This report presents an unusual case of life-threatening massive bleeding in the pulmonary trunk adjacent to the right ventricular outflow tract during resection of a large primary mediastinal nonseminomatous germ cell tumor (PMNSGCT) in the absence of cardiovascular surgeons. The patient was a 21-year-old male whose large mediastinal tumor was diagnosed as an extragonadal PMNSGCT, which was a mixture of a yolk sac tumor and an immature teratoma. Generally, chemotherapy causes extensive peripheral tumor necrosis of PMNSGCTs, thus enabling their complete resection. In this case, surgeons considered the resection as possible by dissecting the peripheral necrotic tissue, and cardiovascular surgeons were thus not consulted. Enlarged modified left hemi-clamshell thoracotomy (HCST) was applied. While dissecting around the pulmonary trunk, the assistant-held forceps accidentally touched the tensed pulmonary trunk, which caused bleeding. We immediately contacted the collaborating cardiac surgery department at another hospital for assistance. Meanwhile, massive bleeding occurred, leading to hemorrhagic shock, and thus direct cardiac massage was required. Our team managed to establish a venoarterial (VA) extracorporeal membrane oxygenation (ECMO). After the arrival of cardiac surgeons, a suction circuit was added, and bleeding was stopped using sutures. Finally, complete resection of the tumor was achieved, and the patient awoke the following day without any brain dysfunction. After discussions with all the members involved in the surgery, we developed an in-hospital consensus on how to perform surgeries for large thoracic tumors safely at our cancer center without the cardiovascular surgery department. We herein present the case and consensus and discuss the relevant issues.