Thanks for clarifying. @lisakinpa's response explains well. In short the sentinel lymph node is removed and analyzed to see if cancer has spread beyond the tumor. The sentinel lymph node would be the first place that cancer cells may spread to.

Here's more information from Mayo Clinic:
- Sentinel node biopsy https://www.mayoclinic.org/tests-procedures/sentinel-node-biopsy/about/pac-20385264

"Sentinel node biopsy is a surgical procedure used to determine whether cancer has spread beyond a primary tumor into your lymphatic system. It's used most commonly in evaluating breast cancer and melanoma.

The sentinel nodes are the first few lymph nodes into which a tumor drains. Sentinel node biopsy involves injecting a tracer material that helps the surgeon locate the sentinel nodes during surgery. The sentinel nodes are removed and analyzed in a laboratory.

If the sentinel nodes are free of cancer, then cancer is unlikely to have spread, and removing additional lymph nodes is unnecessary.

If a sentinel lymph node biopsy reveals cancer, your doctor might recommend removing more lymph nodes."