The surface area effect: How the intermediate dose spill depends on the PTV surface area in SRS

Area of Science:

• Medical Physics
• Radiation Oncology
• Neurosurgery

Background:

• Stereotactic radiosurgery (SRS) is a standard treatment for intracranial targets.
• Planning target volume (PTV) characteristics influence dose spill and potential brain toxicity.
• R50% quantifies intermediate dose spill, a critical factor in SRS planning.

Purpose of the Study:

• To investigate the relationship between PTV surface area (SA_PTV) and intermediate dose spill (R50%) in SRS.
• To introduce a novel understanding of the
• surface area effect
• on dose distribution in SRS treatments.

Main Methods:

• Utilized a phantom model (IROC Head Phantom®) and Eclipse® Treatment Planning System for SRS simulations.
• Employed a Varian TrueBeam® linear accelerator with a 6 MV FFF beam.
• Developed an Eclipse script to extract SA_PTV for arbitrary PTV shapes and compared R50% to an analytical prediction (R50%_Analytic).

Main Results:

• A strong correlation was observed between plan R50% and SA_PTV in SRS phantom studies.
• A near-linear relationship between R50% and SA_PTV was identified, consistent with the R50%_Analytic model.
• Good agreement was found between planned R50% values and R50%_Analytic predictions, especially for larger PTVs.

Conclusions:

• Demonstrated that SA_PTV is a key determinant of intermediate dose spill (R50%) in SRS, termed the "surface area effect".
• The R50%_Analytic model explicitly incorporates this dependence, offering a predictive tool.
• R50%_Analytic can guide SRS treatment plan optimization and evaluation for specific PTVs.