Low-megavoltage electron beams have 4 been utilized during the past fifteen years as one of the major therapeutic modalities in the management of patients with lymphoma cutis (1, 13, 14). Treatment with total-skin electron irradiation has resulted in symptomatic relief of pruritus and burning and frequently has promoted involution of plaques, skin ulcers, and tumors. Low-megavoltage electron beams have been employed in psoriasis and atopic dermatitis but with more variable results (4, 13, 15). The use of conventional superficial x-ray therapy in lymphoma cutis results in a significant degree of cumulative total-body irradiation. An advantage of electron therapy has been that the penetration of electrons can be controlled, concentrating the ionizing radiation to the pathologically involved epidermis and dermis with relative sparing of the underlying hematopoietic and visceral organs. Cumulative experience with this therapeutic modality, however, suggests that a small fraction of radiation delivered to the skin is in the form of x-rays (7, 13, 14). The advantage of therapy with electron beams is diminished if this x-ray contamination represents a significant proportion of the total ionizing radiation. The purpose of this paper is to present an heretofore unreported source of x-ray contamination occurring during the course of large-field electron-beam therapy. Technics and Instrumentation Thirty-four patients with lymphoma cutis (mycosis fungoides, premycosis fungoides, and lymphosarcoma) and severe exfoliative dermatitis have been treated at the University of California at Los Angeles Center for the Health Sciences during the past two years. Morgan and Dowdy (12) and Haimson and Karzmark (5) have previously described in detail the technical aspects of the 6-Mev linear accelerator employed, 2 and have documented the negligible fraction of x- ray associated with the 6-Mev electron beam. The electrons are magnetically deflected away from a gold target and are scattered through an 0.008-in. nickel window. The beam is collimated entirely within the head by a tungsten primary collimator and tungsten-faced movable jaws. The patients are positioned at a distance of 600 cm, at which point the useful diameter of the beam is about 6 feet, with a uniformity of ± 7 per cent. Seven patients received low-megavoltage electron therapy with an initial energy of 6.5 Mev and a maximum penetration of approximately 2.5 cm of tissue at a distance of 600 cm, with the 50 per cent depth dose at 19 mm. Thirteen were treated with the same 6.5-Mev source but with the maximum penetration reduced to approximately 2 cm with the 50 per cent depth dose at 14 mm. The reduction was accomplished by interposing a 1¼4-in. sheet of Masonite over the exit portal of the collimating system. Further reduction of penetration to approximately 1 cm with a 50 per cent depth dose at 5 mm was employed for 18 patients3 by interposing 3¾4-in. of Masonite added filtration.