13.3 ICD-O-3.2 Morphology: Histology, Behavior & the Solid Tumor Histology Rules

Structure of a morphology code

The first three digits group related tumors. For example, 814x–838x are adenocarcinomas, 805x–808x are squamous cell carcinomas, 872x–879x are nevi and melanomas, 959x–972x are lymphomas, and 980x–994x are leukemias. The fourth digit gives the specific type.

Behavior codes

Matrix concept: ICD-O's rule is to use the appropriate behavior digit with a histology code even if that exact combination is not printed in the book. For example, a pathologist's "in situ" diagnosis of a type listed only as /3 is coded /2. Registries then apply edits and the NAACCR update tables, which state which new combinations are valid and reportable.

Reportability by behavior: common traps

• Borderline ovarian tumors (for example, serous borderline tumor 8442/1) are not reportable to SEER, NPCR or CoC for 2001 and later diagnoses. Invasive ovarian carcinoma is /3.
• Non-malignant CNS tumors such as meningioma 9530/0 and vestibular schwannoma 9560/0 are reportable for 2004 and later diagnoses (Public Law 107-260, the Benign Brain Tumor Cancer Registries Amendment Act).
• GIST and thymoma: /3 for 2021 and later diagnoses.
• Pilocytic astrocytoma: coded 9421/1 for 2023 and later, and still reportable as a CNS tumor.
• Microinvasion means invasion, so the behavior is /3.
• A single tumor with in situ and invasive components is coded to the invasive histology with behavior /3.

ICD-O-3.2 and the annual update tables

North American registries adopted ICD-O-3.2 for cases diagnosed in 2021 and later. Each year NAACCR publishes the Guidelines for ICD-O-3.2 Histology Code and Behavior Update, which lists new codes, new terms and changes in behavior and reportability for that diagnosis year. The Solid Tumor Rules editors recommend coding histology using, in order: the Solid Tumor Rules, then the updated ICD-O histology tables, then ICD-O-3.2 itself. If a code still cannot be found, submit a question to Ask a SEER Registrar.

General histology rules (Solid Tumor Rules)

1. Do not use the histology rules to decide reportability.
2. Rules are split into Single Tumor and Multiple Tumors Abstracted as a Single Primary sections. Within each, the rules are hierarchical: use the first rule that applies and stop.
3. Code the histology diagnosed before neoadjuvant therapy. Chemotherapy, radiation, immunotherapy and targeted therapy can change the tumor's appearance. The exception: if the first diagnosis came from FNA, cytology, or a regional or metastatic site, and the resection after neoadjuvant therapy shows a different or more specific histology, code the resection histology.
4. Code the histology assigned by the physician. Do not change histology to make a case fit a staging schema.
5. Priority of documentation: each site module lists its own priority. Tissue pathology from the primary site comes first: addenda and comments, then the final diagnosis or synoptic report, then the CAP protocol. Cytology, imaging and physician statements come after that. When the final diagnosis and the synoptic report differ, use the more specific histology.
6. Most specific histology: code a subtype or variant (Table 3 "column 3" terms in each module) over its NOS parent. For example, invasive lobular carcinoma 8520/3 rather than carcinoma NOS. When a biopsy and a resection give two clearly different histologies, code the histology from the most representative specimen, meaning the one with more tumor.
7. Combination codes: some modules provide combination codes for specific mixtures. For example, breast carcinoma NST (duct) with lobular carcinoma is 8522, and the last-resort breast code for mixed subtypes is 8255. Adenosquamous carcinoma is 8560/3, and mixed adenoneuroendocrine carcinoma is 8244/3. Use a combination code only when the module's table lists that combination.

Histology described with ambiguous terms

The Solid Tumor Rules list of ambiguous terms for histology is: apparently, appears, comparable with, compatible with, consistent with, favor(s), malignant appearing, most likely, presumed, probable, suspect(ed), suspicious (for), and typical (of).

A histology described with one of these terms is coded only when:

• the histology is clinically confirmed by a physician (attending, pathologist, oncologist and so on); or
• the patient is treated for that histology; or
• the case is accessioned based on that single ambiguous histology, with no other histology information.

Otherwise, code the NOS histology. The Solid Tumor Rules also stress that the SEER and CoC ambiguous-terms lists decide reportability (Section 9.4), not histology.

Examples from the breast module:

• "Carcinoma NST consistent with pleomorphic carcinoma," and the oncology consult says pleomorphic carcinoma: code pleomorphic carcinoma, because it is clinically confirmed.
• "Sarcoma consistent with liposarcoma," and the treatment plan is for liposarcoma: code liposarcoma, because the patient is treated for it.
• An outpatient biopsy says "probably apocrine carcinoma," and no further information is available: code apocrine carcinoma, because it is the only histology for the accessioned case.

Worked examples