Assessment of dose inhomogeneities in clinical practice by film dosimetry.

Abstract

To use portal images acquired in routine circumstances for assessment of midplane dose variations in the patient. Optical density readings are performed on routinely acquired Verification films of breast and ear-nose-throat (ENT) cancer patients and these readings are converted into relative doses with the sensitometric curve. ( 1 ) The impact of redistribution is evaluated on films taken close to the patient exit surface and at routine focus film distance. (2) Midplane doses are estimated from film readings to assess dose variations in the patient. The influence of wedges is evaluated. Film measurements doses are compared with calculated exit doses. (1) In regions with large variations in the distance between the patient exit surface and the film but without inhomogeneities in tissue density, the relative doses distributions read on films acquired at large focus-film-distance (FFD) are proportional to exit doses. In regions with flat exit surfaces but with inhomogeneities in tissue density, the redistribution has only a small impact. (2) Large variations in relative midplane doses were found in both breast (85-115%) and ENT (-3.6 to +15%) patients. The application of a wedge was shown to increase dose homogeneity in the midplane. A good agreement (differences < 3%) was found between exit doses obtained from film readings and exit doses calculated with the treatment planning system (TPS). Films acquired in routine circumstances at large FFD can be used to obtain information on exit doses and to assess midplane doses in breast and ENT, without the use of a TPS. Film dosimetry can also provide a quality assurance tool to check actually delivered doses in patients by comparing exit doses estimated on film to expected exit doses calculated by the TPS.