14.1 Solid Tumor Rules: Purpose, Site Groups, Navigation & Timing Principles

What the rules are for

The Solid Tumor Rules replaced the 2007 Multiple Primary and Histology (MP/H) rules, site group by site group. The general instructions set the scope:

• The purpose is to determine multiple primaries and code histology only. The rules are not used for reportability, casefinding, stage or grade.
• Staging systems are not used to decide the number of primaries or the histology.
• The rules do not apply to lymphomas or leukemias of any site (9590–9993). Those use the Hematopoietic manual and database (Section 14.4).
• The rules are revised every year. Use the most recent version as soon as it is released; previous versions are archived and should not be used. From 2025, the rules are published only as one consolidated manual.

Which rule set applies

When tumors are diagnosed in different years, use the later diagnosis year to choose the rule set. For example, a lung tumor from 2016 and a new lung tumor in 2026 are evaluated with the Solid Tumor Rules.

How each site module is organized

1. Equivalent Terms and Definitions
   • Coding notes and important changes.
   • Equivalent terms and terms that are not equivalent.
   • Tables: primary site codes (for example, the breast clock-position table), combination histology codes, histology tables of NOS terms, synonyms and subtypes/variants, non-reportable histologies, paired sites.
   • Illustrations.
2. Multiple Primary (M) Rules, split into three sections:
   • Unknown if Single or Multiple Tumors. Use this only after all sources are exhausted, for example death certificate only or a pathology report alone.
   • Single Tumor. "A single tumor is always a single primary," even if it overlaps subsites or has in situ and invasive or several histologic components.
   • Multiple Tumors. These may be a single primary or multiple primaries.
3. Histology (H) Rules, split into Single Tumor and Multiple Tumors Abstracted as a Single Primary, each with a documentation priority list.

Order of use

For multiple tumors:

1. Use the histology rules to give each tumor a "working" histology.
2. Use the M rules to decide single versus multiple primaries.
3. Code the final histology for each resulting primary using the priority list and the H rules.

The hierarchy

The rules are hierarchical. Start at the first rule of the section that fits, use the first rule that applies, and stop. Do not skip ahead to a rule that seems more specific. Footnotes tell you the outcome: "prepare one abstract" for a single primary, or "prepare two or more abstracts" for multiple primaries.

General instructions that apply everywhere

• Metastases are not counted. Each M rules section begins with a note that the rules are not used for tumors described as metastases. STR example: a left lung tumor diagnosed as metastatic breast cancer in a known breast cancer patient is a metastasis of that primary, so the lung rules are not used.
• Number of tumors: ignore separate microscopic foci when counting tumors. Collision tumors (two tumors that grew together) count as two tumors, so use the Multiple Tumors section.
• Biomarkers: do not code multiple primaries or histology based on biomarkers.
• Physician staging: do not use it to decide multiple primaries.
• Physician's word "recurrence": several rules say to follow the rules even if the physician calls a new tumor a recurrence. For example, colon rule M10 and the in situ-then-invasive 60-day rules make such tumors new primaries.
• Equivalent terms: multicentric = multifocal; simultaneous = synchronous = "prior to first course treatment." Tumor, mass, lesion, neoplasm and nodule are used only to count tumors for multiple primaries, never for casefinding or reportability.

Timing rules

Many modules include timing rules, and the intervals differ by site:

Definitions from the general instructions:

• One year = one calendar year. A tumor diagnosed 1/1/2018 and a second diagnosed 1/1/2019 are exactly one year apart, which is not "more than one year." 1/2/2019 would be more than one year.
• Clinically disease-free means no evidence of recurrence on follow-up.
• The clock restarts at each recurrence. If the cancer recurred within the interval, count the disease-free time from the last recurrence, not from the original diagnosis.
• Unknown recurrence status: if it is not documented whether the patient had a recurrence, use the date of diagnosis to compute the interval.

Worked example

A patient had sigmoid colon adenocarcinoma resected in March 2023. A liver recurrence was treated in June 2023, and a new sigmoid tumor away from the anastomosis is diagnosed in May 2024.

• Colon M10 requires the patient to be disease-free for more than one year from the diagnosis or last recurrence. From June 2023 to May 2024 is less than one year, so M10 does not apply.
• The tumor is in the same segment (C187), with the same histology, so the later rules lead to a single primary. The new tumor is recorded as a recurrence.