Application. As only the absorbed energy can exert biologic
action, accurate approximation of the radium to the lesion is imperative. The
optimal usefulness of radium is limited to a short radius, and its effects
depend largely upon the accessibility of the cells to its emitted energy. Skin
lesions are so readily accessible that one of the most serious handicaps in the
use of radium for the treatment of malignant disease is overcome at the outset.
Superficial
radium therapy deals with lesions not much more than 1 cm. in depth. The
quality and quantity of energy reaching the lesion are the salient factors, and
not the amount in the source. Since the beta rays are absorbed much more
readily than the gamma rays they affect principally the superficial layers of
tissue, while the gamma rays penetrate and produce effects at greater depth.
Penetration is indispensable in the treatment of subcutaneous pathologic
processes, and in cutaneous lesions that are thickened. Screening (filtration)
is afforded by interposing various metals or other substances known as screens
between the radioactive source and the skin, thus relieving the latter from the
absorption of much soft radiation and subjecting it to a minimum amount of
injury when subcutaneous or deep structures are under treatment. Hard radiation
is also reduced to some extent. The indications for screening in radium therapy
are regulated to a large degree by the size, location, and character of the
pathologic condition. The employment of screens introduces the additional
factors of secondary radiation generated in the filtering material, and of loss
of intensity through increased distance due to its thickness.
Screens
are usually made of platinum, brass or lead. As the secondary radiation from
these metals may result in undesirable superficial irritation upon the skin, it
is advisable to insert, between these screens and the skin, further absorptive
materials, nonmetallic or of low density, such as sheet rubber, chamois, wood,
gauze, or filter paper. When only gamma radiation is desired, a primary filter
of 0.5 mm.
platinum or 2 mm. of lead or brass and a secondary filter of
2 mm. gum rubber or an equivalent combination of other materials should be
used.
Types of Applicators. The flat square
applicator is popular among dermatologists because a small amount of radium in
this form permits the use of soft beta rays and, with suitable screening, the
more penetrating effects of hard beta or gamma rays. The "strength"
of the flat applicator is governed by the amount of radium per square
centimeter of surface, while the total area and strength determine its radium
content. Full strength is denoted by 5 mg. of radium per square centimeter;
double-strength and half-strength applicators are measured accordingly. A
full-strength applicator containing 20 mg. has a surface dimension of 2 X 2 cm.
Flat applicators may be made in any shape. The square shape is recommended
because with it a large area such as that of an angioma may be radiated in
sections without overlapping. Round or oval shapes are suited for small
lesions, upon which they may often be used without shielding.
Tubes of
various sizes and materials are also used as containers of radium salt or
radon. They may be employed upon the surface, inserted into cavities, or
implanted interstitially.
Radon tubes are used mainly as follows:
1. Gold and platinum alloy tubes containing glass radon tubes about 0.6 mm. in diameter and 10 to 15 mm. in length, are evenly distributed within the substance of a mold of plastic substance—usually nonmetallic filled dental modeling compound—conforming to the shape and size of the lesion.
2. Some of
these same radon tubes are arranged in grooves in the floor of square or
circular brass boxes or trays, and held in this position by paraffin. The floor
is of desired thickness, so that the total filtration of the wall of the tube
and the brass floor is the equivalent of 0.5 mm. of platinum or 2 mm. of brass.
These flat applicators or trays are used at a distance.
3. Radon in
small pure gold tubes ( gold implants) of 3 to 5 mm. length and 0.55 to 0.75
mm. diameter and with a wall thickness of 0.3 mm., is implanted in the lesion.
Radium salts
are placed directly in hollow needles with soldered screw caps. These needles
are made of platinum iridium. They usually contain 1, 2, 5, 10, 12 1/2 mg.
radium as sulfate, and are from 10 to 30 mm. in length. The amount of radium or
radon placed in each needle varies widely, but there is a tendency to prefer
small quantities.
Platinum
needles, 11 mm. long, having a wall thickness of 0.5 mm. and containing 1 mg.
of radium, or a larger needle, 27 mm. in length and containing 2 mg. of radium,
are popular and have important uses in the treatment of cutaneous cancer. The
radiation from them is harder than that from gold implants.
Hollow needles,
similar to those used for radium, containing a radon tube about 13 mm. in
length and 0.8 mm. in diameter fulfil the same uses as needles of radium salts.