Ultraviolet ray irradiation duration for the pre-exposure of Gafchromic EBT2

The strength and density change of the ultraviolet (UV) ray of Gafchromic EBT2 were investigated. Previous studies suggested that UV-A rays can be substituted for the x-ray double-exposure technique to correct Gafchromic EBT2’s non-uniformity error. In this study, we aimed to determine the appropriate strength of UV-A rays for irradiating an active layer that would correct the non-uniformity error of Gafchromic EBT2. UV-A rays with a wavelength of 375 nm were used to irradiate Gafchromic EBT2 in various durations, and the resulting density change was investigated. To correct Gafchromic EBT2’s non-uniformity error, a pre-irradiation with a UV-A lamp was conducted at a distance of 72 cm for 30 min. To determine the most appropriate irradiation duration, a UV light-emitting diode generating UV-A of 375 nm was used to irradiate the Gafchromic EBT2 film with varying durations of 1, 2, 3, 4, 5, 10, 15, 20, 25, and 30 min at a distance of 5.3 cm. A 12.7 diameter region of interest was set by the irradiation area, and a histogram of pixel values was created. The condition options were decided based on two important requirements: 1) no zero values of the mode and seconds exist, and 2) the 1/10 value of the mode intersects both histogram sleeves. In the case of Gafchromic EBT2, the irradiation strength was 85.43 mJ/cm2 for one minute in which the pixel value of mean ± SD was 255.34 ± 213.29. The irradiation duration of 4 min was the border duration of the above two conditions. When a UV ray of 375 nm wavelength is used to irradiate Gafchromic EBT2 as a substitute for x-ray exposure, the 4-min pre-irradiation duration (341.74 mJ/cm2) is demonstrably sufficient.

Dosimetric evaluation of cobalt-60 teletherapy in advanced radiation oncology

BackgroundRecent investigations demonstrate a strong potential for cobalt-60 (Co-60)-based teletherapy. The influence of the lower energy and penetration of a cobalt-60 beam compared with linear accelerator beams is negligible for intensity-modulated radiotherapy.PurposeThe aim of this research is to investigate source head fluence modulation in cobalt-60 teletherapy by using a three-dimensional (3D) physical compensator and secondary collimator jaw motion.Materials and methodsThe Oncentra treatment planning system was used to develop three hypothetical plans by secondary collimator jaw motion. A clinical MDS Nordion Equinox 80 cobalt-60 teletherapy unit was used to acquire conventional water phantom beam characteristics. Fluence modulation experiments were executed at 5·0 cm depth in a PTW universal intensity-modulated radiation therapy (IMRT) verification phantom using calibrated Gafchromic external beam therapy 2 (EBT2) and RTQA2-1010 film batches. Gafchromic EBT2 film was used to sample intensity maps generated by secondary collimator jaw motion, yet Gafchromic RTQA2-1010 film sampled maps from the 3D physical compensator. The solid-state drives used were 75·0 and 74·3 cm for the Gafchromic EBT2 and Gafchromic RTQA2-1010 film measurements.ResultsA 2D gamma index analysis was coded to compare EBT2 film measurements with Digital Imaging and Communications in Medicine data. This analysis was also used to verify film measurements versus Monte-Carlo simulations.ConclusionLateral beam profiles generated from water phantom measurements were used to establish source head fluence modulation on the film measurements. The source head fluence of a cobalt-60 teletherapy beam could be modulated by secondary collimator jaw motion and using a 3D physical compensator.

Determining optimum wavelength of ultraviolet rays to pre-exposure of non-uniformity error correction in Gafchromic EBT2 films

Gafchromic films have been used to measure X-ray doses in diagnostic radiology such as computed tomography. The double-exposure technique is used to correct non-uniformity error of Gafchromic EBT2 films. Because of the heel effect of diagnostic x-rays, ultraviolet A (UV-A) is intended to be used as a substitute for x-rays. When using a UV-A light-emitting diode (LED), it is necessary to determine the effective optimal UV wavelength for the active layer of Gafchromic EBT2 films. This study evaluated the relation between the increase in color density of Gafchromic EBT2 films and the UV wavelengths. First, to correct non-uniformity, a Gafchromic EBT2 film was pre-irradiated using uniform UV-A radiation for 60 min from a 72-cm distance. Second, the film was irradiated using a UV-LED with a wavelength of 353-410 nm for 60 min from a 5.3-cm distance. The maximum, minimum, and mean ± standard deviation (SD) of pixel values of the subtraction images were evaluated using 0.5 inches of a circular region of interest (ROI). The highest mean ± SD (8915.25 ± 608.86) of the pixel value was obtained at a wavelength of 375 nm. The results indicated that 375 nm is the most effective and sensitive wavelength of UV-A for Gafchromic EBT2 films and that UV-A can be used as a substitute for x-rays in the double-exposure technique.