SCIENTIFIC IMPORTANCE OF X-RAYS—GARLAND 195 
table is often more difficult but, unfortunately, sometimes is over- 
looked. The best way to achieve this protection is to use as small a 
beam as possible in examining patients, to work as expeditiously as is 
consistent with thoroughness, and to stay at the maximum distance 
or behind the safest barrier available. Protective barriers usually are 
made of lead or concrete. If workers are not safeguarded properly 
they may develop injury to the blood or reproductive system with 
consequent dangers of anemia, leukemia, or sterility. The tolerance 
dose is the total X-ray energy that a person may receive continuously 
without suffering damage to the blood or reproductive organs. For 
most workers it is set at 0.2 roentgen per day (a dosage rate not ex- 
ceeding 10 per second). Detailed rules for protection are available 
in the National Bureau of Standards Handbooks 20 (X-ray protec- 
tion) and 23 (Radium protection). 
X-RAYS IN TREATMENT 
X-rays are of considerable significance in the treatment of a large 
number of diseases, ranging from simple infections such as ringworm 
of the scalp to serious processes such as cancer. The physician perform- 
ing Roentgen therapy must strive to be as careful in the calibration 
and handling of his apparatus as a physicist, and as accurate in the 
application of his rays as a surgeon is when applying his knife. The 
therapist has at his disposal a wide variety of equipment supplying 
low-voltage beams for superficial treatment (40 to 100 kilovolt), in- 
termediate voltage for more deeply seated lesions (120 to 150 kilovolt), 
high voltage for deep-seated lesions (180 to 220 kilovolt) and, finally, 
extra-high voltage for a few selected conditions or for biological re- 
search (400 to 1,000 kilovolt). He must select a filter suited to the 
precedure desired, varying from less than 1 millimeter aluminum up 
to as much as 5 millimeters of copper. He calculates his dose in roent- 
gens, and delivers small doses to most inflammatory or benign condi- 
tions, and very high doses to certain localized malignant lesions. A 
small dose ranges from 10 to 100 roentgens and may need to be repeated 
at intervals for weeks or even months. For example, generalized acne 
vulgaris may require weekly doses of 100 roentgens for three months. 
On the other hand a small localized cancer may require 6,000 roentgens 
' delivered either at one session or, depending on the amount of associ- 
ated infection and similar complications, in several sessions. The 
essential effect of X and gamma rays on the tissues is a destructive one. 
Cancer cells are slightly more sensitive to such radiations than are 
normal cells, and for this reason it is possible to destroy some cancers 
without permanent injury to the normal surrounding tissues. 
The following is a partial list of conditions in which X or gamma 
rays are of value: Inflammatory diseases of the skin and adjacent 
