PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

MEDICAL PHYSICS IN THE BALTIC STATES 7 (2009)
Proceedings of International Conference “Medical Physics 2009”
8 - 10 October 2009, Kaunas, Lithuania
APPLICATION OF NEUTRONS IN RADIOTHERAPY
Gediminas ADLYS
Kaunas University of Technology
Abstract: An overview of neutron application for medical purposes from technological and physical point of view is
presented in this article. It discusses the features of fast neutron therapy, neutron capture therapy and production of
radionuclides by means of neutron activation and analyzes the requirements for neutron beams which are produced
using different sources (nuclear reactors, accelerator driven systems neutron spallation sources).
Keywords: neutron therapy, neutron sources, reactors, spallation sources
1. Introduction
Advance of science and technology continually find its
place in medicine practice. Soon after X- rays discovery
it was used for diagnostics and therapy and it continues
until now. Improvements in radiotherapy results were
noted with each technological advance. The field of
radiation therapy began to grow after discovery of the
radioactive elements polonium and radium. Radium was
used until the cobalt and caesium teletherapy machines
came into use. Nowadays, these units are being replaced
by linear accelerators, working without radioactive
sources, which makes them safer in the radiological
point of view.
With invention of computed tomography threedimensional
planning became a possibility. It allows
more accurate determination of the dose distribution
using images of the patient’s anatomy.
Construction of superconducting magnets and progress
in cryogenic technique was applied developing nuclear
magnetic resonance equipment for new imaging
modalities. Advance in nuclear and particle physics
technologies was basis for the positron emission
tomography.
The development of high power compact accelerators
such as radiofrequency quadrupole linacs or cyclotrons
creates the possibility to use neutron related techniques
in medicine at reasonable cost. In proton accelerators
accelerated protons could be used for proton therapy or
for the bombardment of the proper target, which
releases neutrons of sufficient energy to treat tumors at
the modest tissue depths.
Additionally, research nuclear reactors are used as
neutron sources for producing radionuclides for
medicine and for direct neutron therapy as a part of
radiation therapy.
Recently attention of radiotherapists is paid to the new
developing branch of nuclear physics and technology –
nuclear spallation neutron sources.
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2. The place of neutrons in radiotherapy
Radiation therapy is the medical use of ionizing
radiation primarily in the treatment of malignant tumor.
The biological response of a cancer cells to ionizing
radiation is described by its radiosensitivity. The types
of cancer are classified as “radioresistant”; if tumors do
not respond well to low LET radiation [1]. Highly
sensitive cancer cells are rapidly killed by modest doses
of radiation while radioresistant cancer cells require
much higher doses for radical cure than may be safe in
hospital practice. The majority of epithelial cancers are
only moderately radiosensitive, which require 60-70 Gy
dose of radiation to achieve a radical cure.
Radiation therapy of cancer is based upon the basic
effect of ionizing radiation to destroy the ability of cells
to divide and grow by damaging their DNA strands. The
damage is caused by a photon, electron, proton, neutron
or ion beam. It is done directly or indirectly ionizing the
atoms of the DNA chain. For photon, electron and
proton beam the damage is caused as a result of atomic
interactions. It entails primarily the ionization of water,
forming free radicals, notably hydroxyl radicals, which
then damage the DNA. With neutron radiation the
damage is caused by nuclear interactions. Cells have
mechanisms for repairing DNA damage. If radiation is
delivered in small sessions, normal tissue will have time
to repair itself. Tumor cells typically lack effective
repair mechanisms compared to most healthy cells and
are generally less efficient in repair between fractions. It
is one of reasons why the total dose is spread out over
time or fractionated. Accumulating damage to the
cancer cells causes them to die or reproduce more
slowly.
Photon, electron and proton radiation are called low
linear-energy-transfer (LET) radiation. Neutrons are
high LET radiation. If radioresistant tumor cell is
damaged by low LET radiation it has a good possibility