Large radiotherapyfields are being employed with increasing frequency in the treatment of Hodgkin's disease. The rationale for their use has been discussed extensively elsewhere (1–7). One such portal, commonly called the "mantle" field, encompasses all structures of thorax and neck excluding only the lungs (Fig. 1). This study was undertaken to determine the magnitude of the inhomogeneities of dose within such an irregular field and to devise means of making the dose more uniform. Before dealing with inhomogeneities in dose related to the patient's thorax, however, we wished to first examine separately the importance of the penumbra of the lung shields upon dose inhomogeneity. In Part I, therefore, a rectangular water phantom was used in the experiments to be described. Experiment I Methods and Materials The University of Minnesota's Canadian AEC “Eldorado 8” cobalt-60 unit (source diameter = 2.5 cm) and lead lung shields (7 cm in thickness) were used in the same manner as they are employed clinically. A 40 × 40 × 50-cm water tank was positioned 100 cm from the source to serve as a phantom. The lung shields were placed with their closest surfaces 40 cm from the front surface of the water tank, approximating the distance of the blocks from the patient, which is usually employed clinically. This distance is hereafter called the blocks-to-surface distance or “BSD.” The lung shields were separated from each other in a plane perpendicular to the central axis of the treatment beam such that an 8-cm-wide “mediastinal” field was projected onto the front surface of the water tank. This 8 cm width as defined was held constant for all experiments, as was the overall field size of 30 × 30 cm. Several lead disks were mounted on a Lucite strip and placed 10 cm deep to the front of the tank to simulate a mid-medi-astinal mass density which could be related to the image of the lung shields on the portal film. With use of the field-localizing light of the cobalt machine, the margin of the largest lead disk was aligned so that its silhouette was tangent to the shadow of the medial border of the left lung shield as the two were projected onto a film cassette placed 20 cm from the front of the tank. This tangent relationship of the two shadows produced by the light source was adopted primarily as a means of aligning these two key shadows as various other experimental conditions were varied. With this arrangement, all the important elements producing images on the portal film of a patient with a mediastinal mass density were duplicated (Fig. 2). The exposure time producing the best tonal values across the penumbra was then determined, and this exposure time was used for all portal films made. The dosimeter probe of an automatic isodose plotter (Toshiba model MRA-101-3B) was inserted into the water tank (Fig. 2). The position of this probe at different depth-dose readings at 10 cm depth was recorded on “portal” films.