A former publication (1) has aroused sufficient interest to bring forth inquiries as to how the qualitative data there illustrated may be applied in the calculation of depth dosage, the author having advanced this method of depth dose calculation in preference to the various intensities indicated on isodose charts or the still less useful percentage depth dose at the 10th centimeter. There is perhaps no gainsaying the fact that through any single portal the maximum absorption is always in the more superficial layers. Furthermore, the lower the voltage and weaker the filtration, the greater is the absorption, with the most intense biological effect in the superficial layers. When, therefore, in the treatment of superficial lesions a maximum radiation effect is desired, with the least damage to deeper lying tissues and a minimum exit dose, lower voltage and weaker filtration are the choice.2 To follow the same line of thought with regard to subcutaneous and more deeply seated lesions, the most logical method would be to select that quality, i.e., voltage and filtration, whereby the lesion will, if possible, be encompassed within the zone of maximum absorption. This will of necessity be above the 50 per cent intensity level. When lesions are at a depth greater than that encompassed within the first half-value zone (this because of the limitation of the available equipment and technic), then the inclusion of the lesion within a zone of lesser absorption than the half value must suffice. In the treatment of deep-seated lesions in large persons, it stands without argument that (within reason) the highest available voltage with relatively strong filtration affords the maximum depth dosage. The first thing then is to determine the operating conditions under which the best quality factors are to be established. By far the greatest number of therapy installations today are of 200 kv. capacity. There are a number of 400-kv. units and a very few operating at over 600 kv. One of the most frequently employed filters is 0.5 mm. copper faced with one or more millimeters of aluminum. From this, the filter thicknesses vary, with composite and other filters ranging from the equivalent of 0.25 mm. upward to 6.0 mm. and more of copper. A survey of the literature backed by our own experimental work shows that the half-value level, or 50 per cent depth dose, in a large water phantom through a large portal, at the usual focal distance (50 to 70 cm.), varies from d/2 at 6 cm. with 200 kv. and 0.5 mm. Cu filter to d/2 at 9 cm. with 2.0 mm. Cu filter.3 There are a number of items, economic, physical, and physiological, which have a bearing on the quality factors to be employed. The experimental work thus far conducted and the clinical results obtained would hardly appear to justify the heavy expense of a 1,000,000 volt installation.4