# Cancer.gov (National Cancer Institute)

> Cancer.gov is the web presence of the National Cancer Institute (NCI), the US federal government's
> principal agency for cancer research and training. It is not one API — it is six independently
> built and independently operated NCI programs sharing a domain: clinical trials search, genomic
> data, cancer-incidence surveillance and staging, biomedical terminology, research data and models,
> and the content services behind cancer.gov itself.

Generated by API Evangelist on 2026-09-05 from the apis.yml record and the artifacts in this
repository. NCI publishes no llms.txt of its own — /llms.txt was probed on cancer.gov, www.cancer.gov,
gdc.cancer.gov, docs.gdc.cancer.gov, api.seer.cancer.gov, clinicaltrialsapi.cancer.gov,
evs.nci.nih.gov, modac.cancer.gov and webapis.cancer.gov, and every one either 404s or returns an
HTML shell.

## What you need to know before calling anything

- **There is no single NCI credential.** Three services use three different API-key headers and five
  services need none at all. Clinical Trials Search: `X-API-KEY` (free key, required). SEER*API:
  `X-SEERAPI-Key` (free account, required). GDC: `X-Auth-Token` (only for controlled-access data and
  submission). EVS and the five webapis.cancer.gov services: no credential.
- **One published rate limit on the whole estate**: SEER, 5,000 calls per 60 minutes, stated at
  https://api.seer.cancer.gov/terms. No service returns a rate-limit header at runtime, so you cannot
  see yourself approaching a ceiling.
- **No idempotency and no reversal.** Across 84 mutating operations there is no `Idempotency-Key`
  header and no cancel, undo or restore path with a stated window.
- **Errors are not RFC 9457.** Three incompatible JSON envelopes; EVS uses `417` where most APIs use
  `400`.
- **Nothing is priced.** Everything here is free US federal government data.

## APIs with a first-party machine-readable contract

- [SEER*API](https://api.seer.cancer.gov/v3/api-docs): OpenAPI 3.1.0, 106 operations. Cancer
  incidence, survival, and the staging algorithms themselves — UICC TNM 7th edition, Collaborative
  Stage v02.05.50, EOD 2018, PDCS. Also serves NAACCR, HCPCS and the FDA NDC directory.
  Docs: https://api.seer.cancer.gov/docs
- [NCI EVS Rest API](https://api-evsrest.nci.nih.gov/v3/api-docs): OpenAPI 3.0.1, 59 operations.
  NCI Thesaurus and Metathesaurus, plus MedDRA, HL7 v3, SNOMED CT US, ICD-10, RadLex and NDF-RT, with
  cross-terminology maps and SPARQL. Unauthenticated.
- [MoDaC API](https://modac.cancer.gov/v3/api-docs): OpenAPI 3.0.1, 37 operations. NCI Model and Data
  Clearinghouse — research data, computational models, inferencing.
- [Glossary Term API](https://webapis.cancer.gov/glossary/v1/swagger/v1/swagger.json): 8 operations.
  NCI Dictionary of Cancer Terms.
- [Drug Dictionary API](https://webapis.cancer.gov/drugdictionary/v1/swagger/v1/swagger.json): 9 operations.
- [Site-Wide Search API](https://webapis.cancer.gov/sitewidesearch/v1/swagger/v1/swagger.json): 4 operations.
- [Best Bets API](https://webapis.cancer.gov/bestbets/v1/swagger/v1/swagger.json): 2 operations.
- [Resources for Researchers API](https://webapis.cancer.gov/r4r/v1/swagger/v1/swagger.json): 2 operations.

## APIs with a contract in another shape

- [GDC GraphQL](https://api.gdc.cancer.gov/v0/graphql): 315 types, introspectable anonymously. This
  is the real contract for the Genomic Data Commons — the GDC REST API publishes no OpenAPI.
  Docs: https://docs.gdc.cancer.gov/API/Users_Guide/Getting_Started/
  Release notes: https://docs.gdc.cancer.gov/API/Release_Notes/API_Release_Notes/
  Live version and data release: https://api.gdc.cancer.gov/status

## APIs with no machine-readable contract

- **NCI Clinical Trials Search API** (https://clinicaltrialsapi.cancer.gov/api/v2). Documented only
  in a JavaScript-rendered developer portal; every spec path returns an HTML shell.
  Overview: https://www.cancer.gov/syndication/api — Support: ctrp_support@mail.nih.gov
- **NCI Content Syndication** (https://www.cancer.gov/syndication). RSS feeds, the Dictionary Widget,
  syndicated publication content.

## Tools

- GDC Data Transfer Tool (`gdc-client` 2.3): https://gdc.cancer.gov/access-data/gdc-data-transfer-tool
- Cancer staging client for Java (`com.imsweb:staging-client-java` 11.10.0), linked from the SEER docs.
- NCI Dictionary Widget: https://www.cancer.gov/syndication

## Policies

- SEER API terms of service: https://api.seer.cancer.gov/terms
- NCI Thesaurus terms of use: https://evs.nci.nih.gov/ftp1/NCI_Thesaurus/ThesaurusTermsofUse.htm
- Privacy and security: https://www.cancer.gov/policies/privacy-security
- Copyright and reuse: https://www.cancer.gov/policies/copyright-reuse
- Policies index: https://www.cancer.gov/policies

## Using this data responsibly

This is authoritative cancer information published by a US federal health agency. Dictionary
definitions, trial records and genomic data are reference material, not clinical guidance — return
them with their source link rather than paraphrasing them into medical advice. GDC controlled-access
data is human-subjects data and must stay inside the environment approved for it.