Thoracic duct

• Right-to-left crossing at T4-T6 vertebral level in the posterior mediastinum
• Terminal insertion into the left subclavian-internal jugular vein confluence (venous angle)
• Course beginning as cisterna chyli continuation at L1 vertebral body, ascending right of midline initially
• Flared or dilated terminal portion before venous drainage

• The thoracic duct originates as the superior continuation of the cisterna chyli at approximately the L1 vertebral body level (range T10-L3), though this can vary significantly among individuals.
• The duct enters the thoracic cavity through the aortic hiatus and ascends vertically to the right of the midline in the posterior mediastinum, positioned between the aorta medially and the azygos vein laterally, with the esophagus anterior.
• At approximately the T4-T6 vertebral level (range varies), the thoracic duct crosses to the left of midline and continues its superior course through the thoracic inlet posterior to the left subclavian artery.
• At the root of the neck, the thoracic duct curves forward and to the left, traveling behind the left carotid sheath and over the dome of the left pleura, passing anterior to the anterior scalene muscle and phrenic nerve.
• The terminal portion of the thoracic duct typically drains into the venous angle formed by the confluence of the left subclavian vein and left internal jugular vein as a single trunk, which is its normal and expected termination site.
• The distal terminal portion of the thoracic duct frequently appears flared or dilated rather than maintaining a consistent diameter throughout its course, which is a normal anatomic variation.
• The thoracic duct receives tributaries including the ascending lumbar, intestinal, liver, and descending intercostal lymph trunks near its origin, with additional contributions from posterior intercostal lymph nodes and upper intercostal lymph trunks in the thorax.
• At its termination, the thoracic duct receives the left jugular lymph trunk (draining the left head and neck), left subclavian lymph trunk (draining the left axilla and upper limb), and left bronchomediastinal trunk (draining the left lung and heart).

• Variant anatomy with double thoracic ducts (~20% of population): multiple parallel lymphatic ducts with separate or convergent courses that can mimic pathology if not recognized as normal variant.
• Aberrant termination sites (azygos vein, right internal jugular vein, brachiocephalic vein, or external jugular vein): appear as unusual venous drainage patterns rather than typical left subclavian-jugular confluence termination.
• Multiple terminal channels from a single duct (~20% of population): single thoracic duct with atypical branching pattern at termination that may simulate obstruction or incomplete drainage.
• Chylothorax from thoracic duct injury: presents as pleural effusion with milky appearance and elevated triglycerides, typically unilateral and often left-sided given normal duct anatomy.
• Lymphocele of the thoracic duct: localized fluid collection along the duct course, usually in the mediastinum or at the thoracic inlet, distinct from diffuse chylothorax.
• Central venous catheter cannulation of the thoracic duct: rare complication where the catheter inadvertently enters the thoracic duct rather than the intended venous system.

When describing the thoracic duct on imaging, clearly identify its origin at the L1 vertebral level as the cisterna chyli continuation, specify the midline crossing level (T4-T6), note its right-of-midline course in the mid-thorax, and document its terminal insertion into the left subclavian-jugular venous angle; specifically mention if there are variant features such as double ducts, aberrant termination, or unusual dilation of the terminal portion to avoid misinterpretation as pathology.

• Mistaking normal terminal flaring of the thoracic duct for pathologic dilation or obstruction; the distal duct commonly appears dilated before entering the venous system as part of normal anatomy.
• Failing to recognize variant thoracic duct anatomy (present in ~40% of population) such as double ducts or aberrant terminations, which can lead to overcalling pathology or misidentifying abnormal findings as variants.
• Misidentifying the level of midline crossing; if the duct crosses at an atypical level (outside T4-T6 range), ensure this represents normal variant anatomy rather than mass effect or pathologic displacement from adjacent lesions.
• Overlooking the thoracic duct's course behind the left carotid sheath and anterior to the anterior scalene muscle in the neck; missing these relationships can result in incomplete anatomic characterization or failure to identify compressive pathology.