Why Are There Primary Site Coding Instructions for Lymphomas?

Every case abstracted in a cancer registry must include a primary site. For leukemias, this is always coded as C421. For lymphomas, however, standardized coding instructions were created to ensure registrars assign the primary site consistently.

Because lymphomas often involve multiple areas, it is usually impossible to know where the disease first began. The Primary Site coding instructions provide a uniform set of rules so that all registrars record this information in the same way, ensuring consistency across cases.

As a reminder, all leukemias, myelodysplastic syndromes, myeloproliferative neoplasms, and a few other bone marrow neoplasms, have a default primary site of C421 (bone marrow). This information can be found in the Hematopoietic database. Primary site assignments for these neoplasms are not included in the Hematopoietic manual.

When determining which PH rules would apply to a case, go to the Heme DB FIRST to get your histology(ies), AND information on primary site. Many of the lymphomas list the most common primary sites in the database. Also, while working through the M rules, you may be referred you to a specific PH rule.

The primary site and histology rules (PH rules) are presented in 7 different modules. Modules 1-6 are all histology based, so if you do not have one of the histologies listed there, move to the next module.

Module 7 covers the general primary site instructions for lymphomas, and several other neoplasms (excludes leukemia, MDS, MPN, and other bone marrow neoplasms). Several PH rules reference Module 7 for determining the primary site. The PH rules are primarily for lymphomas; however, they can be used for all Hematopoietic neoplasms that are not bone marrow neoplasms or already have an assigned primary site.

Of all the Hematopoietic neoplasms, lymphomas are the hardest to determine a primary site. If lymphomas are limited to a single lymph node, or lymph node chain/region, assigning primary site is straightforward. Yet many lymphomas, especially late stage lymphomas, present with multiple lymph nodes, organs, and/or tissues involved. In these situations, it is usually not possible to determine exactly where the lymphoma originated; however, a primary site still needs to be assigned.

  • Physicians DO NOT determine primary sites for lymphomas this way. They may state an “extranodal lymphoma” or a “nodal lymphoma.” Physicians are more concerned about the histology and the stage, and many times will not try to determine the primary site. As stated above, it can be almost impossible to determine a primary site with precision when there is an advanced or late stage lymphoma.

Prior to using Module 7, use the Hematopoietic Database to determine what the common primary sites are. For example: Follicular lymphomas have common sites of involvement listed as lymph nodes, Waldeyer ring, skin, duodenum, ocular adnexa, breast and testis, whereas primary effusion lymphoma has common sites of involvement listed as body cavities (pleural, pericardial, and peritoneal). Reviewing the common primary sites will help narrow down the primary site. Of course, there can be uncommon primary sites.

The most important rule to follow when assigning primary site for lymphomas, etc. is do NOT assign primary site based on where the biopsy was performed. To determine a diagnosis, the most accessible site will be biopsied. The purpose of the biopsy is to get the histological diagnosis. This does not mean it is the primary site.

Imaging is critical in assigning primary site for lymphomas. Scans will show where there is lymphadenopathy or organ involvement. Bone marrow can only be assessed with a bone marrow biopsy. Imaging will provide the best information in determining the primary site and/or metastatic sites.

Primary vs Metastatic Sites

If you had colon cancer with liver involvement, there would be no question on what the primary site is and what to do with the liver involvement. This same concept applies to lymphomas.

Like Solid Tumors, lymphomas have common metastatic sites which include bone, brain (CNS), liver, lung, distant lymph nodes and/or bone marrow. When trying to determine primary site, if these common metastatic sites are involved, set them aside for determining primary site, as they probably indicate metastatic disease.

Lymphomas originating in these organs, bone, brain (CNS), liver, lung, or bone marrow, are extremely rare. Metastatic site involvement for hematopoietic neoplasms, like solid tumors, are recorded in stage and the Mets at Dx data items. They do not factor into primary site.

Mets at Dx data items include

  • Mets at Dx-Bone
  • Mets at Dx-Brain
  • Mets at Dx-Liver
  • Mets at Dx-Lung
  • Mets at Dx-Distant lymph nodes
    • Note: If primary site is C770-C779, this data item is coded to 8
  • Mets at Dx-Other
    • Note 1: Bone marrow involvement is coded as 1.
    • Note 2: Spleen involvement is NOT metastatic disease for lymphomas.
    • Exception: Do not code 1 when the primary site is bone marrow (C421)
  • All Mets at Dx fields are coded to 8 when primary site is C421

Note: It is possible for lymphomas, like solid tumors, to originate in common metastatic sites.

  • Example: Pathology report states Primary large B-cell lymphoma of the CNS (PCNS-LBCL). Imaging states no other areas of involvement. Assign primary site C719.

Updated: December 2, 2025