2027 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 2027 (October 1, 2026 through September 30, 2027) 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 23 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 2027.
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 2027.
Source: CMS Payment Year 2027 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
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to C78.6: its own notes plus those printed at C78 and Chapter 2. A note printed at a category, block or chapter applies to every code under it.
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
Notes
General instructions on how these codes are used.
- Functional activity
- All neoplasms are classified in this chapter, whether they are functionally active or not. An additional code from Chapter 4 may be used, to identify functional activity associated with any neoplasm.
- Morphology [Histology]
- Chapter 2 classifies neoplasms primarily by site (topography), with broad groupings for behavior, malignant, in situ, benign, etc. The Table of Neoplasms should be used to identify the correct topography code. In a few cases, such as for malignant melanoma and certain neuroendocrine tumors, the morphology (histologic type) is included in the category and codes.
- Primary malignant neoplasms overlapping site boundaries
- A primary malignant neoplasm that overlaps two or more contiguous (next to each other) sites should be classified to the subcategory/code .8 ('overlapping lesion'), unless the combination is specifically indexed elsewhere. For multiple neoplasms of the same site that are not contiguous, such as tumors in different quadrants of the same breast, codes for each site should be assigned.
- Malignant neoplasm of ectopic tissue
- Malignant neoplasms of ectopic tissue are to be coded to the site mentioned, e.g., ectopic pancreatic malignant neoplasms are coded to pancreas, unspecified (C25.9).
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name C78.6, its category, or a range that includes it.
Applicable To 2
These notes mention C78.6.
Code First 1
These codes say to code first a condition named in the note, and C78.6 is one of the conditions named.
- R18.0 Malignant ascitessecondary malignant neoplasm of retroperitoneum and peritoneum (C78.6)
Index to Diseases and InjuriesGuidance
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
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 |
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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, 2026 through September 30, 2027.
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, 2026 through September 30, 2027.
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 375, 374, 376, with relative weights from 0.8925 to 2.1101 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 2027. 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 23 closely related codes in its exclusion list.
Can C78.6 be reported with C7B.02?
Not as a rule. The Excludes1 note at C78, which applies to C78.6, lists "secondary carcinoid tumors of liver (C7B.02)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
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 2027. 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.
