Tumour morphology refers to the type of cell (histology) that has become neoplastic and its biologic activity (behaviour).
ICD-10-AM morphology (M) codes consist of five digits: the first four digits identify the histological type of the neoplasm and the fifth digit, following a slash, indicates its behaviour.
The one digit behaviour code is as follows:
Behaviour Code - Neoplasm category
/0 Benign neoplasms - D10-D36
/1 Neoplasms of uncertain and unknown behaviour - D37-D48
/2 In situ neoplasms - D00-D09
/3 Malignant neoplasms, stated or presumed to be primary - C00-C76, C80-C96
/6 Malignant neoplasms, stated or presumed to be secondary - C77-C79
/9 Malignant, uncertain whether primary or metastatic site
For example a primary neoplasm of the breast (ICD-10-AM code C50.9) should have an associated morphology code ending with a /3 behaviour character.
A morphology code must never be the principal diagnosis.
A morphology code (including the behaviour) must be appropriate to the histological type of the neoplasm.
A morphology code should always be assigned directly after the neoplasm(s) to which it relates. For instances where a /6 metastatic neoplasm morphology and neoplasm is recorded a /3 primary neoplasm site and morphology must be recorded in addition to the metastatic neoplasm and morphology code.
The behaviour of a neoplasm, indicated by the last digit of the morphology code, can change depending on the behaviour description of the neoplasm in the clinical record. For example, a 'superficial spreading non-invasive adenocarcinoma' should be coded to M8143/2 rather than M8143/3 because although the superficial spreading adenocarcinoma is normally classified as 'malignant, primary site' (/3), the description of 'non-invasive' changes the behaviour classification to /2 'carcinoma in situ'.
If a morphological diagnosis contains two histological terms which have different morphology codes, select the highest number as it is usually more specific.
Example: Transitional cell epidermoid carcinoma
Transitional cell carcinoma NOS is coded to M8120/3
Epidermoid carcinoma NOS is coded to M8070/3
In this case, the highest numerical value (M8120/3) should be used.
If the morphology of a neoplasm is unknown and is without any available histology report or clinical clarification, assign M8000/3 Neoplasm, malignant, as advised in Coding Matters Volume 12, Number 3, December 2005.