C21.8 C21.8 is a valid billable ICD-10 diagnosis code for Malignant neoplasm of overlapping sites of rectum, anus and anal canal. It is found in the 2025 version of the ICD-10 Clinical Modification (CM) and can be used in all HIPAA-covered transactions from Oct 01, 2024 - Sep 30, 2025.
↓ See below for any exclusions, inclusions or special notations
C21.8 also applies to the following:
See also, ICD-10 Table of Neoplasms.
ICD-10 code C21.8 is based on the following Tabular structure:
Chapter 2: Neoplasms
Should you use C21.8 or C218 (with or without decimal point)?
DO NOT include the decimal point when electronically filing claims as it may be rejected. Some clearinghouses may remove it for you
but to avoid having a rejected claim due to an invalid ICD-10 code, do not include the decimal point when submitting claims electronically.
NEC Not elsewhere classifiable
This abbreviation in the Tabular List represents “other specified”. When a specific code is not available for a condition, the Tabular List includes an NEC entry under a code to identify the code as the “other specified” code.
NOS Not otherwise specified
This abbreviation is the equivalent of unspecified.
This note further define, or give examples of, the content of the code or category.
List of terms is included under some codes. These terms are the conditions for which that code is to be used. The terms may be synonyms of the code title, or, in the case of “other specified” codes, the terms are a list of the various conditions assigned to that code. The inclusion terms are not necessarily exhaustive. Additional terms found only in the Alphabetic Index may also be assigned to a code.
Certain conditions have both an underlying etiology and multiple body system manifestations due to the underlying etiology. For such conditions, the ICD-10-CM has a coding convention that requires the underlying condition be sequenced first, if applicable, followed by the manifestation. Wherever such a combination exists, there is a “use additional code” note at the etiology code, and a “code first” note at the manifestation code. These instructional notes indicate the proper sequencing order of the codes, etiology followed by manifestation.
A type 1 Excludes note is a pure excludes. It means 'NOT CODED HERE!' An Excludes1 note indicates that the code excluded should never be used at the same time as the code above the Excludes1 note. An Excludes1 is used when two conditions cannot occur together, such as a congenital form versus an acquired form of the same condition.
A type 2 Excludes note represents 'Not included here'. An Excludes2 note indicates that the condition excluded is not part of the condition it is excluded from but a patient may have both conditions at the same time. When an Excludes2 note appears under a code it is acceptable to use both the code and the excluded code together.
A “code also” note instructs that two codes may be required to fully describe a condition, but this note does not provide sequencing direction. The sequencing depends on the circumstances of the encounter.
Certain conditions have both an underlying etiology and multiple body system manifestations due to the underlying etiology. For such conditions, the ICD-10-CM has a coding convention that requires the underlying condition be sequenced first, if applicable, followed by the manifestation. Wherever such a combination exists, there is a “use additional code” note at the etiology code, and a “code first” note at the manifestation code. These instructional notes indicate the proper sequencing order of the codes, etiology followed by manifestation.