[The Dose Effect of Isocenter Selection during IMRT Dose Verification with the 2D Chamber Array]

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Intensity-modulated radiation therapy (IMRT) requires precise dose verification.
• 2D chamber arrays are commonly used for IMRT dose verification.
• The impact of isocenter definition on verification accuracy is not fully understood.

Purpose of the Study:

• To investigate the effect of isocenter positioning errors on IMRT dose verification using a 2D chamber array.
• To determine the optimal isocenter placement for accurate dose verification in IMRT.

Main Methods:

• IMRT plans from 10 patients were analyzed.
• Isocenters were defined at the target center (P(o)) and 8cm offset in x/y directions (P(x), P(y)).
• Dose verification was performed using a PTW729 2D chamber array with beams at 0 degrees, followed by gamma analysis.

Main Results:

• The mean gamma analysis passing rate was highest when the isocenter was placed at the target center (P(o)).
• Overall plan passing rates were better than individual beam passing rates across all groups.
• Dose leakage between leaves increased, worsening verification in the P(y) group with larger isocenter offsets.

Conclusions:

• Isocenter determination critically affects the accuracy of IMRT dose verification with 2D chamber arrays.
• For reliable IMRT dose verification, the planning isocenter should be positioned close to the geometric center of the target.