冲洗时间因素对胶片剂量的影响

目的监测冲洗药水更换周期内HS-126E X线胶片自动冲洗机的性能稳定性，研究冲洗时间因素对胶片剂量测量的影响。方法相同条件下曝光来自同一批的15张Kodak X—Omat—V胶片，随机分成5个组，每组3张。在冲洗药水更换周期的5个工作日内，每天冲洗1组。用RIT113胶片剂量测量系统软件测量每张胶片照射野中心的平均吸光度（A）值，进行组间比较。另取10张胶片，分成5个组，均以垂直法曝光，曝光后每天冲洗1组，用于建立校正文件并绘制感光度曲线。建立校正文件后，对整个冲洗药水更换周期内感光度曲线进行质量控制，并观察胶片敏感性的变化。再取来自同一批的胶片15张，分成5个组，均以平行法曝光，曝光后每天冲洗1组，用当天绘制的感光度曲线测量中心轴上1．5cm深度的点剂量，与相同条件下的电离室测量值比较，并进行组间比较。结果在冲洗药水更换周期内，各组胶片A值的均数有随时间延长降低的趋势，方差分析显示，各组A值的均数间差异均有统计学意义。感光度曲线的质量控制显示出随时间延长向下倾斜趋向，体现了胶片敏感性受冲洗因素影响而降低。各组胶片点剂量的差异〈2％，与电离室实测剂量的差别〈3％。冲洗周期内点剂量随时间的变化较小。结论HS-126E X线胶片自动冲洗机在冲洗药水更换周期的早期是稳定的。用于辐射剂量测量的胶片应当在冲洗药水更换周期内尽早冲洗，这对A值的测量尤其重要。尽管冲洗因素对感光度曲线影响显著，且胶片敏感性在冲洗药水更换周期内明显降低，但点剂量在冲洗药水更换周期内的变化较小。建议同时冲洗用于剂量测量的胶片和用于校正的胶片。

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 VXR-16 bit film digitizer. The average optical density(A) at irradiation field center was analyzed with RIT113 film dosimetry system 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?cm depth on the central axis was calculated with sensitometric curve and compared with the value measured by ion chamber.Results Variations of A value in each group was less than 2%.During the processing period,a trend of decrease along with time in mean A value decreased day 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.