Background <p>For patients with clinical stage III melanoma, a potentially surgically curable disease, neoadjuvant immunotherapy improves event-free survival. Substantial pathologic complete response (pCR) rates in the affected nodal basin have raised interest in response-directed care including de-escalation of the extent of operation and adjuvant therapies. This study prospectively assessed the fidelity of index lymph node (ILN) pathology in predicting pathologic response across the entire nodal basin.</p> Methods <p>The ILN substudy was a preplanned prospective investigation within the multicenter NeoACTIVATE Arm C trial testing neoadjuvant doublet immunotherapy with atezolizumab and tiragolumab followed by therapeutic lymph node dissection (TLND). Eligible patients had resectable clinical stage III nodal disease. Patients were required to have a sonographically visible clip placed in the ILN at diagnosis. Surgeons marked the ILN, which was assessed separately in addition to pathologic assessment of the entire nodal basin using International Neoadjuvant Melanoma Consortium criteria.</p> Results <p>Among 34 enrolled patients, 30 underwent TLND per protocol. Two patients (6.7%) demonstrated discordance between ILN and nodal basin pathology, both with a pCR in the ILN but residual disease in non–ILNs. The false-negative rate of an ILN pCR was 13.3%. Among eight patients with a single involved lymph node at baseline, the pathologic major response rate was 75% and concordance was 100%.</p> Conclusions <p>In this prospective substudy, ILN pathology did not fully reflect nodal basin status for all the patients. This finding supports caution in de-escalating treatment based solely on ILN response and highlight the need for prospective trials evaluating oncologic outcomes after limited nodal resection.</p>