Apparatus and methods for accessing the lymphatic system

A method for diagnosing, monitoring, or treating a patient via a thoracic duct of the patient's body, comprising:

• providing a tubular member comprising a proximal end, a distal end sized for introduction into the patient's body, and an expandable member on the distal end;
• introducing the distal end of the tubular member into the patient's vasculature via a percutaneous access site in a right internal jugular vein or right subclavian vein with the expandable member in a contracted condition;
• advancing the tubular member until the distal end is disposed within a junction of the patient's left internal jugular vein and left subclavian vein;
• manipulating the tubular member to direct the distal end into the thoracic duct;
• expanding the expandable member to substantially isolate the thoracic duct from the left internal jugular vein and left subclavian vein;
• removing lymphatic fluid from the thoracic duct through the tubular member to a location exterior to the patient's body;
• and
• analyzing the removed lymphatic fluid to monitor cancer status of the patient based on detecting or quantifying circulating t-cell surface markers or substances found in circulating lymph that have been released by tumor cells.