AAPM recommendations on dose prescription and reporting methods for permanent interstitial brachytherapy for prostate cancer: Report of Task Group 137

Abstract

During the past decade, permanent radioactive source implantation of the prostate has become the standard of care for selected prostate cancer patients, and the techniques for implantation have evolved in many different forms. Although most implants use 125I or 103Pd sources, clinical use of 131Cs sources has also recently been introduced. These sources produce different dose distributions and irradiate the tumors at different dose rates. Ultrasound was used originally to guide the planning and implantation of sources in the tumor. More recently, CT and/or MR are used routinely in many clinics for dose evaluation and planning. Several investigators reported that the tumor volumes and target volumes delineated from ultrasound, CT, and MR can vary substantially because of the inherent differences in these imaging modalities. It has also been reported that these volumes depend critically on the time of imaging after the implant. Many clinics, in particular those using intraoperative implantation, perform imaging only on the day of the implant. Because the effects of edema caused by surgical trauma can vary from one patient to another and resolve at different rates, the timing of imaging for dosimetry evaluation can have a profound effect on the dose reported (to have been delivered), i.e., for the same implant (same dose delivered), CT at different timing can yield different doses reported. Also, many different loading patterns and margins around the tumor volumes have been used, and these may lead to variations in the dose delivered. In this report, the current literature on these issues is reviewed, and the impact of these issues on the radiobiological response is estimated. The radiobiological models for the biological equivalent dose (BED) are reviewed. Starting with the BED model for acute single doses, the models for fractionated doses, continuous low-dose-rate irradiation, and both homogeneous and inhomogeneous dose distributions, as well as tumor cure probability models, are reviewed. Based on these developments in literature, the AAPM recommends guidelines for dose prescription from a physics perspective for routine patient treatment, clinical trials, and for treatment planning software developers. The authors continue to follow the current recommendations on using D90 and V100 as the primary quantitles, with more specific guidelines on the use of the imaging modalities and the timing of the imaging. The AAPM recommends that the postimplant evaluation should be performed at the optimum time for specific radionuclides. In addition, they encourage the use of a radiobiological model with a specific set of parameters to facilitate relative comparisons of treatment plans reported by different institutions using different loading patterns or radionuclides.

A quantitative assessment of the consequences of allowing dose heterogeneity in prostate radiation therapy planning

“…Hence, removing the PTV UDC ultimately increases dose heterogeneity through the PTV, but the increased levels of heterogeneity are not without clinical precedent. Comparable or greater levels of dose heterogeneity are typically seen in prostate brachytherapy 22, 23. Overall, the results are consistent between optimization algorithms.…”

“…For EBRT,$\text{BED}=D\ast (1+d/(\alpha /\beta ))$For 125 I implant brachytherapy,$\text{BED}=false(R_{0}false/\lambda false)\ast \left[1+R_{0}false/(false(\mu +\lambda false)false(\alpha false/\beta false))\right]$where $R_0=D\ast \lambda ,\lambda =0.693/T_{1/2},$ $T_{1/2}=60\text{days for}{}^{125}\mathrm{I}\text{and}\mu =0.693/t_{1/2}$ 25. From AAPM TG 137, for a prescription dose of 145 Gy for permanent 125 I implants, it is recommended that final dose to 10% of the urethra should be <150% (218 Gy) for an acceptable toxicity rate 22. For fractionated EBRT, we assumed an alpha beta ratio of 5 Gy (some evidence suggests a range of 5–10 Gy for urethra late toxicity26) and tissue repair half‐life of 0.1 h as the worst case scenario.…”

COMP report: CPQR technical quality control guidelines for low‐dose‐rate permanent seed brachytherapy

“…Some have detailed descriptions indicating how to conduct the various quality control tests. The guidelines promoted in this document are based on the experience of the authors and expert practitioners and are broadly consistent with recommendations from other jurisdictions 6, 7, 8, 9, 10, 11…”

“…There are several other publications dealing with the performance, specifications, and quality control of low‐dose‐rate (LDR) permanent seed brachytherapy 4, 5, 6, 7, 8, 9, 10, 11. Most of these publications have extensive reference lists.…”

Providing a fast conversion of total dose to biological effective dose (BED) for hybrid seed brachytherapy

“…TG‐137 states nominal values for α/β ratios when performing BED calculations for the prostate (17) . Within this study, an α/β ratio of 3.1 was used (TG‐137 states to use an α/β ratio of 3.0).…”