Various terms used in radiotherapy are defined as follows: The dose depends on the intensity times the duration of the exposure. One milligram for one thousand hours delivers the same dose as a thousand milligrams for one hour. But the biologic effect upon actively growing cells is not the same; therefore descriptive details of each application are desirable.
The unit of dosage is the roentgen. However, it is difficult to measure the dosage of radium glazed plaques in contact with the skin in physical units. For this reason, the term roentgen is not used much with plaques, but instead the specification of the exposure in milligram hours is serviceable.
In interstitial therapy the dose is estimated as the intensity of the radiation at 1 centimeter distance from 1 milligram point source of radium element (in equilibrium with its products ) filtered by 0.5 mm. platinum. The dose rate of 1 milligram of radium, assuming a point source, filtered through 0.5 mm. platinum at 1 cm. distance is 8.4 roentgens in one hour.
It should be the constant endeavor of the radium therapist to obtain as nearly as possible a uniform dosage in all parts of the lesion. To effect this homogeneous irradiation, the various devices such as cross-firing, the use of radium spread over a considerable area as in the flat plaques, the use of a number of tubes placed about the lesion ( or a number of needles or implants placed into it), or trays at varying distances, have been adopted. Screening, which regulates the quality of radiation, is most important. All these methods have their advantages and limitations. Dosage is thus influenced by the details of the application, the chief of which are the distance from the skin, the screening, the size of the applicator, the quantity and distribution of the radiating material, and whether the source of radiation is a point, a tube, or a unit of another form.
Whenever exposures are repeated the intervals between the treatments influence the dosage. The optimum interval between treatments depends upon the dosage and the quality of the radiation, but most of all upon the disease under treatment. This subject will be taken up specifically in other chapters.
The skin erythema dose is, generally speaking, the smallest quantity of radiation which will produce a faint redness of the exposed skin, which usually appears five to seven days after exposure and lasts two to three weeks. Ideas of what constitutes a faint redness vary widely, but the threshold erythema dose is accepted by most dermatologists. In a general way, it requires considerably more hard radiation than soft radiation to produce an erythema. When combinations of soft x-rays and gamma rays are employed, it requires more of the combined radiation to produce the erythema than it does of either one alone. The erythema dose is of no interest in the treatment of cancer but is of some importance in the treatment of hemangioma.
The distance between the radium and the skin surface, as previously stated, has a tremendous effect upon the dosage. The inverse square law applies strictly only to radiation from a point source. For the tubes, plaques, or other applicators used in practice, the effect of distance is somewhat less than for point sources; the values cannot be calculated by a simple formula. Quimby, and Paterson and Parker have worked out tables for applicators of different sizes for various distances, so that doses from them may be compared directly, and by means of curves or tables a comparison may be made for any practical applicator at any distance. Also, if the dose is known for one applicator under a given set of conditions of distance and filtration, it may be obtained for any other applicator under any ordinary conditions of application.
Common empirical doses for the various types of skin lesions will be but briefly covered in this chapter, receiving more consideration under the captions of the respective diseases.
Flat Applicators. Flat Monel applicators 2 cm. square and containing 20 mg. of radium are in common use. Unscreened, this standard 20 mg. full-strength radium plaque in direct contact with the skin will produce an erythema in about ten minutes, an erythema with temporary epilation in about twenty minutes, and an intense erythema with desquamation and exudation in thirty minutes. The action is largely a beta ray effect extending to the depth of a few millimeters, which is often suitable for the treatment of senile and seborrheic keratoses and plantar warts. The erythema begins a few days after treatment, and persists in mild reactions for two weeks.
Flat square applicators screened through 2 mm. brass are suitable for the treatment of hemangiomas. The output is gamma radiation. Pfahler uses 1 mm. platinum screen and Lamb uses 2 mm. lead, both of which give an output of a little harder quality. Everett Lain and John Lamb have for years used these plaques screened through 2 mm. lead at 1 or 2 cm. distance for the treatment of epitheliomas. The output of gamma rays is so penetrating and spreads out so widely that it catches all the pseudopods of the tumor. After wide experience with all forms of radiotherapy they prefer this method, combining it often with x-ray treatment, giving 3000 to 4000 r with radium and 2000 to 3000 r with x-rays for a total tumor dose of 5000 to 7000 r.
