Question

Name two different types of “correction factors” which must be applied to data taken in an in vivo counter facility. What techniques might be used to obtain values for each of these factors?

Answer 1

Correction Factors :

1. Cumulative factors : which the calibration factors have to be multiplied in order to get the correct dose: if the response of the detector is lower or higher in clinical than in reference conditions, these factors are respectively larger or smaller than unity.

2. Energy correction factors : LiF powder TLDs irradiated in high-energy electron beams and applied to mailed dosimetry for quality assurance networks. Mostly the in vivo measurement of the exit dose is performed concomitantly with that of the entrance dose. It is then important when positioning the exit detector, to avoid the shadowing effect of the entrance detector. In fact, provided the calibration of the detectors be performed with a beam of the same radiation quality as the one used to treat patients, no correction factor has to be used except a correction factor, CD, for non-linearity of dose response with some Thermo luminescent TL material. This makes TL detectors particularly attractive for in vivo dosimetry

            Some of the values recommended are strongly related to the methodology adopted . When the value of the correction factor is equal to unity, it may be either because basically the corresponding parameter has no influence on detector response or because the error is estimated not to exceed 1 %.

The response of a Thermo luminescent (TL ) dosimeter has to be corrected to take into account the photon or electron energy. Correction factors to be used cannot be obtained from theoretical data, except in particular conditions. They have to be determined in the same conditions as those used to treat patients and for the type of dosimeters used in practice and read out on the particular reader. For a given TL dosimeter associated with a given reader, energy correction factors remain constant and can be introduced in the computer associated with some readout systems in order to correct the results automatically. No correction for directional effect is necessary except if the dosimeter container and associated build-up cap have an asymmetric shape; even for the tangential irradiation of the breast or the thoracic wall no directional dependence of detector response is observed.

            It is important to keep in mind that incomplete build-up on an entrance detector might lead to certain correction factors which are only determined by the influence of the head scatter electron contamination and not at all by the intrinsic characteristics of the detector type involved. Those latter category of correction factors are treated in the chapters about diode dosimeters and radio thermo luminescent dosimeters.