Therapy imaging: source sizes of radiotherapy beams.

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Imaging Science

Background:

• Accurate characterization of x-ray sources is crucial for radiotherapy quality assurance.
• Previous methods may not fully capture the complex intensity distributions of modern radiotherapy sources.
• Understanding source characteristics is vital for optimizing imaging and treatment planning.

Purpose of the Study:

• To develop and validate a novel method for measuring x-ray source intensity distributions.
• To quantify the spatial characteristics and intensity profiles of various radiotherapy x-ray sources.
• To assess the impact of x-ray source properties on overall image resolution in radiotherapy imaging.

Main Methods:

• Employed large lead collimators and computed tomography (CT) reconstruction techniques.
• Measured intensity distributions for 60Co, 6-MV, 18-MV, and 25-MV radiotherapy devices.
• Calculated modulation transfer functions (MTFs) for accelerator sources and compared them to detector MTFs.

Main Results:

• Accelerator x-ray sources were elliptical, varying in eccentricity and size (0.7–3.3 mm FWHM).
• The 60Co source was circular (20 mm diameter) but exhibited non-uniform output.
• Source MTFs declined rapidly at high spatial frequencies, indicating significant resolution loss.

Conclusions:

• The novel method accurately measures x-ray source intensity distributions.
• X-ray source characteristics significantly influence spatial resolution in radiotherapy imaging.
• Source-induced resolution loss is comparable to or greater than detector scatter in typical radiotherapy geometries.