ICD-10 Code C18.7: Malignant Neoplasm of Sigmoid Colon

Key Takeaways

ICD-10 Code C18.7 is the billable ICD-10-CM diagnosis code for malignant neoplasm of sigmoid colon, valid for all dates of service from October 1, 2015 onward.

C18.7 includes the sigmoid flexure as an applicable site; synonyms include adenocarcinoma and carcinoma of sigmoid colon.

When sigmoid colon cancer has metastasized, C18.7 codes the primary site – secondary sites require additional codes such as C78.7 for liver metastasis.

Pabau’s claims management software supports accurate ICD-10 code assignment and documentation workflows for oncology and gastroenterology practices.

Sigmoid colon cancer accounts for roughly 25% of all colorectal malignancies, yet coding errors at this site remain a persistent source of claim denials. Whether the confusion stems from choosing between C18.7 and C18.9, mishandling metastatic sequencing, or missing the applicable synonym for sigmoid flexure, the downstream billing impact is real. ICD-10 Code C18.7 covers a specific anatomical site with precise rules that every coder handling oncology encounters needs to know.

This reference covers the official code description, classification hierarchy, applicable synonyms, excludes notes, documentation requirements, related C18 codes, and the MS-DRG billing context for C18.7. All information reflects the 2026 ICD-10-CM tabular list as maintained by the Centers for Medicare and Medicaid Services (CMS) and the National Center for Health Statistics (NCHS).

ICD-10 Code C18.7: Definition and Clinical Classification

Official code description: Malignant neoplasm of sigmoid colon.

ICD-10 Code C18.7 is a billable, specific ICD-10-CM diagnosis code. It falls within the Neoplasms chapter (C00-D49), under the Malignant Neoplasms of Digestive Organs block (C15-C26), and specifically within the Malignant Neoplasm of Colon category (C18). The World Health Organization’s ICD-10 classification establishes the global framework from which ICD-10-CM derives its structure for use in the United States.

The sigmoid colon is the S-shaped distal segment of the large intestine connecting the descending colon to the rectum. Tumors arising here are distinct from those of the descending colon (C18.6) or the rectosigmoid junction (C19). Accurate anatomical site identification in pathology and operative reports is the foundation for selecting C18.7 rather than an unspecified colon code.

Reimbursement claims with a date of service on or after October 1, 2015 require ICD-10-CM codes, making C18.7 the appropriate code for all current sigmoid colon malignancy encounters. The claims management software used by a practice must support ICD-10-CM code sets to process these encounters without rejection.

Applicable To Notes and Clinical Synonyms

The ICD-10-CM tabular list includes an “Applicable To” note directly under C18.7 that expands its valid scope. Coders must treat this note as an official part of the code’s definition, not a suggestion.

Applicable To: Malignant neoplasm of sigmoid flexure. The sigmoid flexure is an alternate anatomical term for the sigmoid colon itself. When a pathology or operative report references the sigmoid flexure, C18.7 is the correct code, not a separate or unspecified code.

Beyond the official Applicable To note, commonly used clinical synonyms accepted by coding references include:

• Adenocarcinoma of sigmoid colon
• Carcinoma of sigmoid colon
• Primary malignant neoplasm of sigmoid colon
• Sigmoid colon cancer
• Malignant tumor of sigmoid colon

These synonyms typically drive index-based lookups in the ICD-10-CM Alphabetic Index under the Neoplasm Table. When searching for “adenocarcinoma, colon, sigmoid,” the index directs coders to C18.7 for the primary malignant column. Coders should verify the histological term used in the operative or pathology report before finalizing the code.

Type 1 Excludes note: C18.7 carries a Type 1 Excludes note for malignant neoplasm of rectosigmoid junction (C19). A Type 1 Excludes means the two conditions cannot be coded together. When a tumor involves the rectosigmoid junction specifically, C19 replaces C18.7. If documentation is ambiguous about whether the tumor is purely sigmoid or extends into the junction, the coder should query the treating physician before assigning the code.

Documentation Requirements for Sigmoid Colon Malignancy

Correct assignment of C18.7 depends on specific information appearing in the medical record. Missing any of these elements is the most common reason auditors challenge a code assignment.

1. Confirmed malignancy. The diagnosis must be documented as malignant, not “suspected,” “possible,” or “rule out.” For inpatient encounters, coders may code conditions documented as probable or suspected per ICD-10-CM Official Guidelines Section II.H. For outpatient encounters, only confirmed diagnoses are coded.
2. Anatomical site specificity. The record must identify the sigmoid colon (or sigmoid flexure) as the primary site. Operative reports, pathology reports, colonoscopy findings, or radiology interpretations are acceptable sources. Vague references to “colon cancer” without site specification default to C18.9 (malignant neoplasm of colon, unspecified).
3. Primary vs. secondary designation. ICD-10-CM Official Guidelines require that the primary site be coded when known. C18.7 codes the primary tumor. If the sigmoid colon cancer has spread, the primary C18.7 code is sequenced first, followed by secondary malignancy codes for each affected distant site (e.g., C78.7 for liver metastasis).
4. Histological type (when affecting code selection). For most epithelial tumors including adenocarcinoma, C18.7 is the appropriate code regardless of histological subtype. However, for mesenchymal tumors such as leiomyosarcoma, the site of origin guides code selection. When the tumor arises from the muscularis propria of the sigmoid colon, coding to the anatomical site (C18.7) may be appropriate; when arising from soft tissue with adjacent colon involvement, C49.4 (malignant neoplasm of connective and soft tissue of abdomen) may apply. Because this is an area of coding ambiguity flagged in community discussions, consult the AHA Coding Clinic or your payer’s policy before assigning one over the other.

