Assessment of dose inhomogeneities in clinical practice by film dosimetry

Aim: To use portal images acquired in routine circumstances for assessment of midplane dose variations in the patient. Material and methods: Optical density readings are performed on routinely a cquired Verification films of breast and ear-nose-throat (ENT) cancer patie nts and these readings are converted into relative doses with the sensitome tric curve. (1) The impact of redistribution is evaluated on films taken cl ose to the patient exit surface and at routine focus film distance. (2) Mid plane doses are estimated from film readings to assess dose variations in t he patient. The influence of wedges is evaluated. Film measurements doses a re compared with calculated exit doses. Results: (1) In regions with large variations in the distance between the p atient exit surface and the film but without inhomogeneities in tissue dens ity, the relative doses distributions read on films acquired at large focus -film-distance (FFD) are proportional to exit doses. In regions with flat e xit surfaces bur with inhomogeneities in tissue density, the redistribution has only a small impact. (2) Large variations in relative midplane doses w ere found in both breast (85-115%) and ENT (-3.6 to +15%) patients. The app lication of a wedge was shown to increase dose homogeneity in the midplane. A good agreement (differences <3%) was found between exit doses obtained f rom film readings and exit doses calculated with the treatment planning sys tem (TPS). Conclusion: Films acquired in routine circumstances at large FFD can be use d to obtain information on exit doses and to assess midplane doses in breas t and ENT, without the use of a TPS. Film dosimetry can also provide a qual ity assurance tool to check actually delivered doses in patients by compari ng exit doses estimated on film to expected exit doses calculated by the TP S. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.