Wedge factor changes with depth and field size on a linear accelerator with a motorized wedge

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• The wedge filter is crucial in radiotherapy for shaping radiation beams, particularly for treating tumors with specific dose distributions.
• Accurate determination of the wedge factor is essential for precise dose calculations in radiation therapy.
• Variations in wedge factors can lead to significant under- or over-dosing of patients.

Purpose of the Study:

• To experimentally determine the wedge factor as a function of field size and phantom depth for 6-MV X-rays.
• To investigate the impact of beam hardening on wedge factor measurements.
• To clarify the behavior of wedge factors for rectangular fields.

Main Methods:

• Measurements were performed on a linear accelerator equipped with a motorized universal wedge filter (60 degrees nominal wedge angle).
• Wedge factors were measured as a function of field size and phantom depth for 6-MV X-rays.
• Percentage Depth Dose (PDD) curves for wedged and open fields were compared to assess beam hardening effects.

Main Results:

• The wedge factor exhibited a linear increase with phantom depth (0.275%/cm), largely independent of field size.
• This depth-dependent variation was attributed to beam hardening caused by the high atomic number (Z) material of the wedge filter.
• For rectangular fields, the wedge factor variation followed the equivalent square law, irrespective of the dimension along the wedged direction.

Conclusions:

• The linear increase in wedge factor with phantom depth necessitates adjustments in dose calculations to avoid systematic errors, especially in deep-seated tumor treatments.
• Understanding the beam hardening effect is critical for accurate radiotherapy planning.
• The application of the equivalent square law for rectangular fields simplifies wedge factor determination in clinical practice.