3 Radiation Protection in Brachytherapy

Radiation Protection in Brachytherapy

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

Fig. 3.3 169 Yb radiation transmission factor T for a lead shielding barrier in broad beam geom- etry conditions. Note the change of slope in the transmission curve at small thickness due to the combined effect of increased attenuation of the lower photon energy emissions by 169 Yb and build up. The transmission curve for monoenergetic photons of energy close to the maximum energy emitted by 169 Yb is also presented for comparison. (Courtesy: P. Papagiannis)

Fig. 3.2 60 Co radiation transmission factor T for a lead shielding barrier in narrow and broad beam geometry conditions. (Courtesy: P. Papagiannis)

data from the literature for the correct (realistic) broad beam geometry. As shown in Fig. 3.2, the use of narrow beam con- ditions employing material attenuation coefficients (i.e. solving the equation: T(x) =I(x)/I0 =exp (-μx) for x in which I stands for the intensity of the radiation beam) is unacceptable due to the build-up factor. In order to augment outdated experimental transmission data for selected brachytherapy radionuclide/material combinations in broad beam conditions, the numerical method of Monte Carlo simulation has been employed in a series of publications (6, 27, 37). This approach however produces discrete values of trans- mission versus material thickness in tabular form or graphs, thus necessitating interpolations. Often recourse is made to the use of transmission curve indices such as the half and tenth value layer ( HVL and TVL , respectively) for particular radionuclide-mate- rial combinations (11, 36). These indices however do not remain constant due to spectral variation with increasing shielding bar- rier thickness (27, 37). An example is shown in Fig. 3.3. It has been suggested (11) that the first HVL or TVL at small R (or large T ) and HVL e or TVL e at large R (or small T ), should be used for the calculation of material shielding thickness to achieve a given R or T value. HVL e and TVL e , called “hard” or equilibrium values, correspond to that penetrating region where the radiation directional and spectral distributions are practical- ly independent of thickness so that a single value of the HVL or TVL is valid. This suggestion however has been shown to intro- duce potentially significant errors (37). It is therefore both con- venient and accurate to fit an ad hoc analytical representation to transmission data for use in practical dose calculations and shield designs. Within the framework of a BRAPHYQS activity, (BRAPHYQS, a physicists’ working group within GEC-ESTRO) Monte Carlo simulation was used to generate photon radiation broad beam transmission data for various radionuclide-materi- al combinations ( 60 Co, 137 Cs, 198 Au, 192 Ir, 169 Yb, 170 Tm, 131 Cs, 125 I,

and 103 Pd photons through concrete, stainless steel, lead, as well as lead glass and baryte concrete). Besides the tabulation of first and equilibrium HVL/TVL values, a three-parameter analytical expression was fitted to results to facilitate accurate and simple radiation shielding calculations (37). The Monte Carlo calcu- lated data sets, as well as fitting coefficient results, are available online at http://www.estro.org/about/governance-organisation/ committees-activities/radiation-protection. Indicative data are presented in Fig. 3.4. This figure also provides a hint on the se- lection of shielding material. A high-Z/high-density material such as lead would require a smaller shielding thickness which could be favorable if space is limited. Depending on the facility workload and layout however, support issues might ensue that could be dealt with by using a combination of materials (i.e. con- crete and lead or stainless steel sheets) or a maze to reduce door shielding. Cost, time to completion, and other technical aspects are also pertinent; the solution to the shielding problem is nei- ther unique nor generally applicable. Two more publications prepared within the framework of BRA- PHYQS activities present data useful for the structural shielding of brachytherapy facilities. Zourari et al. (42) present a method for calculating the transmission of any broad photon beam with a known energy spectrum in the range of 20–1090 keV, through concrete and lead, based on the superposition of corresponding monoenergetic data obtained from Monte Carlo simulation. This method is complemented with a simple program, incorporating a graphical user interface, to facilitate the superposition of monoen- ergetic data, the graphical and tabular display of broad photon beam transmission curves, and the calculation of material thick- ness required for a given transmission from these curves. Pujades et al. (39) present the adaptation of the NCRP 151 meth- odology (35) for estimating the air-kerma rate at the door in BT facilities using Monte Carlo simulation in actual brachytherapy