Myelography with the Use of Thorium Dioxide Solution (Thorotrast) as a Contrast Medium

Abstract

AS with many another substance which may eventually prove to be harmful if used indiscriminately, the status of thorium dioxide solution (thorotrast) in the literature appears to be determined largely by each author's inclination to approve or disapprove of its use. Either contention may be supported by experimental evidence based on the results of the administration of comparatively large doses in both animals and humans. This substance, in small amounts, probably possesses no greater potentialities for harmful effects in the body than many other diagnostic and therapeutic substances now in use. If given in carefully calculated doses which are below the minimal estimated toxic level, we feel that it may be possible to take advantage of some of its valuable properties. In the report of the Council on Pharmacy and Chemistry (1), the properties of thorium dioxide solution (thorotrast) are outlined. Its composition, as given by the manufacturer, is said to be a stabilized thorium dioxide solution containing 25 per cent by volume of thorium dioxide (from 19 to 20 per cent by weight) and from 16 to 19 per cent by weight of dextrin preparation as a protective colloid. There is added to the solution 0.15 per cent methyl-p-hydroxybenzoate as a preservative. It is felt by those who have employed it that, excepting exploratory surgery, it yields diagnostic information not obtainable by other means. The problem of ultimate toxicity is the main consideration, since the immediate reaction to its use is slight. The apprehension regarding the use of thorotrast is based primarily upon its radio-active properties. Thorium and its preparations disintegrate into mesothorium, radiothorium, thorium X, thorium emanation, thorium A, B, C, C′, C″ (2, 3), until the products of disintegration accumulate and reach equilibrium with each other (4). Despite the low penetrating powers of the alpha ray, this ray is believed by Flinn (5) to be of greatest importance in its effects upon tissue, the ratio of toxicity for the alpha, beta, and gamma rays being given by Martland (6) as 10,000 to 100 to 1, respectively, while their relative penetrations vary in the reverse order. The radio-activity of thorotrast has been summarized in the report of the Council as follows: “Alpha ray—Twenty-five c.c. of thorotrast is equivalent to a maximum of one microgram and a minimum of 0.5 microgram of radium (Schlundt). “Beta rays—‘Too feeble to be of physiological significance’ (Schlundt). “Gamma rays—Twenty-five c.c. of thorotrast is equivalent to 0.4 microgram of radium (Radium Institute Bergakademie, Freiberg); equivalent to approximately 0.3 microgram of radium (Flinn). ‘Too feeble to be of physiological significance’ (Schlundt).” The significance of these figures will be considered later but it is sufficient to state that in this study they afford the basis for calculating the radio-activity of the retained thorotrast.