The purpose of the Solid Tumor Rules is to determine the number of primaries to abstract and the histology to code. The most recent Solid Tumor Rules update should be used as soon as it is released and can be applied to 2018+ cases (see General Instructions for start years for each Site-group). If a specific code or instruction has an effective year later than 2018, it will be noted in the text.

The Solid Tumor Rules are revised annually to reflect new terminology, ICD-O codes, and other changes to keep in step with current clinical practice. The 2026 Solid Tumor Update includes significant improvements to the format of the histology tables. Please see the current change log for a complete list of updates.

Download the Solid Tumor Rules 2026 Update (PDF, 8.7 MB) (December 8, 2025)

Revision History

Significant formatting changes were made to the histology tables. See change log for updates made in December 2025. Please see the Revision Archive for earlier changes.

Histology Coding Clarifications

On occasion, data collection requirements of AJCC and NCI SEER have resulted in conflicting cancer coding instructions for cancer registrars. For more information and specific instructions about reviewing cases already coded, please visit the Histology Coding Clarifications page.

Suggested Citation

Dickie, L., Adams, S., Harrison, D., Hofferkamp, J., Traverso-Ortiz, M., Van Dyke, A., Negoita, S. (December 2025). Solid Tumor Rules. National Cancer Institute, Rockville, MD 20850.