]: “Neoplasm, neoplastic, Douglas' cul-de-sac or pouch (Malignant, Secondary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • Neoplasm, neoplastic, mesentery, mesenteric — Neoplasm, neoplastic, mesentery, mesenteric — Malignant, Secondary [Neoplasm table]: “Neoplasm, neoplastic, mesentery, mesenteric (Malignant, Secondary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • Neoplasm, neoplastic, mesoappendix — Neoplasm, neoplastic, mesoappendix — Malignant, Secondary [Neoplasm table]: “Neoplasm, neoplastic, mesoappendix (Malignant, Secondary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • Neoplasm, neoplastic, mesocolon — Neoplasm, neoplastic, mesocolon — Malignant, Secondary [Neoplasm table]: “Neoplasm, neoplastic, mesocolon (Malignant, Secondary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • Neoplasm, neoplastic, omentum — Neoplasm, neoplastic, omentum — Malignant, Secondary [Neoplasm table]: “Neoplasm, neoplastic, omentum (Malignant, Secondary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • and 13 more

Index entries

  • Carcinomatosis, peritonei[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Pseudomyxoma peritonei[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (18)

Change history

  • FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data
CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page
Inpatient payment groups Official source data
CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page
Risk adjustment Official source data
2026 Mid-Year Final ICD-10 Mappings + Model Software (cms.gov/files/zip/2026-midyear-final-icd-10-mappings.zip, 2026-midyear-final-model-software.zip) — CMS-HCC V28, PY2026 mid-year final release Release, file and checksum · Publisher’s page
Change history and date-of-service validity Official source data
CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page
Comparisons, relationships and the contextual map MedCoder structured relationship
Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears
Summary and FAQ answers MedCoder explanation
Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Cite this page

Reference this page in a research guide, syllabus or article. The release is included because code content changes each year.

MedCoder.ai. "C78.6 — Secondary malignant neoplasm of retroperitoneum and peritoneum." ICD-10-CM FY2026. https://medcoder.ai/icd10/code/c78.6-secondary-malignant-neoplasm-of-retroperitoneum-and-peritoneum

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Secondary malignant neoplasm of retroperitoneum and peritoneum

No changes since FY2016 — additions, deletions, description changes and billable-status changes are tracked through FY2027 (effective October 1, 2026), and none are recorded for this code. Note changes are tracked from FY2027 only.

Nearest Codes in This Family

Official ICD-10-CM classifications closest to C78.6 in its code family, with their registry titles.

View all codes in the C78 family