ABSTRACT
Gynaecological intracavitary insertions using 137Cs form the most common examples of treatments using sources taken from a standing inventory. As discussed in chapter 3, one of the last parts of the review includes a check that the correct sources occupy the proper positions. Seeing that such applications use relatively few sources, checking each source individually takes little time. A single source with the wrong strength easily produces errors in dose exceeding 20%. The manufacturers of intracavitary sources have used different approaches to marking their strength. Table 4.1 summarizes the methods used, and each has benefits and disadvantages. Most modern afterloading appliances use one or both of the means employed by the Fletcher-Suit applicator for treatment of cancer of the cervix: a uterine tandem and vaginal ovoids. For this applicator, the sources in the tandem are first slid down a clear plastic tube (sealed at one end), in the source-storage facility. Later, in the patient's room, the person loading inserts the plastic tube containing the source into the hollow tandem previously placed into the patient's uterine canal. For this type of loading, colour coding on the side of the source
Loading for the Fletcher-Suit ovoids proceeds very differently. Because the final orientation of the sources lies almost perpendicular to the handles (and access channels) of the ovoids, the sources sit in metal buckets on double hinges. Once in the bucket, a colour code on the top of the source remains visible, while that on the side becomes hidden. Thus a single marking system fails to provide indication of source strength in one or the other part of the applicator. A common attempt at solving this dichotomy finds the physicist adding enamel colour codes to either the top or side of each source. By choice, manufacturers produce sources with nonreactive jackets, and user-added marks require frequent refreshing, particularly on the side, where the source rubs against the appliance. User-added enamel marks on the side sometimes also cause problems fitting the sources into holders without much clearance. The manufacturer colour codes on the side use enamel-impregnated porcelain laid into machined channels to yield a uniform diameter along the source length. However, with heavy use and time, even these manufacturer markings fade or crack and fall out of their tracks.
