Further Experiences with Chaoul Therapy

Abstract

In 1937 we presented our experiences with short-distance low-voltage roentgen therapy before the Fifth International Congress of Radiology (2). At that time we had treated but 80 patients with various neoplastic and inflammatory conditions. We have had the opportunity of treating many more lesions by this method since then. The purpose of the present communication is to record the results of further experiences with short-distance low-voltage irradiation. Physical Data Our experience has been limited to the use of the Chaoul unit for “contact” therapy which was installed in our department in March 1936. Briefly, the x-ray tube is activated by a constant potential generator using a modified Greinacher circuit. The tube itself is a modification of the old Lenard ray tube in which the electron beam passes down a grounded metal cylinder, striking at the end a target of gold-plated nickel. The rays emerge through the target, which, therefore, acts as part of the filtration. In addition the beam passes through the cooling water (2 mm.) and a thin metal foil window, which forms part of the water jacket. The total inherent filtration, as stated by the manufacturer, is approximately equivalent to 0.2 mm. of nickel. While the rated capacity of the tube is 60 kv., 4 rna., we have chosen to operate it at 50 kv., 4 rna. The original tube is still in use, the output being about the same now as it was when first measured. The half-value layer of the emerging beam is 2.4 mm. aluminum. The effective wavelength was found to be 0.38 Å. The focal spot measured approximately 1.6 em. in diameter (pinhole 0.4 mm. in diameter, 61 em. pinhole-film distance). Mayneord called attention to the radial variation of intensity in the emerging x-ray beam (1). We found that the intensity fell off 30 to 40 per cent toward the periphery of the field. While the inverse square law of variation in intensity with distance could not be assumed to hold with such a large focal spot at short distances, our investigations revealed that a close agreement did exist. The back-scatter of the rays emitted by the Chaoul unit varies directly with portal area, approaching a limiting value of about 20 per cent for the largest cone, measuring 4.6 cm. in diameter. The back-scatter for the smallest cone, 2 em. in diameter, is about 7 per cent. The difference in percentage back-scatter between target-skin distances of 3 em. and 5 em. was found to be negligible. The percentage depth dose varies from 65 to 24 per cent at I-em. depth as the target-skin distance decreases from 5 em. to 3 mm. The differences in depth dose with varying target-skin distances are summarized in Table 1. Not enough emphasis has been placed upon the protection of the operator during the treatment of the patient.