Effects of chemical processing time on the reproducibility of film dosimetry and its optimization

Objective: To monitor the performance constancy of the HS-126E automatic X-ray film processor and to investigate the effects of chemical processing time on the reproducibility of film dosimetry. Methods: With films having identical exposure, 15 ready-pack Kodak X-omat-V films from the same batch were divided into 5 groups and exposed one by one under the identical conditions. One group of test film was processed per day from Monday to Friday with identical temperature and time. All films were digitized with a Vidar VXB-16 bit film digitizer. The average optical density (A) at irradiation field center was analyzed with RIT113 film dosimetry sys- tem software. At the same time, another 10 films in 5 groups from the same batch were exposed with vertical method and processed to get the calibration data and sensitometric curves. The film sensitivities, i.e., ratio of A to delivered dose, were calculated and compared between groups. Another 15 films in 5 groups were exposed with horizontal method and processed similarly to the test films, the point dose at 1.5 em depth on the central axis was calculated with sensitometric curve and compared with the value measured by ion chamber. Results: Variations ora value in each group was less than 2 %. During the processing period, a trend of decrease along with time in mean A value decreased clay by day was observed. Both the standard deviations and coefficients of variation in all groups were within 2%. One-way ANOVA showed that there was a significant difference of the mean A value among all films from Monday to Friday (P<0.05); between the last 2 groups and the former 3 groups (P< 0.05). Quality control of sensitometric curves showed a trend of decline, which gave the expression of film sensitivity reduction with time. The difference of the measured point dose value between each group was less than 2% and less than 3 % when compared with that measured by ion chamber. The point dose measured with film varied little with time during one processing period. Conclusions: Our data suggest that films used for radiation dosimetry should be processed in the early days of the processing period, which is especially important for A measurement. Film processing also has influence on sensitometric curve. Therefore, films used for measurement and calibration should be processed at the same time for the result. (authors)