In March 2004, the recommendations of the American Association of Physicists in Medicine (AAPM) on the interstitial brachytherapydosimetry using and were reported in Medical Physics [TG-43 Update: Rivard et al., 31, 633–674 (2004)]. These recommendations include some minor changes in the dose-calculation formalism and a major update of the dosimetry parameters for eight widely used interstitial brachytherapy sources. A full implementation of these recommendations could result in unintended changes in delivered dose without corresponding revisions in the prescribed dose. Because most published clinical experience with permanent brachytherapy is based upon two widely used source models, the Model 6711 and Model 200 sources, in this report we present an analysis of the dosimetric impact of the 2004 TG-43 dosimetry parameters on the history of dose delivery for these two source models. Our analysis indicates that the currently recommended prescribed dose of 125 Gy for Model 200 implants planned using previously recommended dosimetry parameters [AAPM dose prescription: Williamson et al., Med. Phys.27, 634–642 (2000)] results in a delivered dose of 120 Gy according to dose calculations based on the 2004 TG-43 update. Further, delivered doses prior to October 1997 varied from 113 to 119 Gy for a prescribed dose of 115 Gy compared to 124 Gy estimated by the AAPM 2000 report. For implants using Model 6711 seeds, there are no significant changes (less than 2%). Practicing physicians should take these results into account when selecting the clinically appropriate prescribed dose for interstitial implant patients following implementation of the 2004 TG-43 update dose-calculation recommendations. The AAPM recommends that the radiationoncology community review this report and consider whether the currently recommended dose level (125 Gy) needs to be revised.
The authors would like to thank Dr. Ty Robin, Dr. Mary Napolitano, and Joe Rodgers of Theragenics Corporation for their exceptionally detailed and helpful comments.
Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the AAPM or the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
II. METHODS AND MATERIALS
II.A. Brief history of brachytherapy dosimetry
II.A.1. Theragenics™ calibration standard (1988–1997)
II.A.2. Early single-source dosimetry parameters (1990–1995)
II.A.3. Transition from “heavy” to “light” seed design (1992–1993)
II.A.4. Shift in vendor calibration (Fall 1997)
II.A.5. Implementation of the NIST WAFAC 1999 standard
II.A.6. Revised dosimetry parameters and AAPM 2000 dose-specification guidance (1999–2000)
II.A.7. Discovery and correction of calendar year 1999 WAFAC measurement errors (March 2001)
II.A.8. Revised dosimetry parameters and the updated TG-43 protocol (March 2004)
II.B. Generalized formalism for evaluation of administered-to-prescribed dose ratios
II.C. Air-kerma strength standard revisions
II.D. Reference dosimetry parameters
II.E. Prescription dosimetry parameters
II.F. Dose-averaging procedures
II.F.1. Radial dose function equivalence approximation (RDA)
II.F.2. Geometry-function weighted single-seed approximation (GFSA)
II.F.3. Clinical implant averaging
III.A. Reference-to-prescription dose ratios for brachytherapy
III.B. Administered-to-prescribed dose ratios
III.C. Model 6711 source
IV. DISCUSSION AND CONCLUSIONS
WHOM TO CONTACT FOR FURTHER ASSISTANCE
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