The Multiple Primary Rules (M rules)/Primary site and Histology Rules (PH rules)

The multiple primary (M) rules were developed so that there is a standardized way of counting Hematopoietic neoplasms. This is very similar to the Solid Tumor Rules. These rules may not agree with how physicians count the number of primaries. Registrars should always follow the rules in the Hematopoietic Manual, even when they disagree with the physician.

Unlike solid tumors, where an adenocarcinoma in the breast and the pancreas would be two primaries, a hematopoietic histology occurring in several different locations of the body is the same primary. This type of presentation may be seen at diagnosis or may show up as recurrence or progression.

Use the Multiple Primary (M) Rules summary table found in the Steps for Using the Heme DB and Hematopoietic Coding Manual in the Hematopoietic Manual for a summary of the multiple primary (M)rules. Most of these rules are histology based and can be ruled out very quickly. While working through the M rules, you may also encounter some that refer you to a specific PH rule.

Since the rules are difficult to comprehend at first, it is advisable to read through ALL the rules and familiarize yourself with them. There are brief headers for each rule that provides information on what the rule is about. This will help determine quickly which rules apply to a case or not.

Hematopoietic neoplasms are much more complicated than Solid Tumors in that frequently there will be many provisional diagnoses. These are all part of the initial clinical workup of the patient, which will include lymph node or organ biopsy, bone marrow biopsy, flow cytometry, genetic testing, immunophenotyping and imaging. In other words, a patient may end up with 2 or 3 diagnoses until the final diagnosis is determined.

Example: Bone marrow biopsy done, flow cytometry consistent with myeloproliferative neoplasm, MPN. (At this time, the histology is 9975/3). Genetic testing is done, and a JAK2 comes back positive for Essential Thrombocythemia (ET). (Histology is now 9962/3).

• ET is a specific Myeloproliferative neoplasm. So, this is one primary, the ET (9962/3)
• If you query 9975/3 or 9962/3 in the Hematopoietic database, you will see that these histologies are listed as the same primary for one another.

In situations like this, it is advantageous to know where the lineage tables are in the manual and how to review them.

Updated: December 2, 2025