Adaptation of Computed Radiographic System for Treatment Setup Verification in External Beam Radiotherapy

ABSTRACT

In radiation therapy, the traditional way of providing accuracy of megavoltage radiation treatment fields, is by the use of portal radiographs. In these radiographs the film detector is positioned at the exist point of the beam beside the patient and details of its anatomical parts located within the field borders are taken. There is poor visualization of landmarks in portal films due to the high energy of photons used for treatment. Due to this draw-back, an electronic portal imaging device (EPID) is now being assembled with modern therapy machine. This device uses kilovoltage energies for treatment setup verification, which provides clear anatomical landmarks of patient for treatment. The importance for field edges verification is to:

 Verify the shape of the radiation beam.

 To establish a coordinate system common to both reference and portal image in which to account for deviation in patient setup.

The aim of this study was to assess the possibility of using computed radiography (CR) system instead of the conventional film in the use for treatment setup verification with the telecobalt machine at the radiotherapy department of the Korle-Bu Teaching Hospital. In diagnostic radiology, the use of CR has been long in existence to obtain digital radiographic images which help in providing picture archival communication system (PACS). The film is substituted with a photostimulable phosphor plate (PSP) in CR system. Energies deposited by the X-ray beam on the PSP are read by the scanner laser beam in the CR reader. After the PSP has been scanned by the reader, it can also be ready for reuse by exposing it to light for about forty seconds for the erasure of any remaining image on the plate. A phantom was constructed and fabricated from a perspex which compensates for the human tissue. The phantom was irradiated using cobalt-60 source in which both the CR and the radiographic film were used as detectors to detect the images of the built phantom. These images were scanned using the scan maker 9800 XL. This thesis defines the use of the current image quality matrices such as Modulation Transfer Function (MTF), Contrast to noise ratio (CNR) and Signal to noise ratio (SNR) which show a practical easy-to-use software based program Image J for measurable assessment of digital (CR) and conventional detector system (film).

From the results obtained, the CR demonstrated superior values in CNR and SNR than that of the radiographic film, which shows better quality of images on CR. Notwithstanding these qualities in CR, the film also showed a better resolution than that of the CR.

Therefore digital treatment portal and simulation images, can easily be obtained with CR system which offers a base for a picture archival communication system in radiation oncology. Simulation and digital portal images will ease verification of treatment setup.