A landmark study by Balch et al demonstrated the relationship between positive lymph nodes and survival time
When melanoma cells reach a lymph node (and when sentinel node biopsy is not performed) they may grow and multiply within the lymph node and eventually the node will become palpable and will be noticed either by the patient or the physician during routine examination.
The diagnosis of metastatic melanoma is then usually proven by needle biopsy cytology and after some investigations the patient is advised to have all the lymph nodes in that single regional node group removed.
The operation is called lymphadenectomy.
These graphs were constructed after the lymph nodes in the lymphadenectomy specimen have been investigated by a pathologist and show how the prognosis deteriorates with the number of lymph nodes involved by metastatic melanoma.
Balch et al. Prognostic Factors Analysis of 17,600 Melanoma Patients: Validation of the American Joint Committee on Cancer Melanoma Staging System. Journal of Clinical Oncology, Vol 19, Issue 16 (August), 2001: 3622-3634