Filtered Metallic Tubes. Brass and platinum tubes are especially suitable for treatment of lesions in the aural canals, nostrils, and other body orifices. The dosage for tubes thus used varies with the filtration and the nature of the disease. In general, the dose for brass tubes is between 75 and 100 milligram hours, the tubes being surrounded by sterile rubber tubing to absorb secondary radiation and to maintain a distance of 2 or 3 mm. between the tube and the surface under treatment.
Gold implants are used in amounts of 1 to 3 millicuries per cubic centimeter in diseased tissue, the average being 2 millicuries. The gold capsule, by removing the soft beta radiation, decreases the sloughing and pain that formerly attended the use of bare glass seeds. In particular, the doses of gold implants for carcinoma are 2 to 2.5 millicuries per cubic centimeter, for angioma 0.8 to 1 millicurie per cubic centimeter.
Needles. Low intensity platinum needles containing 1 or 2 mg. of radium are employed to give prolonged interstitial irradiation with hard gamma rays. The needles are implanted at 1 cm. distance from each other and are sutured in place, being allowed to remain for a total of 100 to 167 mg. hours (4-7 days ). The prolonged radiation time and the hardness of the radiation permit sterilization of cancer cells without tissue necrosis. The published tables of Paterson and Parker and of Quimby give data for estimation of the dose in a tumor implanted with needles. From these the total dose delivered to any one point by all of the needles implanted can be calculated. The total tumor dose is usually 5000 to 6000 roentgens uniformly distributed.
Trays. With a filter of 2 mm. brass and a distance of 1 cm., a tray 1.8 cm. square which has a radiating surface of 2 sq. cm., the exposure should be 800 to 1000 mg. hrs. for small epitheliomas. If a round plaque 3.5 cm. in diameter is used at the same distance and with the same filtration the exposure should be 1000 to 1200 mg. hrs. At a distance of 3 cm. the same tray would require an exposure of 2500 to 3000 mg. hrs.
Strontium 90, a radioactive isotope, is used in 5 x 5 cm. plaques. These are pure beta ray applicators. At 1 mm. depth, the dose is 40 per cent of the surface dose and at 3 mm. depth, it is 7 per cent. Plaques contain 25 to 40 mcs. and deliver 35 rep., plus or minus, per second. The erythema dose is 320 rep. According to Lutterbeck, the dose for hemangioma is 600 rep. X 3 = 1800 rep. Because of small depth dose, this applicator is ideal for treatment about the eyes but is only useful in superficial lesions. He has also treated pterygium and leukoplakia successfully with it. Port-wine marks do not respond.
Thorium X is used in an alcoholic solution in a strength of 150 mc. per cc. It has a half-life of 3.65 days. Some of its decay products contain beta and gamma rays, so that the radiation from the solution is mainly alpha, but includes also betas and gammas. The gamma radiation is however significant from a health point of view. Thorium X has been used for many decades. It is dangerous to handle but may be used in treatment with safety if precautions are observed. The danger is not to the patient but to the physician. Lead-rubber gloves are worn. The solution is painted upon patches of psoriasis, upon warts, localized neurodermatitis, mycosis fungoides, pruritus ani, seborrheic dermatitis and other skin diseases. It is applied with a metal applicator on a small amount of cotton at one end. The solution is painted on a lesion and allowed to dry. The painting is repeated twice more. The part is then not washed for forty-eight hours. It is important that the cotton be discarded in a covered metal receptacle or in the toilet at once. This treatment is repeated weekly and may be continued for a year, apparently with very little need for concern. Pigmentation commonly results from the treatment. Erythema and blistering may occur. Slight atrophy has been reported but certainly is extremely rare. A physicist who made thorium X solution for many years is said to have developed carcinomas on his hands from it. One of my patients who painted himself liberally with it for two years for treatment of mycosis fungoides, at autopsy following a dose of nitrogen mustard given by another physician showed no residual radioactive material in his viscera as tested by autoradiograms, in spite of inhalation and application, and having had radioactive phosphorus intravenously two years prior to death.
