ROENTGEN’S DISCOVERY was artificial ionizing
radiation. Two years later, a French physicist,
Henri Becquerel, discovered that certain rocks
emitted natural ionizing radiation with characteristics
much like Roentgen’s X rays. Becquerel’s colleagues
Pierre and Marie Curie refined the naturally radioactive
ores to derive uranium, polonium, and radium.
Radium was perceived to have a value in treating
cancers, already seen to be responsive to X rays. Marie
Curie’s work produced only tiny amounts, with one
ounce of radium being offered for sale at $1 million. The
radium salt (usually radium sulfate) was sealed in hollow
gold or platinum needles and inserted into or against
cancerous lumps to deliver cell-killing doses of radiation.
A decay product of radium, radon gas, was used
in hollow glass seeds for insertion in tumors which could
not be reached with the removable needles.
William Coolidge soon improved his X-ray tubes to
deliver energy levels of 200 kilovolts and more, and as
doctors used radium coupled with the high energy X-ray
beams, they noted the seeming paradox that higher
energies killed more cancer cells and spared more normal
tissue than lower-energy radiation. Radiobiologists came
to understand that the rapid mitosis of cancer cells made
them more susceptible to radiation destruction and less
capable of regeneration than slower-growing normal
cells. But because some normal cells were necessarily
radiated in the process of getting the energy to the
cancers, the success of treatment depended upon the
ability of the radiologist to plan and deliver a dose that
would kill all of the cancer cells without destroying
an unacceptable amount of normal cells.