What are the dose constraints in Stereotactic Body Radiation Therapy (SBRT)?

Dose Constraints in SBRT

For lung SBRT, maintain mean lung dose (MLD) ≤8 Gy and lung V20 <10-15% to keep symptomatic radiation pneumonitis below 10-15%, while for central tumors use 50 Gy in 5 fractions rather than higher dose-per-fraction regimens that caused lethal toxicity. 1, 2, 3

Lung Parenchyma Constraints

The most critical normal tissue constraint for lung SBRT is the mean lung dose, which consistently correlates with radiation pneumonitis across multiple patient cohorts 4, 3:

• MLD should be ≤8 Gy for 3-5 fraction treatments 1, 3
• V20 (lung volume receiving ≥20 Gy) should be <10-15% 1, 3
• V5 should be kept ≤55% when combining conventional radiotherapy with SBRT boost 5

These constraints achieve symptomatic radiation-induced lung toxicity rates below 10-15% 3. Patients with interstitial lung disease are especially susceptible to severe toxicity and require even more conservative constraints 3.

Chest Wall and Rib Constraints

To minimize severe chest wall pain and rib fractures:

• Chest wall volume receiving ≥30 Gy should be <30 cm³ to keep fracture risk below 30% 6
• Maximum dose to 2 cm³ of any individual rib (D2cc) should be <27 Gy for 3-fraction regimens, corresponding to approximately 5% fracture risk 6
• For 54-60 Gy in 3 fractions, maintain D2cc <27 Gy (3 × 9 Gy) 6

Tumor Location-Specific Dosing

Central Lung Tumors

Central tumors (within 2 cm of mediastinal structures) require 50 Gy in 5 fractions rather than the higher doses used for peripheral tumors 1, 2. Early studies using 60-66 Gy in 3 fractions for central tumors reported serious and lethal toxicity 1, 2.

Critical distinction: Avoid SBRT entirely for ultracentral tumors where the PTV overlaps trachea or main bronchi due to unacceptable toxicity risk 1, 2.

Peripheral Lung Tumors

For peripheral early-stage NSCLC, the biological equivalent dose (BED10) should be at least 100 Gy, with optimal ranges of 106-146 Gy showing best outcomes 1, 2.

Spinal Metastases Dose Recommendations

For spinal SBRT, use one of these fractionation schemes to achieve BED ≥50 Gy for approximately 90% local control at 1 year 1:

• 16-24 Gy in 1 fraction
• 24 Gy in 2 fractions
• 24-27 Gy in 3 fractions
• 30-35 Gy in 5 fractions

For renal cell carcinoma spinal metastases specifically, 24 Gy in a single fraction achieves 90% 1-year local control 1. The most common late toxicity is vertebral compression fracture, occurring in 9.4% of patients 1.

Other Critical Structure Constraints

When combining conventional radiotherapy (50.4 Gy in 28 fractions) with SBRT boost 5:

• Esophagus 5 cc dose should be <12 Gy for the SBRT component
• Cumulative esophagus dose: <10% receiving >50 Gy
• Spinal cord cumulative dose <33 Gy
• Heart V25 <5%

For conventional concurrent chemoradiotherapy, doses to central bronchi exceeding 80 Gy increase late toxicity risk 7.

Common Pitfalls to Avoid

• Never use conventional low-dose palliative radiation (8 Gy in 1 fraction) for patients with longer expected survival, as this achieves less than 50% 1-year local control for bulky tumors and increases risk of spinal cord compression 1
• Do not use 60-66 Gy in 3 fractions for central lung tumors - this regimen caused lethal toxicity in early studies 1, 2
• Ensure BED10 reaches at least 100 Gy for lung SBRT to optimize outcomes 1, 2
• Avoid transpleural biopsy approaches for suspected thymomas 7

Planning Technical Requirements

Advanced dose calculation algorithms (type B) are mandatory for thoracic SBRT, as they account for tissue heterogeneity and provide accurate dose distributions 7. Type A and type B algorithm doses cannot be compared directly 7.

Use planning organ at risk volumes (PRV) with appropriate margins around critical serial organs, especially for centrally located tumors 7, 2.