Madame Marie Curie discovered radium in 1898; soon after, a
fellow scientist, Henri Becquerel, inadvertently carried a small quantity of
radium in the chest pocket of his lab coat, and developed an ulcer on the
chest. The discovery that radium could destroy tissue was made serenedipitously. Prior to that, Wilhelm Roentgen discovered X rays in 1895, and the first
person to be “X-rayed” was his wife. The X ray of Mrs Roentgen’s hand is the
stuff of Radiation Oncology folklore. Marie Curie and Wilhelm Roentgen went on to receive the
Nobel Prize for Physics in 1903 and 1901 respectively. They are also
immortalized in the form of units of radioactivity and exposure to radiation,
respectively.
Old timers in medicine continue refer to external beam
radiation as DXT for Deep X ray Treatment and to Brachytherapy or the insertion
of radiation into the tumour / body as radium treatment. Technology and
computers have taken radiation much ahead of the days of DXT and radium
treatments, to linear accelerators, treatment planning computers and remote
controlled brachytherapy.
The work horse of radiation departments was, for many
decades, the telecobalt machine. It
offered the advantage, over DXT, in
being able to deliver radiation with sufficient energy to penetrate deep into
the body and spare the overlying normal
tissues. However, linear accelerators,
developed independently in England and United States during the Second World War,
allowed for radiation beams with sharper beam edges and variations in the
energy of the X-rays generated. In addition, the radiation beam of a linear
accelerator did not emanate from a radioactive source and therefore there were
no problems associated with “decay “ of a radiation source, an inherent
property of all radioactive substances. The beam in a linear accelerator is
composed of X rays, which are produced when a stream of electrons bombards a
target. These electrons can also be harnessed to produce an electron beam,
which is used to treat superficial cancers like skin cancers.
That radiation has a lethal effect on tumour cells is well
known. However , to harness this effect in a safe way, the oncologist has to be
cognizant of the fact that radiation can also harm normal tissues. Broadly, the
harmful effect of radiation emanates from denudation of epithelium in the acute
phase and from ischemia and fibrosis secondary to endarteritis in the late
phase. Simply physically shielding a
normal organ utilizing high molecular weight substances such as lead based
alloys, restricts the dose to normal structures and minimizes the accompanying
effects of radiation. In telecobalt machines and early linear accelerators,
these shield were manually placed in the path of the radiation beam, before the
latter entered the body. This was obviously cumbersome, time consuming and had
the potential for error.
The development of the multi leaf collimator was a significant development in the evolution of modern radiation techniques. The collimator is a device that shaped the radiation beam ; this shape was either a square or a rectangle since the collimators were basically a set of 2 jaws perpendicular to one another. To alter the shape of the radiation beam, one could move the collimators or insert shields.
In a multileaf collimator, one pair of jaws is replaced by a set of bars , called leaves, which therefore allow for flexibility in creating shapes that could match the shape of the tumour. The thinner the leaves, the more “conformal “ the shape of the beam to that of the tumour.
The earlier leaves in the collimator were moved manually. However, with increasing sophistication of computers and their application in every aspect of radiation planning and delivery, the process of driving the leaves of the multi leaf collimator was computerised.
This brings us to computerised treatment planning, which warrants its own blogpost
In
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