How to Use the Histology Coding Rules

We covered most of the content of the Information about the 2007 Histology Coding Rules earlier in this training module. To review these guidelines as they are printed in the 2007 MPH General Instructions.

By now:

  • You have reviewed the MPH General Instructions.
  • You have determined what the primary site is and you have reviewed the site-specific Equivalent Terms and Definitions for that site or body system. The definitions can give you important information about particular cell types or the relationships between various histologies.
  • You have used the Multiple Primaries rules modules for that site to determine how many abstracts to prepare.
  • You are ready to assign the histology code to the first abstract. (Later you will repeat this process to assign the histology code to subsequent abstracts for the case.)

Where to Get Histology Information

There is a note in the General Instructions at the end of the histology coding rules that defines the priority order for documents that may contain a histologic diagnosis.

The best documents from which to obtain histology information are the pathology report, cytology report, and other clinician documentation in the medical record, in that order.

Pathology Report

The pathology report is the most important document you have access to. There are two important concepts to remember:

  1. Take the histology information from the most representative tumor specimen examined. The definition of 'most representative specimen' is the pathologic specimen from the surgical procedure that removed the most tumor tissue. In other words, if the patient had a needle biopsy of a breast lesion followed by a lumpectomy, the lumpectomy would have the most representative specimen since it removed more tumor tissue than the needle biopsy. In contrast, if the patient had a lumpectomy followed by a mastectomy, and the mastectomy showed only a microscopic residual, the lumpectomy would be the most representative because, again, it produced the most tumor tissue.

    Note that this rule for coding the histology differs considerably from treatment rules that say to code the most extensive procedure. Do not confuse or intermingle the rules for coding multiple primaries and histology with rules for coding other parts of the abstract.

  2. Take the histology information from the final diagnosis and associated addenda and comments. This new rule is a big change for experienced registrars who in the past were told to code the most specific histology and carefully read the microscopic description (if it is available) to gather tidbits of data that could be used to assign the histology code—correctly or incorrectly.

The reason for this new rule is that, during the rules development process, the pathologists on the task force stated that when they document the microscopic description they are talking about what they see from slide to slide to slide. When they have finished looking at all the slides they put together the information that they gathered and use their expertise and experience to come up with a final diagnosis. Their comment was that there may be a more specific diagnosis in the microscopic description, but it may have been a small part of a sample seen only in one slide. Because of that, they chose not to put it into the final diagnosis because it was not representative of the entire specimen; they would never make a final diagnosis based on such a small area.

Generally, the histologic diagnosis will be final diagnosis as written. There are a few exceptions:

  • In some cases, the final diagnosis may say 'see comment', in which case the information in a comment or remark may be used to code the histology.
  • Addenda to the final diagnosis, such as additional information from a delayed laboratory report may also be used to code the histology.
  • A revised or amended diagnosis replaces the original final diagnosis, and the histology should be coded from the revised or amended diagnosis. The revised or amended diagnosis is normally issued by the pathologist who originally signed out the case. If there is a report from a consulting pathologist containing a different diagnosis but the original pathologist does not amend or revise his/her final diagnosis, the original final diagnosis is what should be coded.
  • There will be a few occasions when information from the microscopic description can be used to decide the histology code. Such occasions will be identified clearly in the site-specific rules. For example, for colon it is important to know that the carcinoma developed from a pre-existing polyp. The information about the polyp may not be in the final diagnosis but may be reported in either the gross or microscopic description.

Other Documents

  • The second most important medical document from which to code the histology is the cytology report. If there is no pathology report available, code from the cytology report. Cytology in many cases is less specific than pathology, because the cytologist is looking at individual cells rather than a tumor mass, but it is nonetheless a solid source of histology information.
  • If neither the pathology report or the cytology report is available, code from the medical record documentation that references either the pathology or the cytology. In other words, if you see a note in the medical record saying that the patient had a biopsy in the office prior to admission and that biopsy showed adenocarcinoma, the clinician is referencing the pathology.
  • The lowest priority is a medical record mention of the type of cancer. For example, this might be in the history and physical, where there is a statement that the patient is known to have adenocarcinoma of the left lung. The medical record mentions the type or histology of the cancer, but is not specifically referencing the pathology report or the cytology report.

Histology Coding Modules

With these general rules in mind about where to obtain histology information in the medical record, turn to the Histology Coding rules section in the correct set of rules for the site you are abstracting. You will notice that, similar to the modules in the Multiple Primaries rules section, there are two separate modules of histology coding rules (melanoma of skin is the exception).