The unit of dosage is the roentgen. However, it is difficult to measure the dosage of radium glazed plaques in contact with the skin in physical units. For this reason, the term roentgen is not used much with plaques, but instead the specification of the exposure in milligram hours is serviceable.
In interstitial therapy the dose is estimated as the intensity of the radiation at 1 centimeter distance from 1 milligram point source of radium element (in equilibrium with its products ) filtered by 0.5 mm. platinum. The dose rate of 1 milligram of radium, assuming a point source, filtered through 0.5 mm. platinum at 1 cm. distance is 8.4 roentgens in one hour.
It should be the constant endeavor of the radium therapist to obtain as nearly as possible a uniform dosage in all parts of the lesion. To effect this homogeneous irradiation, the various devices such as cross-firing, the use of radium spread over a considerable area as in the flat plaques, the use of a number of tubes placed about the lesion ( or a number of needles or implants placed into it), or trays at varying distances, have been adopted. Screening, which regulates the quality of radiation, is most important. All these methods have their advantages and limitations. Dosage is thus influenced by the details of the application, the chief of which are the distance from the skin, the screening, the size of the applicator, the quantity and distribution of the radiating material, and whether the source of radiation is a point, a tube, or a unit of another form.
Whenever exposures are repeated the intervals between the treatments influence the dosage. The optimum interval between treatments depends upon the dosage and the quality of the radiation, but most of all upon the disease under treatment. This subject will be taken up specifically in other chapters.
The skin erythema dose is, generally speaking, the smallest quantity of radiation which will produce a faint redness of the exposed skin, which usually appears five to seven days after exposure and lasts two to three weeks. Ideas of what constitutes a faint redness vary widely, but the threshold erythema dose is accepted by most dermatologists. In a general way, it requires considerably more hard radiation than soft radiation to produce an erythema. When combinations of soft x-rays and gamma rays are employed, it requires more of the combined radiation to produce the erythema than it does of either one alone. The erythema dose is of no interest in the treatment of cancer but is of some importance in the treatment of hemangioma.
The distance between the radium and the skin surface, as previously stated, has a tremendous effect upon the dosage. The inverse square law applies strictly only to radiation from a point source. For the tubes, plaques, or other applicators used in practice, the effect of distance is somewhat less than for point sources; the values cannot be calculated by a simple formula. Quimby, and Paterson and Parker have worked out tables for applicators of different sizes for various distances, so that doses from them may be compared directly, and by means of curves or tables a comparison may be made for any practical applicator at any distance. Also, if the dose is known for one applicator under a given set of conditions of distance and filtration, it may be obtained for any other applicator under any ordinary conditions of application.
Common empirical doses for the various types of skin lesions will be but briefly covered in this chapter, receiving more consideration under the captions of the respective diseases.
Flat Applicators. Flat Monel applicators 2 cm. square and containing 20 mg. of radium are in common use. Unscreened, this standard 20 mg. full-strength radium plaque in direct contact with the skin will produce an erythema in about ten minutes, an erythema with temporary epilation in about twenty minutes, and an intense erythema with desquamation and exudation in thirty minutes. The action is largely a beta ray effect extending to the depth of a few millimeters, which is often suitable for the treatment of senile and seborrheic keratoses and plantar warts. The erythema begins a few days after treatment, and persists in mild reactions for two weeks.
Flat square applicators screened through 2 mm. brass are suitable for the treatment of hemangiomas. The output is gamma radiation. Pfahler uses 1 mm. platinum screen and Lamb uses 2 mm. lead, both of which give an output of a little harder quality. Everett Lain and John Lamb have for years used these plaques screened through 2 mm. lead at 1 or 2 cm. distance for the treatment of epitheliomas. The output of gamma rays is so penetrating and spreads out so widely that it catches all the pseudopods of the tumor. After wide experience with all forms of radiotherapy they prefer this method, combining it often with x-ray treatment, giving 3000 to 4000 r with radium and 2000 to 3000 r with x-rays for a total tumor dose of 5000 to 7000 r.
