Low-Density Inhomogeneity Corrections for 22-MV X-Ray Therapy

The intent of this paper is to present a status report on the examination of correction factors to be used with 22 MV x rays in low-density inhomo-geneities. The problem arose in an attempt to deal with lung in betatron dose distributions. Some interesting values, which will be described, were obtained when electronic equilibrium had not been established before the beam entered the lung. The experimental data were obtained as follows. A phantom was constructed, using Lucite as the tissue-equivalent material and cork as the low-density material simulating lung. LiF capsules were spaced at l-cm intervals along a polystyrene rod on the central axis of the betatron beam and within the cork around the central axis, so that one capsule would not be in the shadow of another. Experiments were performed, varying the thickness of the cork and the thickness of Lucite overlying the cork. The results were compared with values obtained in a solid phantom. All values were adjusted to account for the non-tissue density of the Lucite and polystyrene. The experiment was repeated, with no buildup, with 60Co radiation to obtain data for later comparison with calculated data. The calculated data were obtained in two ways. The first was derived from Batho's method for 60Co (1) as shown in Figure 1, A. The correction of the dose at a point P, at a distance A from the lung-tissue interface, where the thickness of the lung is B and the thickness of the tissue overlying the lung is C is [TAR(A)/TAR(A + B)]0.65 (1). The density of the lung was assumed to be 0.35 and the exponent represents (I-density of B). In Batho's work the effect of the tissue overlying the lung was neglected and values within the lung were not calculated. We found that if this formula was used with 22 MV x-ray data, a situation was approximated which assumed no tissue overlying the lung, and consequently, if the lung thickness was less than 4-cm tissue-equivalent, a correction factor <1 would appear up to some depth below the lung-tissue interface. To duplicate the experimental conditions, an exponent of 0.79 was used, 0.21 being the measured density of the cork. An allowance for the tissue overlying the cork, C, was introduced into Bathe's expression which then became [TAR (A + C)/TAR(A + B + C)]0.79. The corrected per cent depth-dose values, within and below the cork, were then calculated by this exponent method. The second calculation method (Fig. 1, B) assumed the lung-simulating material divided into an air portion and a tissue-equivalent portion and the correction to be the ratio formed as follows [TAR (A + 0.21 B + C)jTAR(A + B + C)].