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The role of lymph node dissection in the treatment of differentiated thyroid carcinoma remains controversial, and the benefit of therapy is debatable. This study was designed to identify the precise localization of lymph node micrometastases (LNMM) and map their cervical involvement in relation with the tumor location within the thyroid gland.A total of 2551 cervical lymph nodes were obtained from 80 patients with well-differentiated thyroid cancer. They were diagnosed as clear lymph nodes by hematoxylin and eosin stain and then examined immunohistochemically with cytokeratins (AE1/AE3) for evidence of micrometastases.Forty-two patients out of 80 (53%) had LNMM. Forty-eight patients (60%) had the tumor confined to only one third of 1 of the 2 lobes of the thyroid gland or isthmus. The frequencies and locations of LNMM in patients were 50% (3/6) in the deep upper cervical nodes, with tumors localized in the upper third; 31% (5/16) in the paraglandular nodes, with tumors affecting the middle third; 63% (12/19) in the paratracheal nodes, with tumors affecting the lower third of the thyroid lobe; and 71% (5/7) in the pretracheal nodes in the isthmus-located tumor. All the LNMM occurred on the ipsilateral side of the tumor.When thyroid carcinoma is located in the upper third of the thyroid lobe, the LNMM are found in the direction of upward lymphatic flow. When the tumor is located in the lower third or isthmus, LNMM are directed downward. In addition, early thyroid carcinoma micrometastases do not cross the midline but remain on the ipsilateral side of the tumor. (Surgery 2002;131:249-56).