A prototype of a portable <tex-math>${}^{90}{\rm Sr}/{}^{90}{\rm Y}$</tex-math> irradiator for extracorporeal irradiation of the blood (ECIB) is described. Eight patients with chronic uremia received median transit doses of 14-57 rads during an average of 160 hr continuous ECIB. In all patients the lymphocyte concentration remained around 50 to 60% of the pre-ECIB value during the following 200 days of observation. When these results are compared with the development and degree of lymphopenia in patients receiving higher transit doses (100 to 500 rads), a dose-response effect seems likely. It is concluded that the construction of a portable beta unit which can be used in the clinic without any risk of exposure is only compatible with the use of sources which deliver very low transit doses. The duration of treatment required to significantly deplete the entire lymphoid mass using such minimal transit doses would, however, be prohibitive.

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