What are the dose constraints for adult patients with brain lesions or tumors undergoing Stereotactic Radiosurgery (SRS) or Fractionated Stereotactic Radiosurgery (FSRS)?

Dose Constraints for Stereotactic Radiosurgery (SRS) and Fractionated Stereotactic Radiosurgery (FSRS)

For single-fraction SRS, use 15-24 Gy for brain metastases, with 12-14 Gy for benign meningiomas; for FSRS, use 25-30 Gy in 3-5 fractions for lesions >3 cm or near critical structures. 1

Single-Fraction SRS Dose Constraints

Brain Metastases

• Standard dose range: 15-24 Gy single fraction for brain metastases amenable to SRS 1
• Smaller cavities (<3 cm): 15-18 Gy single fraction achieves <85% 1-year local control 2
• Lesion size and proximity to critical structures determine the upper dose limit 1

Benign Meningiomas

• Cavernous sinus meningiomas: 11-14 Gy (median 14 Gy) prescribed to the 50% isodose line 1
• Maximum point dose ranges from 15-32 Gy (median 27 Gy) 1
• Modern practice favors 11-12 Gy marginal dose for cavernous sinus lesions, with older series using higher doses (15-16 Gy) 1
• Doses below 12 Gy may compromise local control 1

Fractionated Stereotactic Radiosurgery (FSRS) Dose Constraints

Brain Metastases

• Most common regimen: 25 Gy in 5 fractions for lesions >3 cm diameter or near critical structures 1
• Alternative regimen: 27 Gy in 3 fractions provides 91% 1-year local control with only 8% radiation necrosis 2
• 30 Gy in 5 fractions is also supported for larger resection cavities 2
• The 3 × 9 Gy (27 Gy total) or 3 × 12 Gy (36 Gy total) regimens have been employed with 88% 1-year local control 3

Benign Meningiomas

• 25 Gy in 5 fractions is the most common FSRS schedule for meningiomas 1
• FSRS schedules range from 14-30 Gy delivered in 2-5 fractions 1
• Prescription isodose line ranges from 44-95% 1

Critical Structure Dose Constraints

Optic Apparatus (Nerve/Chiasm)

• Single fraction: Maximum 10 Gy for <1% risk of radiation-induced optic neuropathy (RION) 4
• 3 fractions: Maximum 20 Gy for <1% RION risk 4
• 5 fractions: Maximum 25 Gy for <1% RION risk 4
• Prior radiation increases RION risk 10-fold; use lower doses in re-irradiation cases 4

Cranial Nerves in Cavernous Sinus

• Permanent radiation-induced cranial nerve injury occurs in <1% of cases with contemporary dosing 1
• Maximum tolerated dose up to 40 Gy for cranial nerves II and IV 1

Internal Carotid Artery (ICA)

• ICA stenosis after SRS is rare with contemporary techniques 1
• The high-dose volume routinely includes the ICA without significant long-term vascular complications 1

Algorithmic Approach to Dose Selection

Step 1: Determine Lesion Characteristics

• Size: Lesions <3 cm favor single-fraction SRS; lesions >3 cm favor FSRS 1
• Location: Proximity to optic apparatus, brainstem, or eloquent cortex favors FSRS 1
• Histology: Melanoma may require higher doses due to radioresistance 3

Step 2: Select Fractionation Scheme

• Single-fraction SRS if:

  • Lesion <3 cm diameter 1

  • 1 cm from optic apparatus 4

  • No prior radiation to the area 4

• FSRS if:

  • Lesion >3 cm diameter 1
  • Adjacent to critical structures (optic apparatus, brainstem) 1
  • Prior radiation to the area 1
  • Large resection cavity 2

Step 3: Apply Specific Dose Constraints

• For brain metastases <3 cm: 18-24 Gy single fraction 1
• For brain metastases >3 cm: 25-30 Gy in 3-5 fractions 1, 2
• For benign meningiomas: 12-14 Gy single fraction or 25 Gy in 5 fractions 1
• For post-operative cavities: 27-30 Gy in 3-5 fractions 2

Common Pitfalls and How to Avoid Them

Pitfall 1: Excessive Single-Fraction Dose Near Optic Apparatus

• Never exceed 10 Gy to optic structures in single fraction 4
• If tumor abuts optic apparatus, switch to FSRS with 20 Gy in 3 fractions or 25 Gy in 5 fractions 4

Pitfall 2: Underdosing Meningiomas

• Do not use <12 Gy marginal dose for single-fraction SRS of meningiomas, as local control deteriorates 1
• Modern data supports 12-14 Gy range for optimal balance of efficacy and toxicity 1

Pitfall 3: Ignoring Prior Radiation History

• Prior radiation increases toxicity risk 10-fold for optic structures 4
• In re-irradiation cases, strongly favor FSRS over single-fraction SRS and reduce total dose by 20-30% 4

Pitfall 4: Inadequate Dose for Melanoma Metastases

• Melanoma histology predicts local failure (HR 6.1) with standard dosing 3
• Consider dose escalation or more aggressive fractionation for melanoma brain metastases 3

Post-Operative Timing Considerations

• Deliver post-operative SRS within 4 weeks of surgery to optimize outcomes 2
• Delayed treatment beyond 4 weeks may compromise local control 2