5.4 Lung & Thoracic Tumors: Anatomy, Spread & Critical Structures

Anatomy and Nodal Stations

The right lung has three lobes (upper, middle, lower) separated by the horizontal and oblique fissures; the left lung has two lobes (upper, lower) separated by the oblique fissure, with the lingula as an analog to the right middle lobe. Each lung is invested by visceral pleura, with a thin pleural space separating it from the parietal pleura lining the chest wall; the mediastinum contains the heart, great vessels, trachea, esophagus, and thymus, and separates the two pleural cavities. The hilum, where the mainstem bronchus, pulmonary artery, and pulmonary veins enter and exit each lung, is a common site of central tumor involvement and lymphatic convergence.

Mediastinal and hilar lymph nodes are mapped using the standardized IASLC lymph node map, grouped into stations that are essential for staging (the N-descriptor) and for designing elective or involved-field nodal irradiation:

The subcarinal station (station 7) is a particularly important convergence point because it can receive drainage from either lung and from multiple lobes, making it a common site of nodal involvement regardless of the laterality of the primary tumor.

Patterns of Spread

Locally, non-small cell lung cancer (NSCLC) spreads by direct extension into adjacent structures -- chest wall, mediastinum, great vessels, esophagus, or vertebral bodies -- and by lymphatic spread following an orderly progression from intrapulmonary/hilar nodes to mediastinal stations, generally respecting lobar and laterality patterns (though the subcarinal station is a notable exception). Malignant pleural effusion indicates pleural surface involvement and, along with pleural or pericardial nodules, typically upstages disease to a status generally not amenable to curative-intent radiotherapy alone.

Small cell lung cancer (SCLC) is far more aggressive, with a strong tendency toward early hematogenous dissemination; it frequently presents with bulky mediastinal adenopathy and has a high rate of subclinical or overt brain metastases, which is why prophylactic cranial irradiation (PCI) or close MRI surveillance has historically been considered for patients responding well to initial chemoradiation.

Distant metastatic spread for lung cancer in general most commonly targets the brain, bone, liver, and adrenal glands, reflecting the pattern expected from hematogenous seeding through the pulmonary venous circulation. Adrenal metastases are common enough that isolated adrenal nodules found on staging imaging require careful workup before assuming a benign etiology (such as a benign adenoma) in a lung cancer patient.

Critical Structures, Motion Management, and Therapist Considerations

Thoracic radiotherapy planning must account for both dose-limiting normal structures and the fact that the thorax is in constant motion from respiration:

• Spinal cord: a hard maximum point-dose constraint avoids myelopathy, particularly relevant since the cord runs immediately posterior to central mediastinal targets.
• Esophagus: acute esophagitis (odynophagia, dysphagia) is common during concurrent chemoradiation for centrally located or mediastinal disease and can affect nutrition and treatment tolerance.
• Heart: mean heart dose and dose to specific cardiac substructures correlate with cardiac events and, in some studies, overall survival, making cardiac sparing an increasing planning priority.
• Lungs: mean lung dose and the volume of lung receiving 20 Gy (V20) are the principal predictors of symptomatic radiation pneumonitis; these constraints often limit how much dose can be delivered, especially with larger target volumes.
• Brachial plexus: critical for apical (superior sulcus/Pancoast) tumors, where the plexus may be immediately adjacent to or invaded by tumor.

Because tumors and nodal targets move with breathing, four-dimensional CT (4D-CT) simulation captures the tumor position throughout the full respiratory cycle, allowing an internal target volume (ITV) to be constructed that encompasses the entire range of tumor motion rather than relying on a single-phase snapshot. Motion management strategies include free-breathing with ITV-based margins, breath-hold techniques, and respiratory gating, where the beam is only active during a defined portion of the breathing cycle. For early-stage, medically inoperable NSCLC, stereotactic body radiotherapy (SBRT) delivers a small number of very high-dose, highly conformal fractions and depends heavily on accurate motion management and image guidance (daily CBCT) because the ablative per-fraction doses leave very little margin for geographic miss. Immobilization typically uses a wing board or vacuum-bag body cradle with arms positioned overhead to keep them out of the beam path, and daily or frequent image guidance verifies both tumor position and any interval changes such as atelectasis, effusion, or tumor regression that could necessitate a replan.

Staging with PET/CT and, when feasible, endobronchial ultrasound or mediastinoscopy for suspicious nodal stations is essential before curative-intent treatment planning, since accurate nodal staging directly determines whether elective nodal coverage, involved-field-only treatment, or a switch to systemic/palliative management is appropriate. Concurrent chemoradiation remains standard for locally advanced, unresectable NSCLC, while SBRT is generally reserved for small, node-negative, peripheral tumors where ablative focal dose can be delivered safely without excessive normal-tissue exposure. Because central tumors sit close to the proximal bronchial tree, esophagus, and great vessels, SBRT to central lesions requires more conservative fractionation than peripheral SBRT to avoid serious airway or vascular injury.