Dosimetry of a linear accelerator under respiratory gating

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Respiratory gating is a technique used in radiation therapy to minimize motion artifacts.
• The impact of gating parameters, particularly duty cycle, on linear accelerator beam characteristics requires thorough investigation.

Purpose of the Study:

• To investigate the effect of respiratory gating on linear accelerator beam characteristics, focusing on duty cycle influence.
• To determine the impact of gating on depth dose curves, beam profiles, and absolute dosimetry.

Main Methods:

• Measurements were conducted on a Siemens Oncor linear accelerator using 6 MV and 15 MV photon beams.
• The Anzai gating system AZ-733V was employed to assess gating effects at various duty cycles (down to 5%) and realistic respiration frequencies.

Main Results:

• Depth dose curves and beam profiles remained largely unaffected by gating, with dose variations below 2.5% for realistic conditions.
• Absolute dosimetry showed significant deviations (>10%) at very small duty cycles, potentially leading to clinical underdosing.
• The number of monitor units per gate was identified as a critical factor for dosimetry accuracy.

Conclusions:

• Treatment planning for respiratory gating can utilize ungated data if gate sizes are sufficiently large.
• Absolute gate sizes must be individually determined for each accelerator to ensure accurate dosimetry.
• Careful consideration of duty cycle and monitor units per gate is essential for safe and effective gated radiotherapy.