Maximum distance in single-isocenter technique of stereotactic radiosurgery with rotational error using margin-based...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Planning

Background:

• Patient setup errors, particularly rotational errors, can compromise treatment accuracy in radiotherapy.
• Single-isocenter techniques aim to simplify treatment delivery but are sensitive to setup inaccuracies.
• Maintaining precise geometrical coverage of the planning target volume (PTV) is crucial for effective dose delivery.

Purpose of the Study:

• To evaluate the impact of patient setup rotational errors on geometrical coverage in single-isocenter radiotherapy.
• To determine the maximum allowable distance between the isocenter and the gross tumor volume (GTV) that ensures PTV coverage under rotational errors.
• To establish safety margins for single-isocenter techniques considering rotational inaccuracies.

Main Methods:

• Geometrical simulations were performed using spherical GTVs of varying sizes (1.0-3.0 cm diameter).
• Simulated rotational errors (0°-2.0°) were applied around the x, y, and z axes.
• The reduction in geometrical coverage was calculated, with a 5% GTV reduction tolerance defined.

Main Results:

• With a 0.1 cm PTV margin and 0.5° rotational error, maximum distances from the isocenter ensuring 5% tolerance were: GTV1 (1.0 cm) 7.6 cm, GTV2 (1.5 cm) 10.9 cm, GTV3 (2.0 cm) 14.3 cm, and GTV4 (3.0 cm) 21.4 cm.
• A 0.1 cm PTV margin with 0.5° rotational error necessitates excluding targets beyond 7.6 cm from the isocenter for the smallest GTV (1.0 cm) to meet the 5% coverage tolerance.
• Larger GTVs showed greater tolerance to distance from the isocenter under the same rotational error and PTV margin.

Conclusions:

• Single-isocenter techniques require careful consideration of target distance from the isocenter when rotational setup errors are present.
• For a 0.1 cm PTV margin and 0.5° rotational error, targets located more than 7.6 cm from the isocenter may not be adequately covered, particularly smaller GTVs.
• Excluding targets beyond a specific distance from the isocenter may be necessary to ensure geometrical coverage within acceptable tolerances in single-isocenter radiotherapy.