2 Brachytherapy Physics-Sources and Dosimetry

Brachytherapy Physics: Sources and Dosimetry

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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part I: The basics of Brachytherapy Version 1 - 01/12/2014

viewpoints of the professional societies for medical physics , the medical physicist is always responsible for such a calibration. The procedure should rely on a properly calibrated standard ra- dioactive source or he/she should perform a calibration based upon a calibrated measuring device. Recommendations for cali- bration procedures have been published by several organisations including the ICRU (27), the IAEA (23), the NCS (38, 39) and the AAPM (36). The aim of the calibration is two-fold: to ensure that the value entered into the treatment planning system agrees with the source calibration certificate to within a predetermined limit and to ensure traceability to (inter)national standards. The traceability is important as it simplifies national and internation- al comparison of treatment results and improves consistency in clinical outcome. As stated before, the quantities recommended to specify the source strength are the reference air-kerma rate K . ref and air-ker- ma strength S K . Other quantities for specifying source strength, such as the apparent activity (Bq or Ci), are now considered ob- solete. Note that a source specification based on contents of activity may be obligatory in the realm of radiation protection, where the RSO should comply with the legal requirements of the license. Generally, the presence of all radioactive material in the institute needs to be described in terms of activity. The calibration chain proposed by the ICRU (27) to guarantee traceability is as follows: • In the primary laboratory , the RAKR of a source of a given radi- onuclide is measured with a primary ionisation chamber. The source so calibrated is a primary standard source . • The secondary laboratory receives a primary standard source to calibrate the secondary ionisation chamber and calibrates secondary standard sources . The only requirements are that the sources have similar shape and size, same filtration and same radionuclide. • The user may order a calibrated source from the secondary standard laboratory in order to calibrate his own (well-type) ionisation chamber and is therefore able to calibrate any source of the same radionuclide, with the same filtration and similar shape and size. A long half-life source can be used to check the reproducibility of the measuring device in the department. High intensity iridium-192 sources used in afterloading devices require special considerations. In the absence of a suitable pri- mary standard in many countries, the interpolative free-air sec- ondary standard method has become the interim standard for measurement of high intensity iridium-192 source strength (17, 21, 23). Briefly this approach consists of measuring air kerma rate on the transverse axis of an HDR source at a distance of 10- 100 cm in a free air geometry using a thimble-ionisation cham- ber with a build-up cap thick enough to establish secondary electron equilibrium at the highest photon energy encountered. The method can be used as an in-house procedure to calibrate a more simple measurement set-up such as a well-type chamber or a solid phantom. Note that some laboratories provide a ser- vice for direct calibration of a suitable HDR well-type chamber instrument for HDR iridium-192 sources. Total reference air kerma rate of the radioactive sources can now be verified using a well-type ionisation chamber, previously cal- ibrated with at least one source provided by a National Standard Laboratory. At the time of calibration, the well-type ionisation

chamber must be corrected to take into account the energy, the dimensions, and the position in the cavity of the radionuclide, if these are different from the calibration source. The methods used for calibration are either based on a so-called in-air measurement technique (e.g., Fig. 2.6), on the use of a well-type ionization chamber (Fig. 2.7), or on a solid phantom dedicated for calibration purposes. In principle, any source can be calibrated with these methods, but there are some practical limitations. With in-air and solid phantom calibrations the sig- nals, typically obtained when using low strength sources (i.e., LDR seed sources) are small, and the final uncertainty in the air-kerma strength or reference air kerma rate may be unneces- sarily high. For HDR sources, however, all calibration methods discussed here may be considered. Most of the present recommendations from national or inter- national organizations rely on the use of a dedicated well-type ionization chamber. Even though such chambers provide an easy, fast and reliable method for source calibration, it must be kept in mind that in-air calibration is a more fundamental meth- od. However, well-type chambers offer the best in practice for a brachytherapy institution: it is a reliable, reproducible and easy to use method. Several primary or secondary standards labo- ratories, or (in USA) ADCLs, provide users all over the world with a calibration factor for their instrument which must be used with its accompanying electrometer and inserts. The procedure for in-house calibration is then simple and can be repeated at a few intervals in the lifetime of the local HDR afterloader source. A general requirement is to use the available dosimetry equip- ment at each source exchange before any clinical treatments take place. In many institutions, the procedure is repeated at the end of the life time of the source. In this way, the old and the new sources are checked in a short period of time, the stability of the measurement system is demonstrated and confidence in the use of the correct decay factor is established. In most commercial well type chambers, a guide tube is provid- ed to hold the source catheter along the axis of the cylindrical well. The sensitivity of the chamber versus the source position along the guide tube must be checked: the determination of the so-called “sweet spot”. This can be done by varying the position of a small source along the length of the guide tube. Usually, the signal is within about 1% over a trajectory of several mm of the guide tube, indicating that it is very easy to have highly repro- ducible readings. It should be noted that well type chambers with thick internal walls may show energy dependence which is particularly em- phasised when calibrating low energy photon sources, such as iodine-125 and palladium-103. For instance, the filtration of low energy photons depends on the thickness of the wall of the source holder. It is important to understand that a well type chamber in general exhibits a larger dependence on the source design compared to Farmer type chambers. The well type cham- ber’s calibration coefficient is valid only for the type of source for which it has been calibrated. This is specifically true for the low energy photon emitters. The instrument (the ion chamber, electrometer, and cables) itself should be checked for linearity, leakage, and for consistency of the readings, with a measurement at regular intervals with a long lived source, e.g. a cesium-137 source, once a year. Any deviation for the source under consideration larger than 3% must be inspected by repetition of the measurement and/or by independent means.