An Integrated Recording Film Densitometer and Isodose Plotter

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Abstract
IN THE RADIATION treatment of carcinoma through either a single portal or 1800 opposed portals, the problems of calculation of isodose distribution and depth dose are not particularly difficult. They become more complex, however, if one adds ports at other angles or inserts wedges or bolus. With full rotation therapy or arc or skip rotation therapy over uneven body contours, the problem of accurate isodose calculation becomes tremendous. A similar question arises in the determination of dose about an intracavitary applicator. To surmount some of the difficulties of determining and recording dose about a radioactive applicator or a therapy beam and to permit rapid calculation of dose distribution produced by multiple beams, an integrated system was designed and built, consisting of an isodose plotter with a photodensitometer and a scintillation probe to be used as alternative methods of measuring dose. Information obtained by the measuring devices is fed through special electronic circuits and is recorded by a modified printout system (Fig. 1). Recording Film Densitometer In the evaluation of dosage by a film system, the film is placed in a suitable phantom (Fig. 2), exposed to the treatment beam, and then processed. The density pattern on the film will then represent the dose distribution in the area under treatment. These films are ordinarily read in a conventional photodensitometer and the results standardized by ionization chamber readings. Reading a complete film by this method is a slow and laborious job fraught with many inaccuracies. To reduce the time required for interpretation and to prevent inaccuracies in recording the data obtained, the automatic electronic film-reading and print-out system to display a complete isodose pattern from the exposed film was devised and constructed. This device uses as a basic starting point the system described by Gatzek and others (1) with considerable improvement and modification to produce what is a simple but at the same time reliable system for isodose plotting. The film isodose contour plotter enables one to place the exposed and processed film into the machine and have the corresponding density printed out in numbers whose interfaces represent isodose contours (Fig. 3), To convert from these contours to actual isodose plots it is necessary to calibrate the numerical plot of density by including ionization chambers in the film phantom system, Since the most significant consideration, however, is the relative distribution of dosage within a treatment field or series of fields, it is sufficient for teaching purposes and daily clinical planning to use the relatively large amount of information obtained from the film by the numerical indication of relative dose without the necessity of ionization chamber correction into absolute dose. The film scanning and printout are done simultaneously by means of a modified IBM electric typewriter.

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