A simple and efficient method to measure beam attenuation through a radiotherapy treatment couch and immobilization devices

We propose a simple and efficient method to measure beam attenuation in one or two dimensions using an amorphous silicon electronic portal imaging device (a-Si EPID). The proposed method was validated against ionization chamber measurements. Beam attenuation through treatment couches (Varian Medical Systems) and immobilization devices (CIVCO Radiotherapy, USA) was examined. The dependency of beam attenuation on field size, photon energy, thickness of the couch, and the presence of a phantom were studied. Attenuation images were derived by computing the percentage difference between images obtained without and with a couch or immobilization devices determining the percentage of attenuation at the center and the mean attenuation. The beam attenuation measurements obtained with an a-Si EPID and an ionization chamber agreed to within ± 0.10 to 1.80%. No difference was noted between the center and mean of an attenuated image for a small field size of 5 × 5 cm2, whereas a large field size of 15 × 15 cm2 exhibited differences of up to 1.13%. For an 18 MV beam, the a-Si EPID required additional build-up material for accurate assessment of beam attenuation. The a-Si EPID could measure differences in beam attenuation through an image guided radiotherapy (IGRT) couch regardless of the variabilities in couch thickness. Interestingly, the addition of a phantom reduced the magnitude of attenuation by approximately 1.20% for a field size of 15 × 15 cm2. A simple method is proposed that provides the user with beam attenuation data in either 2D or 1D within a few minutes

Sensitivity of Electronic Portal Imaging Device (EPID) Based Transit Dosimetry to Detect Inter-fraction Patient Variations

The sensitivity of EPID-based transit dosimetry to detect patient variations between treatment fractions is examined using gamma analysis and a structural similarity (SSIM) index. EPID images were acquired for 3-dimensional conformal (3DCRT) and dynamic intensity modulated (dIMRT) radiation therapy fields in multiple fractions. Transit images were converted to doses, transit dose in the first fraction considered the reference dose. Variations in patient position or weight were then introduced in the subsequent fractions. Positional variations were examined using a lung and a head and neck phantoms. Anatomical variations were examined using a slab phantom in three scenarios, with solid water simulating tissue, medium-density fiberboard simulating fat, and Styrofoam simulating lung. The dose difference between the first and subsequent fractions was computed using various gamma criteria and the SSIM index. Using a criterion of 3%/3 mm, EPID can detect positional variations ≥ 4 mm, and tissue and fat variations ≥ 1 cm, whereas it cannot detect lung variations up to 4 cm. The sensitivity for 3DCRT is higher than for dIMRT. EPID can detect the most variations when using 3%/1 mm. With the SSIM index, EPID can detect a 2 mm positional variation and 1 cm of lung variation. The factor that optimized the sensitivity of EPID was a reduction in the distance to the agreement criteria. Our study introduces the SSIM as an alternative analysis with high sensitivity for minimal variations

Brief histories of medical physics in Asia-Oceania

The history of medical physics in Asia-Oceania goes back to the late nineteenth century when X-ray imaging was introduced, although medical physicists were not appointed until much later. Medical physics developed very quickly in some countries, but in others the socio-economic situation as such prevented it being established for many years. In others, the political situation and war has impeded its development. In many countries their medical physics history has not been well recorded and there is a danger that it will be lost to future generations. In this paper, brief histories of the development of medical physics in most countries in Asia-Oceania are presented by a large number of authors to serve as a record. The histories are necessarily brief; otherwise the paper would quickly turn into a book of hundreds of pages. The emphasis in each history as recorded here varies as the focus and culture of the countries as well as the length of their histories varies considerably

Brief histories of medical physics in Asia-Oceania

The history of medical physics in Asia-Oceania goes back to the late nineteenth century when X-ray imaging was introduced, although medical physicists were not appointed until much later. Medical physics developed very quickly in some countries, but in others the socio-economic situation as such prevented it being established for many years. In others, the political situation and war has impeded its development. In many countries their medical physics history has not been well recorded and there is a danger that it will be lost to future generations. In this paper, brief histories of the development of medical physics in most countries in Asia-Oceania are presented by a large number of authors to serve as a record. The histories are necessarily brief; otherwise the paper would quickly turn into a book of hundreds of pages. The emphasis in each history as recorded here varies as the focus and culture of the countries as well as the length of their histories varies considerably