Determination of isodose curves for supervoltage and cobalt-60 teletherapy machines with x-ray film.

Abstract

Treatment planning with supervoltage and cobalt-60 machines is often hampered by a surprisingly limited supply of suitable published information. Table I summarizes dosage references from four major radiologic journals. Supervoltage data are represented by only one treatment distance (100 cm.) and three square fields. Although there are more curves between actual and nominal field size is accurately known for the particular machine and treatment distance used, considerable errors are possible. A further difficulty is introduced in teletherapy machines by lack of standardization of penumbra factors, such as source sizes and collimator design (17). The effect of penumbra size on dosage for cobalt-60, they must be employed with care because of the variability of field-defining systems in such machines.3 Even the term “field size” is quite ambiguous. Table II shows actual measured separations in air of 50 per cent of maximum dose locations, for nominal 10 × 10-cm. fields, on several machines in the Philadelphia area. A range of 9.1 to 11.3 cm. is observed among commercial units. Unless the relationship distribution across the field is shown in Figure 1 comparing small and large penumbra situations for identical field size. Achievement of a good penumbra requires both sources of high specific activity and collimator blocks placed relatively close to the patient. The latter requirement normally necessitates rotation through less than 180° to prevent collision of the collimator assembly and the treatment table. It is clear from the above considerations that one must check the applicability of published data to the planned use of a particular teletherapy machine. It appears desirable that field size and penumbra characteristics of such units be checked routinely at installation as well as when significant changes occur in optical system, collimator, or source size. In addition, considerably more isodose curve information is evidently needed, for both single and multiple fields. This paper discusses the development of film methods for obtaining isodose curves in a Masonite or Mix-D phantom. If sufficient accuracy can be achieved, the use of film offers certain advantages over alternative ionization chamber and scintillation probe technics (19, 20, 27). For example: Equipment: Suitable film densitometers cost as little as $550; they are relatively easily maintained and serviced, and normally are quite dependable. On the other hand, small ionization chamber and scintillation probe assemblies are best used by persons with adequate physics and electronics background to keep them operating satisfactorily. Resolution: Films are conveniently scanned with light slits 1 mm. or less in width. Probe units are usually 5 to 8 mm. in diameter, with about 3 mm. as a practical lower limit, and thus tend to measure less accurately near the edge of the field.