2026 ICD-10-CM Diagnosis Code C78.6Secondary malignant neoplasm of retroperitoneum and peritoneum

ICD-10-CM Codes›C00–D49›C76-C80›C78

ICD-10-CM C78.6
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

C78.6 is a billable ICD-10-CM diagnosis code for secondary malignant neoplasm of retroperitoneum and peritoneum. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 374 through 376. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Secondary malignancies.

Code Identity

ICD-10-CM Code
C78.6
Billable Status
Yes — Valid for Submission
Code Describes
Secondary malignant neoplasm of retroperitoneum and peritoneum
Short Description
Secondary malignant neoplasm of retroperiton and peritoneum
Parent Code
C78
Secondary malignant neoplasm of respiratory and digestive organs

Code Classification

ChapterC00–D49Neoplasms
SectionC76-C80Malignant neoplasms of ill-defined, other secondary and unspecified sites
CategoryC78Secondary malignant neoplasm of respiratory and digestive organs
This CodeC78.6Secondary malignant neoplasm of retroperitoneum and peritoneum

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Carcinoma of peritoneum
  • Carcinomatosis of peritoneal cavity
  • Carcinomatosis of peritoneum
  • Clear cell adenocarcinoma of peritoneum
  • Invasion of neoplasm to visceral peritoneum
  • Malignant neoplasm of mesocolon
  • Malignant neoplasm of omentum
  • Malignant neoplasm of perinephric tissue
  • Malignant neoplasm of peritoneum and retroperitoneum
  • Malignant neoplasm of retrocecal tissue
  • Malignant neoplasm of the pouch of Douglas
  • Metastatic clear cell adenocarcinoma to peritoneum
  • Metastatic malignant neoplasm to omentum
  • Metastatic malignant neoplasm to parietal peritoneum
  • Metastatic malignant neoplasm to pelvic peritoneum
  • Metastatic malignant neoplasm to periadrenal tissue
  • Metastatic malignant neoplasm to perirenal tissue
  • Metastatic malignant neoplasm to peritoneum
  • Metastatic malignant neoplasm to rectouterine pouch
  • Metastatic malignant neoplasm to retrocecal tissue
  • Metastatic malignant neoplasm to retroperitoneum
  • Metastatic malignant neoplasm to retroperitoneum and peritoneum
  • Metastatic malignant neoplasm to the mesentery
  • Metastatic malignant neoplasm to the mesocolon
  • Metastatic sarcoma to retroperitoneum
  • Metastatic well-differentiated neuroendocrine neoplasm to peritoneum
  • Metastatic well-differentiated neuroendocrine tumor
  • Neoplasm of omentum
  • Neoplasm of periadrenal tissue
  • Neoplasm of perirenal tissue
  • Neoplasm of peritoneum and retroperitoneum
  • Neoplasm of rectouterine pouch
  • Neoplasm of retrocecal tissue
  • Neoplasm of the mesocolon
  • Pseudomyxoma peritonei
  • Renal tumor finding
  • Renal tumor invasion into perinephric tissue macroscopically
  • Tumor invades retroperitoneal structure

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Carcinomatosis
      • peritonei
    • Pseudomyxoma peritonei

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NEO070
Secondary malignancies
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Mesocolon

    the fold of peritoneum by which the colon is attached to the posterior abdominal wall.

  • Omentum

    a double-layered fold of peritoneum that attaches the stomach to other organs in the abdominal cavity.

  • Pseudomyxoma Peritonei

    a peritoneal adenocarcinoma characterized by build-up of mucus in the peritoneal cavity. mucus secreting cells may attach to the peritoneal lining and continue to secrete mucus. the majority of cases represent tumor spread from a primary low-grade mucinous neoplasm of the appendix (nci thesaurus).

Table of NeoplasmsClinical

Anatomical sites in the Table of Neoplasms that reference this code family.

SiteMalig.
Primary		Malig.
Secondary		Ca
in situ		BenignUncertainUnspec.
cavity › peritonealC48.2C78.6–D20.1D48.4D49.0
connective tissue NEC › retroperitoneumC48.0C78.6–D20.0D48.3D49.0
cul-de-sac (Douglas')C48.1C78.6–D20.1D48.4D49.0
Douglas' cul-de-sac or pouchC48.1C78.6–D20.1D48.4D49.0
mesentery, mesentericC48.1C78.6–D20.1D48.4D49.0
mesoappendixC48.1C78.6–D20.1D48.4D49.0
mesocolonC48.1C78.6–D20.1D48.4D49.0
omentumC48.1C78.6–D20.1D48.4D49.0
paranephricC48.0C78.6–D20.0D48.3D49.0
periadrenal (tissue)C48.0C78.6–D20.0D48.3D49.0
perinephricC48.0C78.6–D20.0D48.3D49.0
peripancreaticC48.0C78.6–D20.0D48.3D49.0
perirenal (tissue)C48.0C78.6–D20.0D48.3D49.0
peritoneum, peritoneal (cavity)C48.2C78.6–D20.1D48.4D49.0
peritoneum, peritoneal (cavity) › parietalC48.1C78.6–D20.1D48.4D49.0
peritoneum, peritoneal (cavity) › pelvicC48.1C78.6–D20.1D48.4D49.0
peritoneum, peritoneal (cavity) › specified part NECC48.1C78.6–D20.1D48.4D49.0
rectouterine pouchC48.1C78.6–D20.1D48.4D49.0
retrocecalC48.0C78.6–D20.0D48.3D49.0
retroperitoneal (space) (tissue)C48.0C78.6–D20.0D48.3D49.0
retroperitoneumC48.0C78.6–D20.0D48.3D49.0

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Convert C78.6 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
197.6 Sec mal neo peritoneum
Exact Match The mapping is direct, with no qualifiers.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About C78.6Overview

Is C78.6 a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report secondary malignant neoplasm of retroperitoneum and peritoneum on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does C78.6 group to?

When secondary malignant neoplasm of retroperitoneum and peritoneum is the principal diagnosis on an inpatient stay, it groups to MS-DRG 374, 375, 376, with relative weights from 0.9243 to 2.1387 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of C78.6?

Under the General Equivalence Mappings, secondary malignant neoplasm of retroperitoneum and peritoneum converts to ICD-9-CM 197.6 (sec mal neo peritoneum). The mapping is a direct match.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.