What is mean lung dose and how is it checked during radiation therapy?

Mean Lung Dose in Radiation Therapy

Mean lung dose (MLD) is the average radiation dose delivered to the entire lung volume (excluding the tumor), and it is checked through dose-volume histogram (DVH) analysis during CT-based treatment planning, serving as a critical predictor of radiation pneumonitis risk. 1

Definition and Clinical Significance

MLD represents the average radiation dose across the total bilateral lung volume, calculated after excluding either the gross tumor volume (GTV) or planning target volume (PTV) from the total lung contour. 2 This dosimetric parameter directly correlates with the risk of developing radiation pneumonitis, which can be fatal in severe cases. 1, 3

• The lung V20 (percentage of lung volume receiving ≥20 Gy) and MLD are the most critical parameters for avoiding fatal pulmonary toxicities. 1
• Both parameters are strongly associated with the development of pneumonitis, with MLD showing consistent predictive value across different radiation techniques. 1, 2, 4

How to Check Mean Lung Dose

CT-Based Treatment Planning Process

MLD is calculated during CT simulation-guided treatment planning using specialized radiation planning software that generates dose-volume histograms. 1

• Lung Contouring: The radiation oncologist or dosimetrist contours both lungs on the planning CT scan, following standardized atlases (such as RTOG organ-at-risk atlas). 4
• Tumor Volume Exclusion: The GTV or PTV is subtracted from the total bilateral lung volume to calculate the "normal lung" volume. 2, 4
• DVH Generation: The treatment planning system calculates the radiation dose distribution and generates a DVH showing the volume of lung receiving various dose levels. 1, 2
• MLD Calculation: The planning software automatically computes the MLD by averaging the dose across all voxels in the defined lung volume. 2, 4

Critical Technical Considerations

A small but significant difference exists depending on whether you exclude GTV versus PTV from the total lung volume—excluding GTV results in slightly higher MLD values (average 16.7 Gy vs 14.8 Gy when excluding PTV). 2 This difference must be accounted for when comparing dosimetric information between institutions or optimizing treatment plans. 2

• For ipsilateral, contralateral, or total lung calculations, specify which lung volume definition is being used, as this affects the numerical values and their interpretation. 4
• Dosimetric parameters of total or ipsilateral lung are more predictive than contralateral lung alone for radiation pneumonitis risk. 4

Recommended Dose Constraints

Standard Fractionation (1.8-2.0 Gy per fraction)

For conventional fractionation delivering approximately 2 Gy per fraction, keep MLD < 20-23 Gy, though 10-15% of patients may still develop severe toxicity even below these thresholds. 1, 3

• In lung cancer treated with standard fractionation, both V20 (35-37%) and MLD (20-23 Gy) have been considered safe upper limits. 1, 3
• However, approximately 10-15% of patients still develop severe radiation-induced toxicity when receiving doses below these thresholds, indicating these are not absolute guarantees of safety. 1, 3

IMRT and Advanced Techniques

When using intensity-modulated radiotherapy (IMRT), particularly after extrapleural pneumonectomy or in mesothelioma cases, keep MLD < 8.5 Gy and lung V20 ≤ 7% to avoid fatal pneumonitis. 1

• IMRT can increase the volume of contralateral lung receiving lower radiation doses, requiring strict control of contralateral lung dose. 1
• The volume of contralateral lung receiving low-dose radiotherapy (e.g., 5 Gy) should be minimized. 1, 5
• After lung-sparing surgery with adjuvant tomotherapy, contralateral lung V5 has been associated with radiation pneumonitis risk. 1

Hypofractionated Radiation (4 Gy per fraction)

For hypofractionated regimens delivering 4 Gy per fraction (60 Gy in 15 fractions or 72 Gy in 18 fractions), maintain MLD < 10.6 Gy to keep the risk of grade ≥2 pneumonitis below 20%. 6

• Lung V20 < 17.7% is the primary constraint for 4 Gy per fraction regimens. 6
• V5 < 41.3% can be considered as a lower-priority constraint. 6

SBRT/SABR

For stereotactic body radiation therapy, constrain mean dose of total lung to < 6 Gy and ipsilateral lung to < 20 Gy to limit clinically significant pneumonitis to < 10%. 4

• Dosimetric factors of total or ipsilateral lung are significant risk factors, while contralateral lung parameters (mean dose, V5, V10, V20) are not significantly associated with symptomatic pneumonitis after SBRT. 4

Common Pitfalls and How to Avoid Them

Definition Inconsistency

Always specify whether MLD is calculated excluding GTV or PTV, as this creates a systematic difference of approximately 2 Gy in the reported values. 2 When comparing plans between institutions or reviewing published constraints, verify which definition was used. 2

High-Risk Patient Populations

Patients with idiopathic interstitial pneumonitis have markedly elevated risk of severe and even lethal radiation pneumonitis—exercise extreme caution and consider alternative treatments. 1, 3

• History of respiratory comorbidity and previous thoracic radiation significantly increase pneumonitis risk. 4
• Patient factors such as baseline lung function, age, and sex do NOT adequately predict radiation pneumonitis risk and should not be used for patient selection. 1, 3

Chemotherapy Interactions

Concurrent chemotherapy with platinum, etoposide, taxanes, and vinorelbine does NOT appear to increase radiation pneumonitis risk. 1, 3 However, gemcitabine is not recommended for routine concurrent use with radiotherapy. 1

IMRT-Specific Risks

IMRT increases low-dose bath to larger lung volumes while sparing high-dose regions—this can paradoxically increase pneumonitis risk if contralateral lung constraints are not strictly enforced. 1, 7

• In esophageal cancer, V20 was significantly larger in IMRT compared to 3D-CRT plans (p = 0.002). 7
• The risk of fatal pneumonitis with IMRT is excessively high if strict limits are not applied to contralateral lung exposure. 1

Verification During Treatment Planning

Review the DVH for both lungs during every treatment planning session, confirming that MLD and V20 values meet institutional constraints before plan approval. 1

• Document the specific lung volume definition used (total lung minus GTV vs. minus PTV). 2
• For complex cases with extensive treatment fields (such as mesothelioma), limiting radiation doses to organs at risk is paramount to minimize patient risk. 1
• The dose-volume histograms of all target volumes and critical organs should be clearly stated in the treatment plan. 1