Phantom used for the irradiation of a film sheet in a megavoltage radiation beam entering the phantom from the left side. The film (4) has partly been withdrawn from the phantom when making the picture.
In the first step to produce simulated dose distributions for the Dosimulator (see previous item), dozens of beams were recorded by irradiating graphic black-and-white film.Before those measurements started, a study was performed of the characteristics of the film material and the relationship between the radiation dose and the blackening of the film. The relationship between the film blackening and the radiation dose is not linear. Also the processing of the film influences this relationship. Lines with equal grey values on the film do not agree with lines with equal dose. The specific radiation sensitivity of the chosen type of graphic film was well documented and the processing procedure was made stable. The result is a reproducible method of film dosimetry of megavoltage radiation beams that was used for a long period at the Netherlands Cancer Institute for research purposes and for verification of the characteristics of the radiation beams in the Radiotherapy Department.
The "Film translator".
The principle of the Dosimulator is, however, based on a coincidence of lines of equal grey values with lines having equal dose. The study showed that this is not the case with the films processed in the usual way. The "Film translator" was developed as the solution for this problem. The grey values of the original film are scanned with a densitometer having a light spot with a small diameter. Measured relative grey values are then spot by spot corrected for their known deviation from the real dose percentage.
The measurement results are used for steering a digital film exposure system. In this way a second film is exposed with pixels having a size of 1 x 2 mm. Each pixel gets a grey value determined by the ratio of fully black and un-exposed film according to the percentage dose that has been calculated at that position. In this way a digital film format is obtained of which the grey values are linear with dose.