2. Confirmation of N0 status by frozen section examination. Right sided tumors require that node levels 4, 7, and 10 be sampled and diagnosed as negative on frozen section. Left sided tumors require that node levels 5 or 6, 7 and 10 be sampled and diagnosed as negative on frozen section. Levels 4 and 7 nodes may be sampled by mediastinoscopy, endobronchial ultrasound (EBUS) and/or endoscopic ultrasound (EUS), or at the time of thoracotomy or VATS exploration. Nodes previously sampled by mediastinoscopy (or EBUS and/or EUS) either immediately prior to or within 6 weeks of the definitive surgical procedure (thoracotomy or VATS) do not need to be resampled.