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

Table 2.7 Sensitivity of commonly implanted anatomic sites to dosimetric limitations of the TG-43 formalism. Items flagged indicate the authors’ opinion that significant differences between administered and delivered dose are possible due to the highlighted dosimetric limitation. See Rivard et al. (46).

PHOTON ENERGY

ANATOMIC SITE

ABSORBED DOSE ATTENUATION SHIELDING

SCATTERING BETA/KERMA DOSE

high low high

Prostate

xxx

xxx

xxx

x

xxx

Breast

low

xxx

xxx

xxx xxx

high

Gyn

low

xxx

xxx

high

xxx xxx

xxx xxx xxx xxx xxx xxx xxx xxx

Skin

low

xxx

high

xxx

Lung

low

xxx

xxx

high

Penis

low

xxx

high

xxx xxx

xxx

Eye

low

xxx

xxx

of Task Group 167’ is under re-review by the AAPM Therapy Committee as of Oct 2014. Chair R. Nath. Accurate dosimetry is of utmost importance when clinical knowledge obtained with one system is to be transferred into another, such as with trial participation. Interest in the research community to study the presence of heterogeneities in calculated dose distributions has resulted in implementation of model based dose calculation algorithms (MBDCAs) in clinical treatment planning systems (4,9,46). Giv- en the situation that at the time of writing of this chapter, there is only one commercially available treatment planning system on the market, able to perform calculations for only one type of source, iridium-192, the authors decided to restrict themselves to presenting only a number of observations in agreement with these and to add a few other references from the international literature for the interested reader. Fig. 2.18 shows very schematically the main steps of the dose cal- culation process for treatment planning using the TG-43 meth- od and model-based dose calculation algorithms. In the TG-43, method a dose distribution is quickly obtained from the model parameters leading to a single source (or single source position) dose distribution. By simply summing the individual dose dis- tributions (the superposition step), the final dose distribution is obtained. The accuracy of the calculation is determined by the limiting factors discussed in the previous paragraphs. Monte Carlo techniques imply that statistical techniques are used to calculate a large number of tracks of the photons that are emit- ted by a source. The ‘history of each track is followed along the possible pathways through the material, and for the end result, all dose depositions are summed and assigned to the volume elements. Practical application in treatment planning requires that the fast calculation is fast and that the dose is derived in all voxels of interest in a 3D region around the defined target. Dif- ferent codes are available in research environments. Accuracy of the calculations is largely determined by the number of histories that are followed. Due to its nature, the process is (still) relatively slow, and therefore it cannot be expected that in the next few years, MC techniques will be useful for optimization routines

Dose differences > 5% are possible for high-energy sources with- in 5 cm of the skin. Equivalence of dose and kerma within 5% does not hold true within a few millimeters of high-energy pho- ton-emitting sources. Table 2.7 shows which of the commonly implanted anatomical sites are potentially most affected by these dosimetric limitations. With the increasing use of 3-D planning techniques and the desire to incorporate CT information into brachytherapy treat- ment planning, the ability to account for tissue heterogeneities becomes of interest. New algorithms are designed to facilitate tissue heterogeneity corrections. A further discussion of ongoing developments and projects, from which the summary of the pre- vious paragraph is taken, can be found in a Vision 20/20 paper in Medical Physics Journal by Rivard et al. (46). 5.6 Model based dose calculation algorithms Inhomogeneities have been taken into account in external beam therapy treatment planning since the early use of CT-based im- aging. One could argue why this was not followed in brachyther- apy. There are, however, several reasons why heterogeneity cor- rections have not been routinely incorporated in brachytherapy. First, the inverse square fall-off in primary fluence with distance from the source dominates all brachytherapy dose distributions: the heterogeneity corrections are in comparison a second order effect. Furthermore, brachytherapy was an early modality for ra- diation therapy and has a successful tradition based on surgical implant skills and empirical knowledge. As long as a successful and reproducible system is unchanged, it can be safely handled by empirical knowledge. Nowadays, modern brachytherapy is in an era of rapid evolution, including use of image guidance, introduction of new radiation sources, and application of new techniques, among other developments (see, among others, (48). A new report, from AAPM TG-167 ‘Recommendations by the AAPM and GEC-ESTRO on the use of new or innovative brachytherapy sources, devices, applicators, or applications: Report