Dosage Table for Linear Radium Sources

The expression of tissue dosage in terms of roentgens is becoming general in x-ray and radium therapy. For interstitial radium or radon sources, the literature contains data for several satisfactory methods of determining the minimum dose delivered within a volume implanted with needles or seeds in accordance with certain general principles. In the case of single linear sources, however, such as are commonly employed in the treatment of cervical and rectal cancer, information is not so accessible. The most generally used charts, those of Paterson and Parker (1), give the number of milligram-hours necessary to deliver 1,000 rγ at points in a plane perpendicular to the source at its mid-point, up to 3 cm. distance. For positions not in this central plane, or for greater distances, they are not helpful. Sievert (2) and Laurence (3) have presented extensive data, but in rather complicated mathematical forms. Mayneord and Honeyburne (4) have published a general method for drawing isodose curves around any particular linear source; this is also too involved for ready use in the radiological department. Wolf (5, 6) gives data for the dosage in roentgens per milligram-hour in a plane perpendicular to the implant at its end, for a wide range of lengths of source and distances from the radium. The advantage of having the data for the end rather than for the middle of the needle is that any implant can be considered a combination of two segments, and the doses along lines perpendicular to the source at any desired point can be obtained by choosing the segments so that their ends come at that point. For instance, doses along a line perpendicular to a 4-cm. needle at a point 1 cm. from either end would be obtained as the sum of doses from a 3-cm. and a 1-cm. needle, the former containing three times as much radium as the latter. For ready use in the radiological department, there appears to be a need for a more straightforward table, from which, within practical limits, doses for a given treatment can be read directly, not only along perpendiculars to the source at central or end-points, but anywhere in the vicinity. Such a table is presented herewith. Some years ago the author published charts and tables giving relative doses at different distances on perpendiculars erected at different positions along the linear sources (7). This was prior to the general acceptance of the gamma roentgen as a dosage unit, but in order to obtain the values in roentgens it is necessary only to establish the dosage for some one particular source and position. This has been done by taking the dose from a point source at 1 cm. distance, filtered by 0.5 mm. Pt, as 8.4 rγ. The material is presented in Table I. Doses are given in gamma roentgens per 100 mg.-hr., at various distances along perpendiculars to the sources, at 0.5-cm. intervals along their lengths, as indicated in Figure 1.