Sterilization of medical disposable devices with electron beam vs cobalt 60

Radiat. Phys. Chem. Vol. 33, No. 2, pp. 179-180, 1989 Int. J. Radiat. Appl. lnstrum. Part C Printed in Great Britain. All rights reserved

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LETTER TO THE EDITORS STERILIZATION OF MEDICAL DISPOSABLE DEVICES WITH ELECTRON BEAM VS COBALT 60 (Received 7 December 1987; in revised form 27 January 1988)

In previous publications, ~]'2) the author has tried to establish the technology of Electron Beam (EB) sterilization rather than using gamma-rays from cobalt 60, which is what most industrial radiation sterilization "machines" use today. He has indicated that there is a possibility of performing such sterilization, even on rather thick or deep boxes which contain medical devices (which typically may have densities of about 0.15-0.2g/cm2), provided that there is sufficient voltage or energy available to the electrons of the EB, and provided, of course, that there is a regulatory climate permitting the use of such voltages. It is recognized that there is at least one linear accelerator in existence which routinely sterilizes such devices at an energy of 12 MeV, (6) but it is this author's understanding that voltages are not generally to be permitted in excess of 10MeV, whether or not one established beyond a reasonable doubt that artificial radioactivity will not be induced by EB's of such energy. Some doubt has been expressed as to the usefulness of EB's for this purpose by virtue of the relatively small penetrating power of electrons as compared with cobalt 60 gamma-rays. It has occurred to the author that it may be possible to show the facts of the situation in a way which has not been published hitherto, and this note is an attempt to do so. In the first place, there are some actual data which show the degree of penetration which is characteristic

of both cobalt gamma-rays and electron beams of various energies.(3,4) Further, it is possible to make corrections to the cobalt 60 data for "inverse-square" losses.(5) The resulting curves do look a bit artificial, in that (a) the attenuation curve from Brynjolfsson's work gives the appearance of having been made under typical radiographic conditions rather than under those which actually prevail in an ordinary cobalt irradiator; and (b) if one corrects for decrease in dose rate with inverse-square of the distance from the source (5) and by simply subtracting 22% of the gamma-ray intensity, to correct for an average density of a "typical" medical disposable box as about 0.2, the final curve does not go through the origin, as it should (Fig. 1); it is clear that 22% of 0.9 is more than 22% of 0.5. Of course it is possible to scale the curves so that the "corrected" curve does go through the origin, but this does not seem to be worth the trouble in this case, since the "radiographic" information, if that is what it is, does give the reasonable penetration which can be compared with that from electrons. And one is not certain that the average density of 0.2 is always reasonable either. Now let us look at Fig. 2. The data for Curves I and II were taken from a paper already referred to (2) which does show the penetration of electron beams of a number of energies, on an equal-entrance and exit basis, in g/cm 2 (cm of water). Since this is directly comparable with the cobalt data, which are in terms

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Fig. I. Penetration of gamma-rays from cobalt 60, showing decrease in intensity with thickness;(3) including both absorption in water and inverse-square losses. 179

Fig. 2. Electron beam penetration (after Brynjolfsson(4) and Bly(2)): curve I--water, one side irradiation; curve l I - water, two side irradiation; curve I I I - - a box o f average density 0.15.

180

Letter to the Editors

of penetration in water, we have chosen to use the same scale in the figure. It will be obvious to even a casual reader that the resulting penetration values are far less than that obtainable from cobalt 60 gamma radiation, even though the depth--dose curve and the attenuation curve from cobalt 60 gamma-rays are not identical. Initially, then, we appear to have proved the obvious, namely that cobalt 60 gamma radiation has inherently far better penetration than can be achieved by electron beams of reasonable energy. Now we should look at Curve III in Fig. 2, which a priori does seem to cover more reasonable thicknesses. It transpires that Curve III is a replot of the data in Curve II, "corrected" for a typical box average density of 0.15, not 0.2. This author's experience indicates that there is little to choose between the two density values, even though 0.1, which has been used with some medical devices, would seem to be too low. However, many medical disposable devices are packed so as to present an average density of 0.15, and many are packed as to represent an average density of 0.2. And between 0.15 and 0.2 there is only a small difference in electron beam penetration; e.g. at 10 MeV an electron beam would still penetrate a distance considerably more than 35 cm, even at a product-density of 0.2. At any rate, the figure shows that the penetration achievable by electrons at a density of 0.15, is fully comparable with what can be had using cobalt 60 gamma-rays. One must admit

that the gamma-ray curve represents penetration in water, and that therefore we have shown the gammarays from cobalt 60 can penetrate practically any thickness likely to be encountered in the medicaldisposable industry. We have shown however, that disposable medical device packages of bulk densities up to 0.2 g/cm 2 of real thicknesses up to 35 cm can be sterilized with a relatively uniform dose distribution, using electron beams of 10MeV accelerating potential. Radiation Dynamics, Inc., 316 South Service Road, Meeville, N Y 11747, U.S.A.

JAMES H. BLY

REFERENCES

I. J. H. Bly, Radiat. Phys. Chem. 1979, 14, 403. 2. J. H. Bly, J. lndust. Irradiat. Tech. 1985, 3 (1), 63. 3. A. Brynjolfsson, Technical Developments and Prospects for Sterlization by Ionizing Radiation (Edited by E. R. L. Gaughran and A. J. Goudie). Multiscience, Montreal, Canada, 1974. 4. A. Brynjolfsson, in Proc. Int. Conf. on Radiation Research, U.S. Army Natick Laboratories, from Radiation Research, U.S. Department of Commerce, Office of Technical Services, 1983, p. 116. 5. J. P. Farrell, S. M. Seltzer and J. Silverman, Radiat. Phys. Chem. 1983, 22, 469. 6. C. B. Williams, private communication, 1978.