Lungs

Paired organs filling most of the thoracic cavity: the right lung has three lobes (upper, middle, lower), the left has two (upper, including the lingula, and lower). Each is surrounded by pleura. On CT, the main airways and hilar vessels run through the lung contour and are handled differently by different definitions.

The lungs bring oxygen into the blood and remove carbon dioxide.

The lung is the classic parallel organ: function is spread across many independent units, so what matters is how much lung receives dose rather than the maximum at any one point. That is why lung limits are usually volume-based — mean lung dose and Vx metrics in conventional fractionation, and a “critical volume” approach in SBRT tables such as Timmerman 2022. The main toxicities are radiation pneumonitis and fibrosis; QUANTEC summarizes the evidence for conventional fractionation.

In esophageal treatment the lungs surround the target along its whole length. When surgery follows, protocols often ask for lower lung doses than for definitive treatment, because lung dose has been linked to complications after the operation.

Kong 2011 recommends:

• Contour only the air-inflated lung, leaving out fluid and collapsed lung (atelectasis).
• Auto-contouring is fine with a threshold chosen for each scan, but always review and edit the result.
• Exclude the proximal bronchial tree; include small vessels (under 1 cm, or beyond the hilum).
• Right and left lungs can be one structure or two. Limits are usually based on both lungs minus the GTV — only the part of the GTV inside the lung, so mediastinal nodal GTVs are not subtracted.
• With collapsed lung, IV contrast or PET can help separate tumor from lung.

Timmerman 2022 (Tables 8–11, written for its SBRT dose tables): right and left lung as one structure, including all lung tissue but excluding the GTV and the major airways (trachea and main/lobar bronchi). Sources differ on which target volume is subtracted, so match the source behind your limit.