Million-Volt Therapy 261 



ilar cases, it will be impossible to draw any dosage conclusions, 

 or to attempt by models to simulate the actual patient and so 

 solve the troublesome features mathematically. At a million 

 volts, the variations are disappearing, and an assessment of 

 results of different geometric methods of treatment is consider- 

 ably helped by a full physical investigation. 



Aids to Accurate Technique 



The light beam indicating the position and size of the X-ray 

 beam can be made to travel along the axis of the tube and, by 

 rotation of the outer sheath of the tube, at right-angles to the 

 tube-axis. These two movements are often of assistance, giving 

 an accurate idea in many cases of the position of the emergent 

 beam. Beam direction has been kept as simple as possible, there 

 being no three-dimensional angulation of beams if it can be 

 avoided, and the patient is either parallel or at right angles to 

 the tube-axis. The "pin-and-arc" device* of Dobbie ^ is used 

 for all angular directions, while a very simple device** is used 



* The pin-and-arc device is, in effect, a large protractor, mounted on a stand with 

 its center removed and a retractable central pointer fitted. In the sketch (figure 6), 

 the pointer is shown dotted at the center of the protractor (point A). Rays are 

 marked on the protractor panel at one-degree intervals radiating from A, with zero 

 vertical. The protiactor is set in the correct position with the aid of a plumb-bob 

 attached at the right-hand top corner. If it is required to direct the center of a 

 beam at a definite angle through a point inside a patient, the location of this point 

 relative to a skin-mark vertically above it being known, the device is used as follows. 

 In the sketch, the point to be aimed at is A, and it is, say, 9 centimeters below the 

 skin mark B. The retractable protractor central point is raised 9 centimeters from 

 its zero point, as indicated on the scale at C, and the device is arranged so that the 

 point is in contact with the skin mark B. The point A in the patient is then at the 

 center of all the protractor rays and, if the required angle is produced backwards 

 onto the patient's skin, the central point of entry, D, of the X-ray beam is obtained. 

 The depth (AD) of the point A from the central point of entry (D) of the beam 

 is obtained by measuring the distance of^D from a 30 centimeter arc E, inscribed 

 on the protractor from the center A. (AD = 30 centimeters less DE centimeters.) 

 ** See figure 7. A hoop, U-shaped, is fitted with a fixed point A, and an adjustable 

 pointer B on the other arm of the hoop, adjustable so that the distance between A and 

 B can be varied. On the hoop is fitted a protractor and plumb-bob C, which reads 

 degrees when AB is vertical. If in the sketch the center line of a beam has to 

 enter at A and emerge at B on a patient's head, the hoop points are adjusted to 

 these points and the plumb-bob protractor reading is taken. This gives the angle 

 required relative to the vertical. With the known divergence of a beam's edge the 

 device can also be used if the required in-and-out positions of the beam edge are 

 known. 



