Film dosimetry in the peripheral region using multiple sensitometric curves.

Abstract

We present a method for applying film dosimetry to the peripheral region utilizing multiple sensitometric curves. There are many instances when the dose to the peripheral region outside the field edges is of clinical and/or research interest. Published peripheral dose data may be insufficient if detailed dose modeling is required, and in those cases measurements must be performed. Film dosimetry is an attractive approach for dose measurement in the peripheral region because it integrates dose, overcoming the low-dose-rate problem, and is time efficient, as it acquires an entire plan of data in a single exposure. However, film response increases at energies below approximately 300 keV. As the scattered photon spectrum changes with distance from the field edge, this increased film sensitivity causes changes in the film response along profiles perpendicular to the field edge. A single sensitometric curve is therefore no longer sufficient for accurate conversion of the optical density to dose. Our new method uses multiple sensitometric curves defined at increasing distances from the field edge. To convert an optical density profile, the dose at each point in the profile is defined as a linear combination of the doses calculated using the two sensitometric curves that bracket the point of interest. A single set of sensitometric curves derived at one field size and source-to-surface distance (SSD) can be applied to density profiles for other field sizes and SSDs. We verified our new method by comparison to ion chamber measurements using three different types of film. Agreement with chamber measurements was within 7%, or less than 2 mm in regions of high gradient, over a wide range of field sizes and SSDs.