A 56-year-old woman with breast carcinoma has pericardial invasion. Which structure is likely involved?

In cases of breast carcinoma with pericardial invasion, the right phrenic nerve is often the structure that may be involved. The right phrenic nerve runs in close proximity to the pericardium, which is the double-walled sac containing the heart. When a cancerous tumor invades the pericardial tissue, it can lead to a direct impact or infiltration of adjacent structures, including the right phrenic nerve.

The right phrenic nerve is responsible for innervating the diaphragm, and its involvement can result in symptoms such as referred pain to the shoulder due to phrenic nerve irritation, a phenomenon known as Kehr's sign. Additionally, pericardial disease can lead to changes in diaphragm movement due to nerve involvement, which can be critical in the context of cancer spreading.

While the other structures listed are located in the thoracic cavity, they have different associations and relationships with the pericardium that make them less likely to be directly involved in the case of pericardial invasion by breast carcinoma. For instance, the coronary sinus primarily drains the heart itself, the thoracic duct is the main lymphatic vessel and is positioned more posteriorly, and the right vagus nerve is