Practices using AI-assisted clinical documentation tools can improve the specificity of their notes at the point of care, reducing the need for retrospective queries. When encounter notes consistently capture anatomical site, laterality, and primary versus secondary malignancy status, downstream coding becomes more straightforward for the billing team.

C18.7 vs. Related Colon Cancer Codes

The C18 category contains ten codes for colon malignancies, each tied to a distinct anatomical segment. Selecting the wrong subcode is one of the most common audit findings in oncology billing. The key differentiator is always the documented primary site.

C18.7 vs. C18.9 (the most common coding decision): C18.9 is appropriate only when site-specific documentation is genuinely absent. If the physician’s note, colonoscopy report, or pathology identifies the sigmoid colon, C18.7 is required. Assigning C18.9 when C18.7 is supportable is a coding error that can trigger payer audits and post-payment recoupments. The CDC/NCHS ICD-10-CM web tool allows coders to verify the official tabular list hierarchy for both codes before finalizing an encounter.

Metastatic sigmoid colon cancer sequencing: When a patient presents with sigmoid colon cancer that has spread to a distant organ, sequence C18.7 as the principal diagnosis for the primary tumor encounter. Code the metastatic sites (e.g., C78.7 for liver, C78.5 for large intestine) as additional diagnoses. Per ICD-10-CM Official Guidelines Section I.C.2, when the primary malignancy is still present and under treatment, it retains principal diagnosis status. If the primary has been excised and the patient is receiving treatment for metastatic disease only, the secondary code becomes the principal diagnosis.

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Billing Context and MS-DRG Groupings

ICD-10 Code C18.7 carries specific billing implications beyond the outpatient encounter. For inpatient claims, CMS groups diagnosis codes into Medicare Severity Diagnosis Related Groups (MS-DRGs) that determine reimbursement for hospital stays. According to icd10data.com, C18.7 is grouped within MS-DRG v43.0, though the specific DRG assigned depends on the procedures performed during the inpatient stay and the presence of major comorbidities or complications (MCC/CC).

For outpatient and physician billing, the code is paired with procedure codes to justify medical necessity. Common CPT code pairings for sigmoid colon malignancy encounters include:

• 45378 Colonoscopy, diagnostic (for surveillance or initial detection)
• 45385 Colonoscopy with removal of tumor by snare technique
• 44143 / 44145 Colectomy, partial, with sigmoid colectomy
• 77427 Radiation treatment management (when radiation is the treatment modality)
• 96413 Chemotherapy administration, intravenous infusion

Each procedure must be supported by clinical documentation matching the diagnosis. Practices should confirm payer-specific local coverage determinations (LCDs) and national coverage determinations (NCDs) before submitting, as some payers require pre-authorization for surgical or chemotherapy encounters billed with C18.7. The AAPC Codify ICD-10-CM lookup provides chapter-specific coding guidelines useful for reviewing official guidance before claim submission.

HIPAA mandates use of the current ICD-10-CM code set for all covered electronic transactions. Practices that have not integrated ICD-10 updates into their practice management software risk submitting claims with outdated or invalid codes, leading to rejections that extend the revenue cycle. Proactive annual code set reviews, aligned with each October 1 update cycle, protect billing accuracy.

Additional Coding Guidance: ICD-9-CM Predecessor and Code Updates

For practices that maintain historical records or conduct retrospective analyses, the ICD-9-CM predecessor to C18.7 was 153.3 (Malignant neoplasm of sigmoid colon). Claims with dates of service before October 1, 2015 used this code. No crosswalk issues affect current billing, but coders working on appeals for pre-2015 encounters should reference the legacy code to match historical documentation.

C18.7 has remained stable since ICD-10-CM implementation. The ResDAC guidance on ICD codes in Medicare files provides context on how ICD-10 transition was handled in claims data, which is relevant for practices conducting longitudinal billing audits or retrospective outcome studies. No revisions have affected C18.7 in the 2026 update cycle.

When coding for clinical trials or registry submissions involving sigmoid colon malignancy, additional Z-codes may be required alongside C18.7. Z85.038 (personal history of other malignant neoplasm of large intestine) applies when a patient has a history of sigmoid colon cancer that is in remission or has been excised. Z80.0 (family history of malignant neoplasm of digestive organs) may be relevant for preventive screening encounters. These supplemental codes do not replace C18.7 but provide payers and registries with fuller clinical context. The patient record management tools a practice uses should accommodate these multi-code encounters without truncating the diagnosis list.

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Conclusion

Sigmoid colon cancer coding hinges on a single documentation detail: whether the record explicitly names the sigmoid colon or sigmoid flexure as the primary site. Without that specificity, coders default to C18.9 and practices absorb the billing consequences.

Pabau’s claims management software helps oncology and gastroenterology practices capture site-specific diagnoses at the point of care and map them accurately to ICD-10 Code C18.7 during claim submission, reducing audit risk and denial rates. To see how Pabau handles complex multi-code encounters, book a demo with the team.

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