Up to date there are no measurements of patient doses directly related to risk and irrespective of imaging method,
exposed body part,
patient's size,
age and gender.
Comparisons are possible only when all these factors are similar.
For radiography,
standard situations have been chosen for evaluation.
We propose a chart that gives an one-blink overview of a practice (see sample in Fig. 1 ): the logarithmic graph of the the measured incident doses,
together with the reference levels for all standard plain X-rays and standard patients. The use of that chart is demonstrated in real cases.
The Effective Dose (with the unit Sievert <Sv>)
measures radiation absorbed in the body,
balanced by risk-related beam- and tissue-coefficients,
adding together radiation effects on different organs.
It is proportional with risk (i.e.
deaths because of late cancers).
But age- and gender-related risk coefficients do not exist (yet),
so doses in non-similar situations cannot be added nor compared [1],
despite the initial hopes that the Effective Dose could be an overall and lifelong indicator.
The Effective Dose cannot be directly measured and it is hard to be calculated/evaluated.
In an experimental setting,
a standard plastic phantom is used,
with many thermoluminescent detectors (TLDs) inserted inside the phantom.
In a clinical setting,
the TLDs are sticked on the patient's skin and a computer mathematical phantom evaluates the dose.
The Incident (Entrance) Dose (with the unit Gray <Gy>)
measures the incident radiation (that “enters” the body).
It is not an estimation of the absorbed dose or related risk.
It isn't the best tool to be used in radioprotection,
but it is simple,
reproducible and easy to be measured,
although not on a routine basis.
To measure the incident dose,
a single TLD is sticked on the patient skin in the center of X-ray beam.
Therefore,
that dose was chosen as Reference for Plain X-Rays.
It has to be underlined that the Incident/Entrance Dose unavoidably includes the back-scatter!
The incident dose is not additive (we cannot add the dose for a chest radiography with the one for an abdominal radiography,
nor with a chest CT or other imaging method),
so keeping records of these has no individual benefit.
Reference Levels
were established for the Incident (Entrance) Dose <Gy> to evaluate patient doses at radiography,
:
- for “standard patients”: 70 Kg adult,
5 year old child,
baby
- for common plain X-Rays: Chest,
Skull,
Spine,
Pelvis,
Abdomen,
...
With reference levels,
comparisons are made possible: between a practice and the reference-levels or between practices .
Are they alternatives to TLDs?
Personal Film-Dosimeters (F.D.) may be used as an alternative to TLDs [2].
Their dose-range covers all common X-rays and are (almost) invisible on radiographs ( Fig. 2 ).
Dose-Area-Product Dosimeters (DAP-meters) are NOT recommended to measure incident doses at plain X-Rays.
They miss the back-scatter ( Fig. 3 ) and geometrical errors may be high ( Fig. 4 ).
Doses displayed in the DICOM tags of the Direct Digital X-Ray units may be inconsistent.
In one unit the displayed values were three times greater than those recorded in the beam ( Fig. 5 ).
The service engineers were not aware on any calibration procedure and the calculation algorithm is proprietary....