Technique for verifying treatment fields using portal images with diagnostic quality

Area of Science:

• Medical Physics
• Radiotherapy Imaging

Background:

• Portal imaging quality for megavoltage photon beams using the double-exposure technique is suboptimal compared to diagnostic X-ray images.
• Accurate visualization of the treatment area is crucial for effective radiotherapy.

Purpose of the Study:

• To develop a technique for producing diagnostic-quality portal images for radiotherapy.
• To superimpose megavoltage portal images with diagnostic X-ray images on a single film.

Main Methods:

• A commercially available X-ray tube was mounted on a 60Co unit head.
• A leveling device ensured precise alignment between the X-ray focal spot and the 60Co source.
• Film-screen combinations were evaluated, with therapy verification film and rare earth intensifying screens identified as optimal.
• The relationship between off-axis dose and the portal image penumbral region was analyzed.

Main Results:

• The described technique successfully produced superimposed megavoltage portal and diagnostic X-ray images with diagnostic quality.
• Therapy verification film in a rare earth intensifying screen cassette proved most suitable.
• The analysis of off-axis dose and penumbral regions provides valuable insights for portal verification film interpretation.

Conclusions:

• The developed technique significantly enhances portal image quality for radiotherapy.
• This method offers improved visualization for radiotherapists, aiding in treatment verification.
• Understanding the dose-image relationship is key for accurate interpretation of portal verification films.