2 Brachytherapy Physics-Sources and Dosimetry

Brachytherapy Physics: Sources and Dosimetry

21

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

expect a period of several years in which manufacturers enthu- siastically advertise their enhanced dose calculation capabilities, and professional societies do not yet have in place recommen- dations for clinical integration of MBDCA. Now that manufac- turers have responded to the clinical needs of the brachytherapy community by developing these MBDCA, additional efforts by professional societies are required for uniform clinical integra- tion into the medical community. Until then, we feel that med- ical physicists should proceed cautiously and share the differ- ences between the old and new approaches with their radiation oncologists while not neglecting the TG-43 formalism used to generate the doses used to determine the current brachytherapy prescription dosage.” • Publication of AAPM TG-43 report was a major break- through in avoiding the drawbacks associated with the earli- er (‘conventional’) dose calculation methods in brachyther- apy treatment planning. TG-43 presented a parameter based method that has become a world-wide standard for treatment planning systems. However, several important issues in brachytherapy treatment planning, such as how to deal with shielding and inhomogeneities, remain unsolved in the TG-43 approach. • Model Based Dose Calculation Algorithms are now devel- oping and have the potential to perform the calculations with a significantly higher accuracy. Different solutions are proposed. At the time of writing of this chapter, not all do- simetric issues that are connected to these new approaches have been solved. International cooperation among profes- sional societies is needed and agreement must still be found on key elements, such as the way of dose reporting and pre- scription.

few papers that assess an entire treatment modality (…) toward providing a simple means for correcting the current prescrip- tion standard of care based on the TG-43 formalism to the more accurate dosimetry provided by MBDCA. Consequently, the brachytherapy community may need to gather data (either ret- rospectively or ideally prospectively) for determining the differ- ences between prescribed and administered doses as calculated with MBDCA. In some instances, a simple scalar value to correct current prescriptions may suffice. However, it is expected that new prescriptive approaches will be needed for large implants with varying scatter conditions and material heterogeneities. Leadership by radiation oncologists and medical physicists will be paramount for a safe and consistent shift to MBDCA. One can • A wide variety of sources have been developed for the pur- pose of clinical application in brachytherapy treatments. The physical properties such as half life and energy of the emitted radiation of the employed radionuclides determine how they can be used in clinical practice, e.g. in LDR, PDR, or HDR application, for permanent or temporary implants, for interstitial or intracavitary applications. • The quantity source strength of a brachytherapy source needs to be clearly defined. For that purpose it is essential to follow international recommended units. • Calibration of each new source with an in-house instrument with traceability to a primary standard is standard practice in a brachytherapy department. 6. KEY MESSAGES