This may be defined as the dose received by the skin opposite the irradiated surface, such as that received by the palm from irradiation of the dorsum of the hand. The exit dose is usually expressed as per cent of the dose received by the skin undergoing treatment. This percentage depends on a number of factors, the most important of which are voltage and filtration, target skin distance, size of field, and thickness of part treated. The air dose alone does not express adequately the amount of radiation received by the skin, and the back scatter and the exit dose, if any, must be taken into consideration. Lack of recognition of these and other physical factors may account for the discrepancy often found with respect to the number of roentgens required to produce an erythema or epilation and the common localization of radiodermatitis on the hands, feet and middle of the face.
When both surfaces of the hand are irradiated, the exit dose and back scatter increase the skin dose over the air dose to each surface of the hand by the following average percentages: 25 per cent with radiation produced at 65 kv., 45 per cent at 100 kv. and 78 per cent at 135 kv. with a 3 mm. Al filter. These percentages may be applied in practice to any air dose given to both surfaces of the average hand. While there will be variations due to differences in thickness, these are generally too small to require correction.
The exit dose on the feet should also be taken into consideration when both sole and dorsum are irradiated.
The calculated air dose that should be given both the palmar and the dorsal aspect of the hand when it is desirable to give 100 r to the skin of each surface is 80 r when 65 kv. radiation is used, approximately 70 r at 100 kv. and only 55 r at 135 kv. and a 3 mm. Al filter. If one wishes to give 75 r to the skin of each surface, about three quarters of these doses should be used.
Tables 3 and 4 illustrate the advantage of 60 kv. over 100 kv. for dermatologic roentgen therapy. When the lower voltage is employed, the total skin dose received by the alae nasi from treating both sides of the face is about 90 per cent of the air dose applied to each cheek. This figure is well within the limits of safety and of therapeutic efficacy. On the other hand, if a voltage of 100 kv. is used each ala receives a dose of 112 per cent of the cheek dose measured in air. To illustrate this in a more practical manner, let it be assumed that patients are given twelve roentgen ray treatments for acne of the face, 75 r being used for each treatment. For patient A a voltage of 60 kv. is used and for patient B a voltage of 100 kv. At the end of twelve treatments, patient A will have received a total dose of 816 r to each ala nasi, which compares with a cheek dose of 900 r measured in air; and so each ala nasi received about the dose desired. However, patient B will have received a total of 1080 r to each ala, which compares with a cheek dose of 900 r. Thus the lower voltages tend to protect the nose from excessive radiation. For instance, at a voltage of 100 kv. the proximal ala nasi receives 12 per cent more and the exit dose on the distant ala 10 per cent more radiation in terms of the air dose than at a voltage of 60 kv. This is a total increase to each ala of 22 per cent if both cheeks are treated. These percentages are independent of the air dose.
It appears that low voltage radiation (at about 60 kv., half value layer 0.7 to 0.75 mm. Al) has definite advantages over higher voltage radiation for roentgen therapy of skin diseases.
Gonad Dose. The gonad dose is a subject of considerable importance, especially in persons under forty years of age. It is difficult to measure accurately by physical means. The dose which is safe for the gonads to receive is largely conjecturable. With x-rays the back scattering from the table or floor is an important consideration. The direction of the primary beam should be focused away from the genitals as much as possible. Cones should always be used and lead rubber sheeting should be beneath the patient to reduce the back scatter. With these precautions the average-sized adult with an epithelioma on the face may be treated with perfect safety. Also young men and women with acne of the face may be safely treated. When the shoulders, buttocks or feet are treated, soft radiation, shielding with a cone and ample use of lead rubber to reduce back scatter are important.
