LWBK1006-07 LWW-Govindan-Review November 22, 2011 13:54
Chapter 7•Principles of Radiation Oncology 93effect is greater than additive lethal effect. However, in a clinical setting,
the effect appears to be additive or independent.Answer 7.13. The answer is C.
Early or acute effects typically occur within weeks after irradiation. They
often occur in tissues that have rapid turnover, and it is thought to result
from the depletion of the clonogenic or stem cells within that tissue. The
frequencies of late effects depend strongly on radiation fraction size. There
are fewer late effects with smaller fraction size. Large a/b ratio has a
large “shoulder” in the low-dose portion. The a/b ratio represents the
dose at which the quadratic (b) and linear (a) components of cell kill are
equivalent. The dose response to radiation can be described by the formula
S=exp (–aD–bD2), where S is surviving fraction and D represents dose.
Typical human tumors and early-responding normal tissues have large
a/b ratios (9 to 13 Gy).Answer 7.14. The answer is A.
The relative probabilities of photoelectric, Compton, and pair produc-
tion interaction depend on the photon energy and the atomic number
of the irradiated material. In modern treatment machines with greater
than 4 MeV photons, the Compton interactions and pair productions are
commonly seen. At diagnostic equipment energy range (25 kvp), pho-
toelectric effect predominates. In photoelectric effect, an incoming x-ray
transfers all its energy to an inner orbit electron, which is ejected from
the atom. A photon is produced as an outer shell electron fills the vacant
hole. In Compton scattering, energy from the x-ray is both absorbed and
scattered. The photon emerges with reduced energy and a change in direc-
tion. In pair production, an electron and a positron are produced, which
then deposit energy through collisions with other electrons. The threshold
energy for pair production is 1.02 MeV.Answer 7.15. The answer is D.
Higher-energy photons deposit less dose to the skin surface. Thus, it is
called the skin-sparing effect of high-energy photons. Depth of maximum
dose increases as the energy of the incident beam increases. It is often
desirable to use high-energy photons (>10 MeV) to reach deeply located
tumors. For a given energy, electrons do not penetrate deeper in tissue
compared with photons. It is for this reason that electron beams are often
used to treat superficially located tumors such as skin cancer. Electron
beams, unlike photons, deposit more dose to the skin surface as the inci-
dent electron energy increases.Answer 7.16. The answer is A.
Isotopes with properties of very short half-lives and low energy are
used for permanent implants, such as for prostate cancer treatment with
iodine-125 with a half-life of 59.4 days and an x-ray energy of 27 to
35 keV. Brachytherapy is a form of treatment that uses direct placement of
radioactive sources or materials within tumors (interstitial brachytherapy)