The local enhancement of radiation dose from photons of MeV energies obtained by introducing materials of high atomic number into the treatment region

Abstract

With the advent of therapeutic radiation treatment machines with photon end point energies of several MeV, a new channel is available to transfer the photon energy to biological material, namely, pair production. This process has a photon threshold energy of 1.02 MeV. The probability of pair production, which depends on the square of the atomic number (Z) of the interacting material, increases markedly as the photon energy is further increased. As the goal of treatment planning in radiation therapy is to locally maximize the absorbed dose in abnormal cells and minimize the dose in surrounding normal cells, in this study the authors measured the dose enhancement which could be expected if a high-Z material such as gold was present adjacent to tumor sites during irradiation. The authors used photon beams produced by electron accelerators with energies ranging from 6 to 25 MV. They chose either gold or lead foils as high-Z materials, the measurements being repeated using the same geometry but replacing the high-Z materials with a low-Z material (aluminum). The comparison of the experimental results using low- and high-Z materials verified the theoretical prediction of the expected dose enhancement. The effect of finite range of the electron-positron pairs was also studied by varying the spacing between two foils placed parallel or orthogonal to the incident photon beam. Using an 18 MV photon beam, the authors observed a maximum dose enhancement of 44%. They intend therefore to proceed from these phantom studies to animal measurements.

Effect of secondary electron generation on dose enhancement in Lipiodol with and without a flattening filter

“…5(c) and 5(d). Wayne D Newhauser, et al reported that the electron energy range at 500 keV in the water is approximately 2 mm 22. The electron range in the Lipiodol is shorter than the water because the Lipiodol is high density and high Z material.…”

Study of dose enhancement effects in the vicinity of high Z atoms with Geant4 Monte Carlo simulation

“…In a study done by Ahmad, etc. [11], a maximum dose enhancement of 44% using an 18 MV photon beam was observed with Au foils imbedded in the phantom.…”

Polymer gel dosimetry for measuring the dose near thin high‐Z materials irradiated with high energy photon beams

“…The current study is an extension of Alkhatib et al, who studied the dose enhancement by increased electron-positronpair production of a high-Z material with high energy photon beams. 18 By using a 3D PGD technique, we measured full 3D dose distribution in the vicinity of the lead foils. The results showed, for example, that the dose enhancement between foils in the orthogonal configuration occurred at a point closer to the first foil and the range of the increased dose on the upstream side of the first foil extends much further compared to the range of increased dose on the downstream side of the second foil, which fell off rapidly.…”

“…In fact, Alkhatib et al proposed such an idea and demonstrated that there is as much as a 44% dose increase when an 18 MV photon beam is used in a water-like medium including thin gold foils. 17,18 The biological effects of the local dose enhancement near metallic materials should be carefully considered for radiation therapy of patients with an implant made of high-Z material. 19 Since more and more patients receive implants for health reasons, this will become a significant treatment-related issue in the radiation oncology community in coming years.…”

“…34,35 Our group has been studying the potential utility of thin foils made of a high-Z material such as gold for dose enhancement with very high energy photon beams. 17,18 The initial study, however, was limited to dose measurements at points by TLD chips and on planes by radiographic films. Hence, we began a comprehensive 3D dosimetric study by PGD as well as a detailed MC-based study to characterize the effects of many different foil arrangements on the dose enhancement.…”