Centre to Lung Distance in 2D Planning
To minimize the risk of radiation pneumonitis in 2D planning, maintain a centre to lung distance of 1.0 to 1.5 cm minimum, with a maximum lung depth of 3.0 to 3.5 cm as projected on the radiograph at isocenter. 1
Specific Distance Parameters
The American College of Radiology and American College of Surgeons established clear geometric constraints for 2D tangential field planning:
- Minimum lung inclusion: 1.0 to 1.5 cm of lung tissue must be included in the tangential fields 1
- Maximum lung depth: Not more than 3.0 to 3.5 cm of lung (as projected on the radiograph at isocenter) should be treated 1
- Measurement technique: These distances are measured on the radiograph at the isocenter level, representing the 2D planning approach 1
Rationale for These Constraints
These specific distance parameters were established to minimize radiation pneumonitis risk while ensuring adequate breast tissue coverage 1. The minimum inclusion requirement ensures proper dose distribution to the chest wall, while the maximum depth constraint directly limits lung exposure to prevent symptomatic pneumonitis.
Modern Context and Dose-Volume Relationships
While the question specifically addresses 2D planning distances, understanding the underlying dose-volume relationships provides important context:
- V20 threshold: The percentage of lung volume receiving ≥20 Gy should ideally remain below 35-37% 1
- Mean lung dose (MLD): Should be kept as low as possible, preferably <8.5 Gy for mesothelioma cases 1, and below 20-23 Gy for lung cancer 1
- Low-dose volume: The V5 (volume receiving 5 Gy) should be minimized, as this correlates with pneumonitis risk 1
Critical Caveats
Important limitation: Even when these distance constraints are respected, approximately 10-15% of patients may still develop severe radiation-induced toxicity 1. Patient-specific factors such as pre-existing interstitial lung disease dramatically increase pneumonitis risk and require more intensive monitoring 2, 3.
For left-sided lesions, efforts should be made to minimize cardiac exposure within the tangential fields while maintaining the lung distance parameters 1.