Sperduto_Khan's Treatment Planning in Radiation Oncology, 5e

CHAPTER 20 Treatment Planning Algorithms: Photon Dose Calculations 439

Electron beam

Primary collimator X-ray target

Flattening filter

Forward peaked X-ray beam

Scattering foil

Carrousel

Ion chamber

Secondary collimator

Flattened X-ray beam

Slot for wedges, blocks, compensators

A Patient FIGURE 20.1. Components of the treatment head of a linear accelerator. A: A cross-sectional view of the treatment head operat ing in X-ray therapy mode. B: A cut-away diagram of the linac. (Image(s) courtesy of Varian Medical Systems, Inc., Palo Alto, CA. Copyright [2021]. All rights reserved.) B

set in motion from the site of the photon interaction. At megavoltage energies, the range of charged particles can be several centimeters. The charged particles are mainly set in forward motion but are scattered consider ably as they slow down and come to rest. Electrons lose energy by two processes: inelastic collisions within the media (primarily with target electrons) and radiative interactions (primarily with target nucleus). Inelastic collisions that ionize the target atom can lead to second ary electrons, known as delta rays. Radiative interactions occur via Bremsstrahlung, which effectively transfers the energy back to a photon. Equations thatmodel these coupled electron–photon interactions are described later on. The indirect nature of photon dose deposition results in several features in photon dose distributions. Initially, the superficial dose increases or “builds up” from the sur face of the patient because of the increased number of charged particles being set in motion. This results in a low skin dose, the magnitude of which is inversely propor tional to the path length of the charged particles. The dose builds up to a maximum at a depth, d max , characteristic of the photon beam energy. At a point in the patient with a depth equal to the penetration distance of charged parti cles, charged particles coming to rest are being replenished by charged particles set in motion, and charged particle equilibrium (CPE) is said to be reached. In this case, the

although the collimator jaws themselves contribute little forward scatter. The photons scattered in the primary col limator and field-flattening filter also add to the fluence just outside the geometrical field boundary. Similar to the accelerator-produced scatter, the phan tom scatter primarily occurs in the forward direction and increases with the size of the field. However, for phantom generated scatter, the penetration characteristics of the beam are also altered. As the field size increases, the phantom scatter causes the beam to be significantly more penetrating with depth. This effect is significant enough that this energy difference must be included in the dose computations. The behavior of scatter from beam modifiers such as wedges must also be considered within the photon model. When the field size is small, a beam modifier mainly alters the transmission and does not contribute much scatter that arrives at the patient. However, when the field is large, beam modifiers begin to alter the pene tration characteristics of the beam, much as the phantom scatter does. This effect is exemplified by the increase in the wedge transmission factor with increasing field size and depth. 6,7 Electron Transport Photons are indirectly ionizing radiation. The dose is deposited by charged particles (electrons and positrons)

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