Photosensitive epilepsy and image safety.

Photosensitive epilepsy came to prominence in the 1950s with the advent of television. Photosensitive epilepsy occurs in 1 in 4000 of the population. The incidence is 1.1 per 100,000 per annum, however amongst 7-19 year-olds the incidence is more than five times as common. Photosensitive epilepsy is twice as common in females as in males. The onset is around puberty, but less than 25 per cent of patients lose their photosensitivity in their twenties. Patients are investigated in the EEG laboratory using intermittent photic stimulation. Peak sensitivity is between 16 and 20 flashes/s but 49 per cent of patients are sensitive to 50 flashes/s, explaining the sensitivity to PAL television systems. From 1993 the development of broadcast guidelines was developed restricting both flash rates and the areas of screen involved, as well as the use of long-wavelength red. Automatic analysis systems can now test material for compliance with guidelines in real time.

A comparison of polyacrylamide gels and radiochromic film for source measurements in intravascular brachytherapy.

For intravascular brachytherapy with catheter-based systems, AAPM Task Group 60 has recommended measurements that should be made to characterize the sources. Beta emitters, including (90)Sr/(90)Y are ideal for intravascular brachytherapy, but problems arise in measuring dose distributions in the high dose gradient region at short distances from the source. In this paper, measurements of radial and orthogonal dose distributions and dose profiles for a (90)Sr/(90)Y source train using polyacrylamide gel (PAG) dosimetry and a high-field 4.7 Tesla MRI scanner are presented and compared with measurements made with two types of radiochromic film, MD-55 and HD-810. For the PAG system, the dose distributions were determined with in-plane resolutions of 0.4 mm and 0.2 mm. The measurements of absorbed dose distributions both orthogonal and parallel to the source axis show good agreement between the PAG and radiochromic film. The absolute dose at a radial distance of 2 mm in the central 32 mm of a line parallel to the axis was measured. For the PAG the measured absorbed dose was 1.25% lower, for MD-55 4% higher and for the HD-810 1.6% higher when compared with the value given by the source calibration. These results confirm that both absorbed dose and dose distributions for high gradient vascular brachytherapy sources can be measured using PAG but the disadvantages of gel manufacture and the need for access to a high resolution scanner suggests that the use of radiochromic film is the method of choice.