How to estimate R50% for cranial SRS/SRT cases with overlapping 50% isodose volumes: A proposed system

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiosurgery

Background:

• Clinical stereotactic treatments with multiple targets often result in overlapping 50% isodose clouds (IDC50%).
• Accurately assessing dose conformity and intermediate dose spill becomes challenging in these overlapping scenarios.
• Existing methods struggle to consistently apportion dose when IDC50% from multiple planning target volumes (PTVs) intersect.

Purpose of the Study:

• To propose and validate a systematic method for apportioning overlapping IDC50% in single-isocenter cranial stereotactic treatments.
• To enable consistent evaluation of intermediate dose spill against emerging universal standards.
• To provide a reliable approach for cases involving multiple PTVs with intersecting IDC50%.

Main Methods:

• A novel "fair value estimate" (FVE) system was developed, apportioning IDC50% based on the relative surface area of each target volume.
• The FVE system's reasonableness was tested by comparing its results to actual R50% values in widely spaced target cases.
• The application of the FVE system to overlapping IDC50% scenarios was demonstrated.

Main Results:

• The FVE system demonstrated reasonable apportionment, yielding results similar to actual R50% in non-overlapping cases.
• The study provides a clear methodology for applying FVE to complex cases with overlapping IDC50%.
• The proposed method offers a consistent way to handle intermediate dose spill apportionment.

Conclusions:

• The fair value estimate (FVE) provides a systematic and consistent method for apportioning overlapping IDC50% in multi-target cranial stereotactic treatments.
• This method facilitates accurate assessment of intermediate dose spill, crucial for meeting proposed universal standards.
• The FVE system is recommended for consideration by the stereotactic community to address challenges in dose apportionment.