A measure of SRS/SRT plan quality: Quantitative limits for intermediate dose spill (R50%) in linac-based delivery

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiosurgery

Background:

• Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) are advancing for multiple cranial targets using single isocenters on C-arm linacs.
• Current clinical practice lacks universal guidelines for acceptable intermediate dose spill limits.

Purpose of the Study:

• To propose an intermediate dose spill guidance range for cranial SRS/SRT delivered on C-arm linacs.
• To establish benchmarks for plan optimization and assessment in single PTV and single isocenter multiple target plans.

Main Methods:

• Quantified intermediate dose spill using the R50% metric (R50% = volume of 50% prescription isodose cloud / PTV volume).
• Tested the proposed R50% range on three clinical datasets using 6 MV and 10 MV beams.
• Focused on plans with planning target volumes (PTV) ranging from 0.02-57.9 cm³.

Main Results:

• Proposed lower (LowerR50%) and upper (UpperR50%) limits for R50% were established.
• The proposed range bounds over 90% of the clinical data analyzed.
• The limits provide a challenging benchmark for treatment planning.

Conclusions:

• The study provides the first proposed intermediate dose spill guidance range for cranial SRS/SRT on C-arm linacs with MLC collimation.
• These R50% limits aid in optimizing and assessing treatment plans for various PTV sizes.
• The findings support the development of more standardized and effective SRS/SRT delivery techniques.