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Dosimetry for Ir-192 HDR Brachytherapy - Present Status and Future Direction

L DeWerd

J Seuntjens

M Rivard

L DeWerd1*, J Seuntjens2*, M Rivard3*, M McEwen4, (1) University of WI-Madison/ADCL, Madison, WI, (2) McGill University, Montreal, QC, (3) Tufts Medical Center, Boston, MA, (4) National Research Council, Ottawa, ON

TU-A-213AB-1 Tuesday 8:00:00 AM - 8:55:00 AM Room: 213AB

Reference dosimetry for high doserate 192Ir brachytherapy in North America is based on an interpolated air kerma standard maintained at the U.S. ADCLs. Clinical well-chambers are calibrated in terms of air kerma and the AAPM TG-43 protocol is used to obtain absorbed dose to water.

This first part of the presentation will review:
i) The calibration chain for the dissemination of the quantity air kerma strength for 192Ir, from the primary standards maintained by NIST, through the intermediate realization at the ADCLs, to the calibration of clinical well-type ionization chambers.
ii) How the accuracy of dose measurements may be affected in situations where the source design differs for calibration and use.
iii) What uncertainty components contribute to the overall uncertainty in the measurement of air kerma strength in the clinical situation, focusing on the recently published AAPM/ESTRO TG-138 report.

There are significant developments worldwide focused on moving to an absorbed dose-to-water basis for HDR 192Ir dosimetry. Various techniques have been proposed – water and graphite calorimetry, Fricke dosimetry, and ionization chambers – and all offer the potential for increased accuracy and a simpler protocol for users. The second part of the presentation will:
i) Outline how absorbed dose standards provide a potential improvement in dosimetry for HDR 192Ir. An absorbed dose standard would mean that the calibration the clinical physicist obtains for their well chamber would be much closer to what they need, thereby potentially reducing the uncertainty on the clinical reference dose.
ii) Review the new approaches for direct absorbed dose to water realization in detail. Water calorimetry would seem to be the most direct option for a primary standard but other approaches under investigation may ultimately offer a lower uncertainty.

The AAPM TG-43 protocol is the standard dosimetry protocol used worldwide for brachytherapy clinical dose calculations. While there have been a number of updates refining the approach, the basic formalism based on superposition of single-source dose distributions in a fixed water sphere has remained. The joint AAPM/ESTRO HEBD report on high-energy (> 50 keV) photon-emitting brachytherapy source dosimetry further refines the basic TG-43 formalism. Specific to high-energy radionuclide sources such as 192Ir, the report includes the following:
a) consensus datasets of brachytherapy dosimetry parameters and recommended methods for evaluating these consensus datasets,
b) recommendations on dosimetry methods to characterize the source dose distribution (based on experimental procedures and Monte Carlo methods), and
c) interpolation/extrapolation techniques for the 2D anisotropy function and radial dose function, specific to high-energy sources, which differ from the low-energy techniques in the 2004 and 2007 AAPM updates.

In addition to explaining this report, the final part of the presentation will discuss other advances such as using model-based dose calculation algorithms (AAPM/ESTRO/ABG TG-186 report) and clinical implementation of primary HDR 192Ir dose rate-to-water calibrations.

Learning Objectives:
1) Understand the present basis for HDR 192Ir dosimetry in North America
2) Understand how proposed standards of absorbed dose will benefit clinical dosimetry
3) Understand how the TG-43 protocol is being refined through joint AAPM/ESTRO activities.

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