  • Single Tumor (one primary site)
  • Multiple Tumors abstracted as a single primary site

Each module is an independent, complete set of rules. The rules are numbered consecutively to avoid confusion, but each module stands on its own.

  • The 'single Tumor' module is used when you have one lesion for which to code the histology. The single tumor might overlap contiguous sites, or it might have a mix of cell types, or it might be a mix of invasive and in situ cancer. All of these issues are addressed in the priorities of the 'single Tumor' rules module.
  • The "Multiple Tumors abstracted as a single primary" module is used when the multiple primaries rules indicated that the multiple tumors were regarded as a single entity and only one abstract should be prepared. (If the multiple primaries rules said that you have to prepare more than one abstract for the case, from this point forward work on only one abstract at a time.) For example, the multiple primaries rules say that a patient with multiple malignant polyps and a history of familial polyposis coli (a hereditary disease) is a single primary. To determine the histology code for this case, use the "Multiple Tumors abstracted as a single primary" module.

Priority of Histology Coding Rules

  • Once you are in the appropriate module, start reading with the first rule in that module.
  • Keep reading until you find a rule that applies to your case.
  • Use the first rule that applies and stop. Do not skip down through the rest of the rules to see if another rule might apply.

Almost invariably, the first two histology coding rules are the same in every module. Rule H1 in the 'single Tumor' module and the corresponding first rule in the "Multiple Tumors" module reiterate the priority of histology information when there is no pathology or cytology specimen (no resection or biopsy) or when the pathology or cytology report is not available (biopsy or resection performed elsewhere): code the histology documented by the physician. The documentation can be in the medical record as a statement referring to pathologic or cytologic findings, as a physician reference to a particular type of cancer, or even in a CT, MRI, or PET imaging report. If there is a specific histology mentioned in any one of these sources, code it that. If nothing more specific than "cancer" or "carcinoma" is documented, code 8000 Malignant neoplasm, NOS or 8010 Carcinoma, NOS as appropriate.

Rule H2 in the 'single Tumor' module and the corresponding second rule in the "Multiple Tumors" module establish that it is acceptable to code the histology from a metastatic site when there is no pathology or cytology specimen from the primary site.

In nearly all cases, however, there will be microscopic confirmation of the tumor either in the form of tissue (pathology) or fluids and cells (cytology). The really site-specific are prioritized after the first two baseline rules.

The theory behind prioritizing the histology coding rules is to deal with the most common situations first. As the rules go farther down the list, they apply to fewer and fewer cases. For example, in head and neck cancers, many times there is only a single histology (squamous cell carcinoma) identified, whether the abstract is covering a single tumor or multiple tumors abstracted as a single primary. When that is the case, you only have to read down a couple of rules before you find the rule that fits ("Code the histology when only one histologic type is identified") and you're done. If there is more than one histologic type identified, subsequent rules say what code to use when both invasive and in situ cancer is present, when one tumor is more invasive than another, when one of the types is a more specific term than another, and so forth, in descending priority.

In general, there are many more histology coding rules than there are multiple primaries rules, because for most sites there is a wide range of histology codes to choose from. Many of the rules deal with one or more specific histology codes. These specific histology codes are arranged according to the cell types that are most important prognostically. For example, in breast carcinoma in situ, non-infiltrating comedocarcinoma has a worse prognosis than other types of in situ carcinomas, so one of the higher priority rules is to code a case with a mix of intraductal carcinoma and non-infiltrating comedocarcinoma to 8501/2 non-infiltrating comedocarcinoma. That way, the histology code for the abstract can reflect something of the patient's prognosis.

In other sets of rules, there are specific instructions when and when not to use certain histology codes. Pathologists have specific criteria for calling a tumor mucinous or signet ring cell adenocarcinoma in the colon, adenocarcinoma versus large cell carcinoma in the lung, or regressing melanoma in the skin. Adenocarcinoma, intestinal type is a diagnosis that should not be used for colon cancers. The 2007 MPH rules were carefully developed and tested to give the proper priority to the many histologies that can develop in a given primary site.

Within each module of histology coding rules, the very last rule, "Code the histology with the numerically higher ICD-O-3 code", is intended to cover any situation not handled in rules with higher priority. This final 'catch-all' rule will be applied very infrequently; it is the 'last resort' rule in ICD-O-3 as well.