Biomedical Engineering Reference
In-Depth Information
or when regional spread has occurred, surgery may not be a viable
option, and radiation therapy will then be the preferred approach.
Distant metastases can, for the most part, only be treated through the
use of systemic approaches such as chemotherapy, immunotherapy,
or, more futuristically, molecular targeting. Combination therapy -
the use of two, or even three, of the approaches just described - is
commonly undertaken to manage optimally the local and proven or
likely systemic components of the disease. An important rationale for
improving local therapy is the observation that the longer a patient
has a viable malignant tumor, the more likely that a metastatic “break
out” of that cancer will occur - which generally badly compromises
the outcome of treatment. Thus “local control” of tumors is necessary
for achieving long-term survival.
Overly aggressive surgery or very high doses of radiation and/or
chemotherapy can eradicate a cancer with high probability - but, at
the cost of causing unacceptable morbidity. Thus,
the art of cancer
treatment is in finding the right balance between tumor cure and
injury to normal tissues
. Much of the motivation for improving the
technology of radiation therapy stems from the desire to reduce the
probability of morbidity - which in turn may allow higher doses to be
delivered to the tumor with an associated increase in tumor control
probability.
T
YPES OF
R
ADIATION
U
SED IN THE
T
REATMENT OF
C
ANCER
Research in physics has contributed directly and indirectly to cancer
therapy over the past century. Only months after their discovery by
Röntgen in 1895, X-rays were employed to treat a patient with breast
cancer. At present, the most commonly employed radiotherapy
treatment employs a beam of high-energy X-rays (often described as a
photon beam
) generated external to the patient and directed toward
the tumor. Machines containing radioactive
60
Co sources are also still
in active use in many parts of the world.
Other forms of radiation which have been used in radiation therapy
are: electron beams; implanted or inserted radioactive sources (
γ
,
β,
and even
emitters are used); neutrons; pi-mesons; protons; and
heavier charged ions such as
12
C and
20
Ne. The bulk of the material
in this topic relates to the use of external beam therapy using photons.
α
External beam therapy with protons is discussed in Chapters
10 and 11.
