2026 ICD-10-CM Diagnosis Code C78.6Secondary malignant neoplasm of retroperitoneum and peritoneum
ICD-10-CM Codes›C00–D49›C76-C80›C78
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 17
- Chronic Condition
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. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 22 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Secondary malignancies.
For Medicare Advantage risk adjustment, C78.6 maps to CMS-HCC Category 17 (Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic) under the V28 model, adding a risk factor of about 4.209 for a community, non-dual, aged beneficiary in payment year 2026.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
C78.6 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2026.
Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
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
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
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.
| Site | Malig. Primary | Malig. Secondary | Ca in situ | Benign | Uncertain | Unspec. |
|---|---|---|---|---|---|---|
| cavity › peritoneal | C48.2 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| connective tissue NEC › retroperitoneum | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
| cul-de-sac (Douglas') | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| Douglas' cul-de-sac or pouch | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| mesentery, mesenteric | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| mesoappendix | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| mesocolon | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| omentum | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| paranephric | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
| periadrenal (tissue) | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
| perinephric | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
| peripancreatic | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
| perirenal (tissue) | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
| peritoneum, peritoneal (cavity) | C48.2 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| peritoneum, peritoneal (cavity) › parietal | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| peritoneum, peritoneal (cavity) › pelvic | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| peritoneum, peritoneal (cavity) › specified part NEC | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| rectouterine pouch | C48.1 | C78.6 | – | D20.1 | D48.4 | D49.0 |
| retrocecal | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
| retroperitoneal (space) (tissue) | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
| retroperitoneum | C48.0 | C78.6 | – | D20.0 | D48.3 | D49.0 |
Patient EducationClinical
Cancer
Cancer is not just one disease but many diseases. Cancer begins in your cells, which are the building blocks of your body. Usually, your body forms new cells as needed, replacing old cells that die. Sometimes this process goes wrong. New cells grow even when you don't need them, and old cells don't die when they should.
The full article covers:
- What is cancer?
- What are the types of cancer?
- How does cancer develop?
- What are the symptoms of cancer?
- How is cancer diagnosed?
- What are the treatments for cancer?
- Can cancer be prevented?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert C78.6 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About C78.6Overview
What is the ICD-10 code for secondary malignant neoplasm of retroperitoneum and peritoneum?
The ICD-10-CM code for secondary malignant neoplasm of retroperitoneum and peritoneum is C78.6 (sometimes written as C786). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
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.
Is C78.6 a CC or MCC?
CMS lists C78.6 as a CC (complication or comorbidity) for FY 2026. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 22 closely related codes in its exclusion list.
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.
What HCC is C78.6?
C78.6 (secondary malignant neoplasm of retroperitoneum and peritoneum) maps to CMS-HCC Category 17 (Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic), commonly written as HCC 17, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2026. It mapped to HCC 8 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 20.
Does C78.6 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, C78.6 adds a risk adjustment factor of about 4.209 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 1.952 to 4.235 depending on the payment segment). HCC 17 sits at the top of its hierarchy, so no other condition category supersedes it. See the full factor table on the HCC 17 category page.
