Radiobiological evaluation considering setup error on single-isocenter irradiation in stereotactic radiosurgery

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiosurgery

Background:

• Single-isocenter stereotactic radiosurgery (SRS) is used for treating multiple brain metastases.
• Patient setup errors, specifically six degree-of-freedom (6DoF) errors, can impact treatment accuracy and efficacy.
• Understanding the dosimetric consequences of these errors is crucial for optimizing treatment planning.

Purpose of the Study:

• To calculate dosimetric indices and estimate tumor control probability (TCP) in SRS.
• To evaluate the impact of 6DoF patient setup errors on TCP using a single-isocenter technique.
• To determine the optimal isocenter position to mitigate the effects of setup errors.

Main Methods:

• Simulated spherical gross tumor volumes (GTVs) of varying sizes (1.0-3.0 cm) and distances from the isocenter (0-10 cm) were used.
• Dose distributions were calculated, and D95, conformity index (CI), and conformation number (CN) were evaluated under different 6DoF setup error conditions (rotation up to 1.0°, translation up to 1.0 mm).
• TCP was estimated, and optimal isocenter coordinates were derived to minimize TCP reduction.

Main Results:

• Dosimetric indices (D95, CI, CN) decreased with increasing isocenter distance, decreasing GTV size, and increasing setup error.
• TCP without setup error was 77.0%. With a 10 cm distance, 1.0 mm translation, and 1.0° rotation, TCP dropped significantly (25.8%-53.0% for GTV 1-3).
• At the optimal isocenter position, TCP improved to 52.3%-66.1% for GTV 1-3 under the same error conditions.

Conclusions:

• TCP in single-isocenter SRS for multiple brain metastases can decrease with increasing isocenter distance and decreasing GTV size when 6DoF setup errors exceed 1.0 mm and 1.0°.
• Utilizing an optimal isocenter position may help maintain TCP in the presence of 6DoF setup errors.
• These findings highlight the importance of precise patient setup and planning in SRS for maximizing tumor control.