Beam Focus and Its Image on Beam Focus Radiogram

Abstract

The actual dose distribution of the beam focus in the body and that of the result obtained by the densitometry of the beam focus image at the position of the turn table was studied. When the fixed field therapy was conducted with the same arrangement of the phantom and film as the beam focus radiography, the patterns of the isodose curves in the phantom and that at the position of the turn table were different each other. The edges of the beam in the body as well as at the turn table, however, diverged and went continuously.
In full rotation therapy with coincidence of the rotation center of the source with center of the phantom, the beam focus image showed nearly the same figure of the actual beam focus delineated by the 80% curve of the isodose curves. If the rotation center was made eccentric from the center of the phantom, the dose distribution at the turn table became less homogeneous than that in the phantom. But, the loss of homogeneity counted 10% at the largest, when the rotation center shifted less than 4cm from the center of the axial transverse cross section of normal sized human body.
Beam focus image increased in size with the increase of the exposure dose. It was concluded to be desirable to employ an exposure dose of from 6 to 12 R for the deduction of the actual beam focus from the beam focus image.