Dosimetry of X-rays

Kienbock Quantimeter application 1918 klein.jpg 

Photograph: The Quantimeter of Dr. Kienböck

In 1914, the radiation output from the ion tube is low due to the limited current of about 3 mA and to the thick 5 mm aluminium filter. Irradiations last about half an hour for a delivery of the equivalent of 250 r (r is the unit Röntgen, see below). The irradiation time is however not an accurate measure of the delivered dose because during the long irradiation, in spite of the gas regulation, the X-ray intensity decreases and the beam quality ("hardness") increases.

In 1914, radiation dosimetry is still in its infancy.  Ionisation measurements are still unknown in Radiology and it will take until 1928 for the concept of the unit Röntgen, "r", to be used as the basis for radiation dosimetry. In 1914, chemical and photographic in vivo measuring systems become available to estimate the delivered dose as a percentage of the skin tolerance dose. 

In practice, the cumulative effect of a series of irradiations is mainly assessed by monitoring the biological changes in the irradiated tissue. In particular, erythema or redness of the skin is an important indicator. But the physician is not wholly dependent on observation of erythema. Initially, Dr. Gaarenstroom uses both the Quantimeter system of Kienböck and the Chromoradiometer of Holzknecht, both from the Viennese school of radiotherapy [6].  A few months later, only the measurements with the chromoradiometer are recorded in the irradiation charts.

Kienbock readout.JPG

the Quantimeter system by Kienböck

In the Quantimeter system by Kienböck, strips of photographic paper (silver bromide) are positioned in the beam on the skin during the treatment. The resulting blackening is compared to a standard scale calibrated in increasing grey values of Kienböck's unit, the "X".   With an unfiltered X-ray beam, it is assumed that the dose at which hair loss occurs is reached at a measurement of 10X, approximately equal to a measurement of 5H (see below).  With heavily filtered radiation, this ratio no longer applies. Therefor Gaarenstroom had to calibrate his own dosimeters in practice.

In the Chromoradiometer system by Dr. Guido Holzknecht, a disc of potassium salt is irradiated.  The disc is bright yellow and becomes darker the more radiation is absorbed.  The irradiated disc is compared to a standard series of yellow shades. Consecutive shades are numbered as multiples of the unit "H". 1H is the dose at which coloration is just visible and is approximately equal to 1/3 of the dose at which erythema occurs. 

Holzknecht Chromoradiometer.bmp.jpg


The accuracy of the Quantimeter and the Chromoradiometer was low.  The coloration of salt or paper strip is energy dependent and sensitive to the surrounding temperature and air humidity.  In measurements in 1932, the sensitivity of the in vivo dosimeters of Holzknecht, Kienböck and Sabouraud is compared to the biological effect and to the new unit in use since 1928, the röntgen (r). 

1932 comp dosi methods klein.jpg

At a tube voltage of 170 kV, there is a large difference in sensitivity of the old in vivo methods between unfiltered and filtered radiation.  This will also have been the case for the beam of about 100 kV in 1914.

In 1914 in the Netherlands Cancer institute, radiologist Frans Gaarenstroom determines in practice that, with skin irradiation, he cannot go further than 20 irradiations of 6.5 "X" on the scale of Kienböck.  That is 130 "X", while under the same conditions in Berlin one reports applying 300 to 400 "X" before major skin damage occurs.  The lesson here is that experiences elsewhere are not easy to repeat.  Local equipment and checking methods must first be carefully examined on efficacy; and so, in 1914, Gaarenstroom develops his own standards for dose-effect relationship.



Bronnen & Publicaties

  • [6] Ned. Tijdschr. v. Gen. (Netherlands Medical Journal) 17. II, 1915, pp 1900- 1922 : G F Gaarenstroom, Behandeling van Kwaadaardige gezwellen met röntgenstralen. ,