SBRT Dosing for 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, 2
Definition of Central 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 important to distinguish between "moderately central" tumors and "ultracentral" tumors (where the planning target volume overlaps the trachea or main bronchi) 1
Dose Recommendations Based on Tumor Location
Moderately Central Tumors
- 50 Gy in 5 fractions is the most commonly used and safe regimen 2
- Alternative regimen: 48 Gy in 4 fractions (used in about 30% of cases) 2
- These doses provide a 5-year local control rate of 89% 2
Ultracentral Tumors
- SBRT is not appropriate for ultracentral tumors due to increased toxicity risk 1
- For these tumors, conventional or hypofractionated radiotherapy schemes are recommended instead of SBRT 1
Safety Considerations
- Central tumor location carries higher risk of treatment-related toxicity compared to peripheral tumors 1, 3
- 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) have demonstrated significantly lower toxicity rates 1, 2
- Toxicity profile with 5-fraction regimens:
Biological Equivalent Dose (BED) Considerations
- The optimal BED10 (biological equivalent dose with α/β = 10) for SBRT should be at least 100 Gy 1
- BED10 ranges of 83.2-106 Gy and 106-146 Gy show the best outcomes 1
- Reducing the BED below 100 Gy to decrease toxicity risk may result in increased local failure 4
Special Considerations
- Tumor size is a significant factor - larger tumors (>4 cm) have higher risk of grade 3+ toxicity 4
- 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, radical RT using more conventional or accelerated schedules is recommended over SBRT 1
Treatment Planning Considerations
- Advanced dose calculation algorithms (type B models) should be used for treatment planning 1
- Planning target volume (PTV) should account for respiratory motion 1
- Planning organ at risk volume (PRV) margins should be used for critical serial organs in centrally located tumors 1
By following these dose recommendations and considering the specific location and characteristics of central lung tumors, SBRT can be safely and effectively administered with high rates of local control while minimizing the risk of severe toxicity.