Filtered Metallic Tubes. Brass and platinum tubes are especially suitable for treatment of lesions in the aural canals, nostrils, and other body orifices. The dosage for tubes thus used varies with the filtration and the nature of the disease. In general, the dose for brass tubes is between 75 and 100 milligram hours, the tubes being surrounded by sterile rubber tubing to absorb secondary radiation and to maintain a distance of 2 or 3 mm. between the tube and the surface under treatment.
Gold implants are used in amounts of 1 to 3 millicuries per cubic centimeter in diseased tissue, the average being 2 millicuries. The gold capsule, by removing the soft beta radiation, decreases the sloughing and pain that formerly attended the use of bare glass seeds. In particular, the doses of gold implants for carcinoma are 2 to 2.5 millicuries per cubic centimeter, for angioma 0.8 to 1 millicurie per cubic centimeter.
Needles. Low intensity platinum needles containing 1 or 2 mg. of radium are employed to give prolonged interstitial irradiation with hard gamma rays. The needles are implanted at 1 cm. distance from each other and are sutured in place, being allowed to remain for a total of 100 to 167 mg. hours (4-7 days ). The prolonged radiation time and the hardness of the radiation permit sterilization of cancer cells without tissue necrosis. The published tables of Paterson and Parker and of Quimby give data for estimation of the dose in a tumor implanted with needles. From these the total dose delivered to any one point by all of the needles implanted can be calculated. The total tumor dose is usually 5000 to 6000 roentgens uniformly distributed.
Trays. With a filter of 2 mm. brass and a distance of 1 cm., a tray 1.8 cm. square which has a radiating surface of 2 sq. cm., the exposure should be 800 to 1000 mg. hrs. for small epitheliomas. If a round plaque 3.5 cm. in diameter is used at the same distance and with the same filtration the exposure should be 1000 to 1200 mg. hrs. At a distance of 3 cm. the same tray would require an exposure of 2500 to 3000 mg. hrs.
Strontium 90, a radioactive isotope, is used in 5 x 5 cm. plaques. These are pure beta ray applicators. At 1 mm. depth, the dose is 40 per cent of the surface dose and at 3 mm. depth, it is 7 per cent. Plaques contain 25 to 40 mcs. and deliver 35 rep., plus or minus, per second. The erythema dose is 320 rep. According to Lutterbeck, the dose for hemangioma is 600 rep. X 3 = 1800 rep. Because of small depth dose, this applicator is ideal for treatment about the eyes but is only useful in superficial lesions. He has also treated pterygium and leukoplakia successfully with it. Port-wine marks do not respond.
Thorium X is used in an alcoholic solution in a strength of 150 mc. per cc. It has a half-life of 3.65 days. Some of its decay products contain beta and gamma rays, so that the radiation from the solution is mainly alpha, but includes also betas and gammas. The gamma radiation is however significant from a health point of view. Thorium X has been used for many decades. It is dangerous to handle but may be used in treatment with safety if precautions are observed. The danger is not to the patient but to the physician. Lead-rubber gloves are worn. The solution is painted upon patches of psoriasis, upon warts, localized neurodermatitis, mycosis fungoides, pruritus ani, seborrheic dermatitis and other skin diseases. It is applied with a metal applicator on a small amount of cotton at one end. The solution is painted on a lesion and allowed to dry. The painting is repeated twice more. The part is then not washed for forty-eight hours. It is important that the cotton be discarded in a covered metal receptacle or in the toilet at once. This treatment is repeated weekly and may be continued for a year, apparently with very little need for concern. Pigmentation commonly results from the treatment. Erythema and blistering may occur. Slight atrophy has been reported but certainly is extremely rare. A physicist who made thorium X solution for many years is said to have developed carcinomas on his hands from it. One of my patients who painted himself liberally with it for two years for treatment of mycosis fungoides, at autopsy following a dose of nitrogen mustard given by another physician showed no residual radioactive material in his viscera as tested by autoradiograms, in spite of inhalation and application, and having had radioactive phosphorus intravenously two years prior to death.