Radiation
Therapy (External, Internal & Systemic Radiation), How It Works,
Side Effects
(August 5th, 2008)
 Radiation
therapy or Radiotherapy is the use of ionizing radiation to kill
or control the growth of malignant or cancerous cells. Radiotherapy
is used as a palliative treatment option for patients suffering
from mesothelioma lung cancer because no definite cure is available
for this disease. The basic theory behind radiotherapy is that it
uses energy waves or particles to destroy cancer cells & tumor
masses. In other cases, radiotherapy is used to prevent future tumor
cells from growing; where no present cancer cells can be found.
There are 3 types of radiation therapy; external, internal, and
systemic and each of them is discussed below. Radiotherapy is usually
confused with radiology which is the use of radiation in medical
imaging techniques; they are two different things. Before conducting
a radiotherapy campaign on a patient, doctors have to determine
the type of tumor they are trying to kill, location & what stage
it has developed to. Overall well being of the patient & previous
side effects is also taken into account.
 How
Radiation Therapy Works
Radiation therapy functions by destroying the DNA of cancer cells.
The destruction is caused by an electron beam that ionizes the atoms
that reside in the DNA chain both directly and indirectly. Ionizing
in this context is the process of converting atoms or molecules
that reside in cancer cells into ions by adding/subtracting charged
particles such as other electrons and ions. Indirect ionization
occurs when radiation therapy forms water as a result of ionization,
which then forms free radicals that damage the DNA of cancer cells.
Because Cancer cells are stem-like and undifferentiated, they tend
to reproduce more and faster. Cell division forces damage to the
DNAs of these cells, forcing them to die and not be able to reproduce.
For example, Proton radiotherapy works by sending protons with varying
kinetic energy to directly kill the tumors.
One of the disadvantages of using radiotherapy is that cells that
contain solid tumor masses become deficient in oxygen. This low-oxygen
condition is known as 'hypoxia' and when hypoxia occurs, tumor masses
are more resistant to the effects of radiotherapy due to lack of
oxygen. The role of oxygen in this process is so important that
it is oxygen that makes DNA damage to cancer cells permanent.
Types of Radiation Therapy
i) External Radiation
 External
radiation is conducted by a two-dimensional beam outside of the
body that uses linear accelerator machines delivered to the patient
in several different angles. The beam resides on a calibrated diagnostic
x-ray machine (also known as a 'simulator'). The simulator aims
to target cells that contain tumor masses and kill these cells via
the beam. The specific beams that are used to kill lung cancerous
cells are called 'Megavoltage X-rays.' This beam may also be given
after surgery to reduce the chance of solid tumor masses from forming
again.
ii) Internal Radiation or Brachytherapy
Brachytherapy is also known as the 'sealed source radiotherapy'
where a radioactive material is placed next to solid tumor masses
that are waiting to be destroyed. This radioactive material is placed
in a small tube, capsule or a seed. For instance electronic brachytherapy
places a miniature low energy (<50kVp) x-ray tube into a position
nearby the tumor masses to rapidly deliver high doses of targeted
cancerous tissues and destroy them, while maintaining low doses
to non-cancerous nearby tissues. The image above is of Seeds (small
radioactive rods) that are placed directly near the tumor masses
in the abdomen or in the lungs. Common side effects of Brachytherapy
include becoming infertile & impotent. Infact, between 6% to
50% of patients who have received Brachytherapy have reported becoming
impotent. It is due to this reason that internal radiation is not
commonly used.
iii) Systemic Radiation (Intensity-Modulated Radiation Therapy
(IMRT))
 Intensity-Modulated
Radiation Therapy (IMRT) is a fairly new & sophisticated type
of radiation therapy where Computer-controlled x-ray accelerators
spread precise radiation doses to malignant tumor masses. The pattern
of how radiation is delivered is determined using highly-tailored
computing applications to perform optimization and treatment simulation.
The dose of radiation is made same as the 3-D shape of the tumor
by modifying the radiation beam’s intensity. The dose of radiation
is increased near solid tumor masses while it is decreased near
non-malignant tissues. This customized form of radiation is done
to ensure the malignant tissues get destroyed while preserving the
nearby non-malignant & healthy tissues. Because the doses of
radiation are modified according to how tumorous the tissues are,
this type of radiotherapy is better than the conventional type &
results in better tumor targeting, less side effects and improved
treatment results.
|
