Detection of primary lung tumor metastasis to regional lymph nodes affects prognosis and therapeutic choices. Surgical biopsy of intrathoracic lymph nodes is recommended during lung tumor resection, but specific guidelines for dogs are lacking. In humans, there is a standard lymph node mapping and classification system used for surgical staging of lung cancer. This study investigated the validity of this anatomic classification system for regional lymph nodes in normal dogs, intraoperatively and postmortem. The utility of lobar methylene blue dye and technetium-99m for identifying nodes was also explored.
A unilateral intercostal thoracotomy was performed in 10 adult Walker hounds; 2 additional dogs underwent bilateral thoracotomies. Dogs were injected subpleurally at several sites with either methylene blue dye alone or in combination with technetium-99m filtered sulfur colloid. Time to first visual appearance of blue-tinged lymph node and/or detectable radioactive lymph node signal was noted. All blue, radioactive, or palpable lymph nodes were removed. At postmortem, all intrathoracic structures were removed and any remaining lymph nodes identified. In the 14 node stations searched, investigators were only able to identify a median of 2 (range, 1–4) lymph nodes per hemithorax in vivo versus a median of 6.5 (range, 2–8) lymph nodes ex vivo. The authors concluded that localizing thoracic lymph nodes in dogs following the human schema is challenging but warrants further investigation. They suggested CT imaging may be a better alternative.