Depth dose curves for grids in x-ray therapy.

Abstract

The use of treatment grids (i.e., radiopaque material with alternate open and closed areas) in x-ray therapy requires depth dose information not given in the conventional depth dose tables. It is the purpose of this paper to show that the essential information can be derived from these tables. A detailed analysis of the grid depth doses will be presented which tends to corroborate the general clinical impression that the function of the grid is to preserve areas of the skin, which will serve as centers of regrowth, while still delivering adequate and sufficiently homogeneous radiation at a depth. The use of deep therapy treatment grids and an understanding of their effect on the distribution of radiation are not new. As early as 1909 Köhler (1) proposed the use of a grid as a method of delivering a deep tumor dose while protecting against radiodermatitis. In 1925 Abeles (2) discussed the distribution of radiation for a grid from a geometrical point of view (without, however, considering the effect of scattered radiation). In 1933 Liberson (1, 3), who was not aware of the previous work, published depth dose measurements and animal and clinical studies with “multiperforated screens” essentially the same as those being reported here. His back-scatter and depth dose measurements were made with ionization chambers sufficiently large so that they averaged over both the maxima and minima in radiation dose-rate. His measurements verified his expectation that the ratio between the average dose-rate with and without the grid equals the ratio of the open area of the grid portal to the total area. He concluded that, “since the remote effect upon the skin of both rabbits and man is the same when three or four times as much radiation is delivered through the perforator as without it, the underlying tissue actually receives one and a half to two times as much radiation with the perforator as without it.” This statement summarizes essentially our present view on the depth dose effect of the treatment grid. Similar reports followed in 1934, by Haring (4) and Woenckhaus (5). In 1945, Grynkraut (6) described biological and clinical experiments with a lead grill having 3 × 3-cm. openings separated by 5 mm. of lead. He made photographic tests of the dose distribution and showed that the sharp picture of the grill obtained on a film at a distance of 10 cm. in air was greatly blurred when water was placed between the grill and the cassette. He concluded that the grill “prevents the vasomotor phenomena of erythema” by protecting areas of the skin, while still giving a more or less homogeneous dose at a depth. He advised that the x-ray air dose must be doubled when using the grill. In 1949, Jolles (7) reported the clinical use of an “alternating chessboard” type of treatment grid. Part of the treatment was given through a certain grid, and the remainder through a complementary grid.