What are the dose constraints for Stereotactic Body Radiation Therapy (SBRT) in central lung tumors?

Dose Constraints for Stereotactic Body Radiation Therapy (SBRT) in Central Lung Tumors

For central lung tumors, the recommended SBRT dose is 50 Gy in 5 fractions, as this provides optimal tumor control while minimizing toxicity to critical structures. 1

Definition of Central Lung Tumors

• Central tumors are defined as those located within 2 cm in all directions of any mediastinal critical structure, including the bronchial tree, esophagus, heart, brachial plexus, major vessels, spinal cord, phrenic nerve, and recurrent laryngeal nerve 1
• It's essential to distinguish between "moderately central" tumors and "ultracentral" tumors (where the planning target volume overlaps the trachea or main bronchi) 1

Dose Recommendations and Constraints

General Dose Recommendations

• 50 Gy in 5 fractions (12.5 Gy per fraction) is the most commonly used and recommended dose regimen for central tumors 1, 2
• The optimal biological equivalent dose (BED10) should be at least 100 Gy for adequate local control 1, 2
• BED10 ranges of 83.2-106 Gy and 106-146 Gy show the best outcomes 1
• SBRT is not recommended for ultracentral tumors due to increased toxicity risk 1, 3

Critical Structure Dose Constraints

• For the bronchial tree: Limit maximum point dose and dose to 5cc to minimize risk of severe toxicity 4
• For the esophagus: PTV overlap with esophagus significantly increases risk of esophageal toxicity (50% risk of grade ≥2 toxicity when overlap occurs) 4
• For main bronchus: Mean dose (Dmean) BED3 should be kept below 91 Gy to reduce risk of grade ≥3 toxicity, including fatal bronchopulmonary hemorrhage 3

Treatment Planning Considerations

• Advanced dose calculation algorithms (type B) are strongly recommended for thoracic radiotherapy as they allow for more accurate computation of dose distributions 5
• Planning target volume (PTV) should account for respiratory motion and daily setup errors 5, 1
• Planning organ at risk volume (PRV) margins should be used for critical serial organs in centrally located tumors 5, 1
• The use of 4D-CT scan data is strongly preferred for treatment planning to account for respiratory motion 5
• Online corrections based on tumor positions derived from cone-beam CT are superior to set-up with bony anatomy 5

Safety Considerations

• Early studies using 60-66 Gy in 3 fractions for central tumors reported serious and even lethal toxicity 1
• Lower doses per fraction (such as 50 Gy in 5 fractions) demonstrate significantly lower toxicity rates 1, 2
• Larger tumor size (>4 cm) is associated with higher risk of grade 3+ toxicity 2
• For ultracentral tumors, protracted regimens such as 60 Gy in 12 fractions may be considered, though still carry substantial risk of fatal bronchopulmonary hemorrhage 3

Special Considerations

• For tumors adjacent to critical structures, "risk-adapted" fractionation schemes should be employed 1
• For medically inoperable patients with tumors >5 cm and/or moderately central location, conventional or accelerated radiotherapy schedules may be preferable to SBRT 1
• Efforts to decrease toxicity risk by decreasing the biologically equivalent dose below 100 Gy result in increased local failure 2
• Stereotactic body proton therapy (SBPT) may be considered for high-risk central tumors to reduce integral dose to organs at risk while maintaining target coverage 6