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HISTORY OF IONIZING RADIATIONS IN MEDICINE
At the end of the 19th century several discoveries concerned
ionizing radiation.
From the very beginning these innovations had an enormous
impact on the medical practice for the two complementary
applications to which radiation is connected: the medical
imaging for diagnostics, and radiotherapy for cancer (and
other diseases) treatment.
Medical Imaging for diagnostic (detectors used to
reproduce images of the internal organs)
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The X-rays. In November 1895 Röntgen
discovered
that accelerating electrons against a lead target some “unknown” rays were produced and he called
them X-rays.
He presented their diagnostic properties the first time only one month later taking
the first radiography of his wife’s hand.
Today X-rays and
CT scans are indispensable in diagnostic.
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The Cyclotron. In 1929 Ernest Lawrence invented the
cyclotron, a particle accelerator which was intended for
research in physics but was soon recognized as a unique
tool for medical and biological studies.
Today these machines
are producing the radioisotopes for the PET (Positron Emission
Tomography) and in general for nuclear medicine scanning
in hundreds of facilities around the world.
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Radiotherapy (cancer and other diseases treatment
with ionizing radiation)
| The first pioneering treatments with X-rays. After
Röntgen’s discovery, it did not take long to the physicians to apply the x-rays in pioneering cancer treatments.
The first attempts of therapy of skin cancer took place at the beginning of the last century. Today
40% of cured tumors were treated with either X-rays alone or in combination with surgery or chemotherapy.
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Natural radioactivity. Only few months after the discovery
of the X-ray, Henri Becquerel (left) discovered the natural radioactivity.
Pierre and Maria Curie discovered
in 1898 the radium, a radioactive source of gamma rays.
Radiotherapy based on radioactive cobalt 60 would have
been used many years later after the second world war for treatment with cobalt units.
Today the majority
of the cobalt units have been replaced by electron linear
accelerators.
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| Nuclear medicine. Ernest Lawrence’s cyclotron was
used in pioneering studies by his own brother John to produce radioactive phosphorus 32 to treat leukemia.
He also used the cyclotron to produce neutron for solid
cancer treatments. Today radioactive material is widely
used in treatments named Brachytherapy where the sources
are placed directly in contact with the tumor.
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IONIZING RADIATION AND CANCER
The primary goals of radiotherapy are the delivery of
the dose to induce the death of the tumor cell preserving
the harm to the surrounding healthy tissue due to the radiation
itself.
- The efficacy of the ionizing radiation for the local
control of the tumors has been proven soon after the discoveries
of the radiations. Although the interaction between radiation
and matter are well known it is particularly difficult to
understand processes and mechanisms that link the delivery of the
radiation to a target to the local control of the tumor. This complex
task is still ongoing nowadays and requires an wide multidisciplinary approach.
- The ionizing radiations used for therapy are photons
(X-rays and gamma rays) and electrons in conventional radiotherapy
and basically protons and carbon ions in today’s hadron
therapy (often also called particle therapy). The radiation interacting
with the atoms of the body (tumor and healthy tissue)
induces ionization. Radiation of different nature produce ionizations
of different density and characteristics.
- The ionization mainly interacts with the biological
matter through the formation of free radicals which, due to
their high reactivity, may induce DNA brakes. The complexity of
the DNA damage is linked to the ionization characteristics.
- The cell reaction, which depends on the extension
and severity of the damage, involves mechanisms to lead to repair
or death.
- The ultimate goal of radiotherapy is to deliver to
the target the minimum dose capable of killing up to the last tumor
cell and, at the same time, to preserve organ at risk and
healthy tissue which surround the tumor. The local control of
the tumor is the result of this chain of effects.
CONVENTIONAL RADIOTHERAPY (X-ray) AND THE
ORIGINAL APPROACH OF HADRON THERAPY
The primary goal of radiotherapy is to deliver to the
tumor the necessary dose and, at the same time, to avoid the damage of the surrounding
healthy tissue.
The dose necessary to cure the patients is evaluated
in dose escalation studies where the total dose and number of fractions for a certain
tumor are optimized to reach the highest rate of cure limiting
secondary effects. The dose is the amount of energy delivered to the tumor per unit
mass. Fractionation is the term used to indicate the number of times in which
the total dose is delivered. Generally a patient receives a fraction of
the dose per day and the treatment can last from few days to several weeks depending
on type and stage of the tumor.
- In conventional radiotherapy with photons, the level
of dose to the healthy tissue is maintained low by increasing the number of irradiation fields. This result
is obtained rotating the linear accelerator around the patient. In the most recent methods of Intensity
Modulated Radiation Therapy, IMRT, the radiation is optimized by distributing the dose trough up to 10 nonisocentric
fields.
- The curve which describes the energy released from
the hadrons shows a peak right at the end of the penetration
of the beam. It is called Bragg peak.
- Because of the Bragg peak and, in the case of carbon
ions, of the very well defined the lateral shape of the
beam, it is possible to conform the dose precisely to
the shape of the tumor with a limited number of irradiation
fields. It is possible to drastically limit the dose
to the sensitive organs, which could be otherwise severely
damaged from the irradiation. In general the total dose
delivered to the healthy tissue is sensibly reduced in respect to conventional therapy.
- The idea of using hadrons for cancer treatment dates
back to 1946, and was originally proposed by the Robert Wilson. Hadron therapy was first put into
practice at the Lawrence Berkeley Laboratory (LBL) in the USA, where approximately 30 patients were treated
with protons in the period 1954–1957.
- Heavy particle radiotherapy is performed today
basically with protons and carbon ions. The number of medical centers which perform the therapy in the work
is rapidly increasing for both particles.
- Carbon ions are more effective in the interaction
with the biological targets when the are close to the end
of their path. Because of this characteristics is it possible
to further decrease the dose to the healthy tissue and therefore reduce the side effects.
- Tumors which have reduced vascularization and
therefore less oxygenation are less sensitive to photon
and proton irradiation. Instead the level of oxygen in
the treated tissue is not a critical parameter for irradiation
with carbon ions. This therapy becomes the indicated
treatment for all the solid tumor lacking of oxygen.
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