Electron-Beam Treatment Planning in Inhomogeneous Tissue

Abstract

THE PURPOSE of this report is to describe measurements with high-energy electrons in homogeneous and inhomogeneous media and applications of these measurements to the treatment planning of specific cases. These measurements had been incorporated in the procedure for treatment planning of inhomogeneities previously described (1, 2). The procedure was originally developed for use with high-electron therapy as carried out at both the University of Illinois College of Medicine and the Memorial Hospital (3–7). The Memorial Sloan-Kettering betatron is an Allis-Chalmers machine and is used at energies from 6 to 24 Mev. All energies throughout this range have proved useful for the treatment of lesions in various sites. The energy of the electron beam is easily adjustable from 6 to 24 Mev. Field sizes are determined by either aluminum or Lucite collimators which range in size from a 3 cm diameter circle to 24 × 20 cm. Some details of this betatron installation, the beam-extraction features, monitoring methods, and measurement procedures for dose distribution as well as our measurements on scattering and collimation have been reported previously (5, 6, 8). As noted before, the amount and material of the scatterer have a limited effect on the shape of the depth-dose distribution. This dependence illustrates the desirability of the specification of scattering material and indicates one reason why real differences can exist between various installations with regard to depth-dose distributions. The material of the cone or diaphragm system employed to define the field size also affects the depth-dose distribution. This has been reported by Turano (9) and Loevinger (10), as well as by us (8). Depth-dose data obtained with aluminum and Lucite cones for 10, 15, and 20 Mev electrons demonstrated a limited decrease in depth for the higher dose percentiles for the Lucite cone as compared with those of aluminum. The effect is more pronounced for small field sizes where the scatter contribution from the cone is more important. The dose distributions reported here have been made either in water, Presd-wood, or Presdwood containing various amounts of specified inhomogeneities. Central axis depth-dose data in homogeneous media, as well as isodose distributions for many different field sizes, different energies, and different angles of incidence had been previously measured. Even with the axis of the electron beam at an angle of 30 degrees with the normal to the water surface, the contours of the isodose distribution are essentially parallel with the surface. This is a feature of the method of absorption of electrons. The use of a polystyrene wedge to control the configuration of isodose contours was previously illustrated (6).