Report of the Task Group 186 on model‐based dose calculation methods in brachytherapy beyond the TG‐43 formalism: Current status and recommendations for clinical implementation

Abstract

The charge of Task Group 186 (TG-186) is to provide guidance for early adopters of model-based dose calculation algorithms (MBDCAs) for brachytherapy (BT) dose calculations to ensure practice uniformity. Contrary to external beam radiotherapy, heterogeneity correction algorithms have only recently been made available to the BT community. Yet, BT dose calculation accuracy is highly dependent on scatter conditions and photoelectric effect cross-sections relative to water. In specific situations, differences between the current water-based BT dose calculation formalism (TG-43) and MBDCAs can lead to differences in calculated doses exceeding a factor of 10. MBDCAs raise three major issues that are not addressed by current guidance documents: (1) MBDCA calculated doses are sensitive to the dose specification medium, resulting in energy-dependent differences between dose calculated to water in a homogeneous water geometry (TG-43), dose calculated to the local medium in the heterogeneous medium, and the intermediate scenario of dose calculated to a small volume of water in the heterogeneous medium. (2) MBDCA doses are sensitive to voxel-by-voxel interaction cross sections. Neither conventional single-energy CT nor ICRU∕ICRP tissue composition compilations provide useful guidance for the task of assigning interaction cross sections to each voxel. (3) Since each patient-source-applicator combination is unique, having reference data for each possible combination to benchmark MBDCAs is an impractical strategy. Hence, a new commissioning process is required. TG-186 addresses in detail the above issues through the literature review and provides explicit recommendations based on the current state of knowledge. TG-43-based dose prescription and dose calculation remain in effect, with MBDCA dose reporting performed in parallel when available. In using MBDCAs, it is recommended that the radiation transport should be performed in the heterogeneous medium and, at minimum, the dose to the local medium be reported along with the TG-43 calculated doses. Assignments of voxel-by-voxel cross sections represent a particular challenge. Electron density information is readily extracted from CT imaging, but cannot be used to distinguish between different materials having the same density. Therefore, a recommendation is made to use a number of standardized materials to maintain uniformity across institutions. Sensitivity analysis shows that this recommendation offers increased accuracy over TG-43. MBDCA commissioning will share commonalities with current TG-43-based systems, but in addition there will be algorithm-specific tasks. Two levels of commissioning are recommended: reproducing TG-43 dose parameters and testing the advanced capabilities of MBDCAs. For validation of heterogeneity and scatter conditions, MBDCAs should mimic the 3D dose distributions from reference virtual geometries. Potential changes in BT dose prescriptions and MBDCA limitations are discussed. When data required for full MBDCA implementation are insufficient, interim ...

Brachytherapy treatment planning commissioning: effect of the election of proper bibliography and finite size of TG‐43 input data on standard treatments

“…Despite the dose differences found (over the international recommendations), the gamma analysis shows results in agreement with the criterion proposed by the TG‐186 (36) . Therefore, further studies are required to exclude the possibility of undesirable effects in the DVH caused by ODIN in the $\mathrm{F}{(\mathrm{r},\theta )}_{\text{VID}}$, associating the conclusions obtained with the sensibility of the gamma analysis, emphasizing in the standard treatments most affected by this error (i.e., one dwell and Fletcher setup).…”

“…The TG‐186 of the AAPM (36) recognize the necessity of define a dose accuracy tolerance requirements considering a gamma‐index metrics for brachytherapy as an important first step, proposing an incipient criterion of $2\%/2 \text{mm}$ with a 99% pass rate for clinically relevant points. On the other hand, a growing amount of literature is warning us about how the gamma analysis may overlook important errors in the TPS causing nonnegligible deviations in the DVH 37 , 38 , 39 , 40 .…”

Patient's specific integration of OAR doses (D2 cc) from EBRT and 3D image‐guided brachytherapy for cervical cancer

“…In this study, all physical HDR plan doses (D2cc and D0.1 cc) were calculated based upon the AAPM TG 43 formula49, 50, 51 without heterogeneity‐corrections. The uncertainties of TG 43 dose calculations have been validated through a model‐based dose calculation algorithm that accounts for tissue and applicator heterogeneity 52. The uncertainties of OAR D2 cc in HDR BT plans for cervical cancer have been reported as on average 1%~3% for plastic tandem‐and‐ring (T&R) applicators calculated using a Grid‐Based Boltzmann Solver (GBBS) algorithm (Acuros, Varian Medical System, Inc.)53 or by a collapsed‐cone convolution algorithm (ACE, Elekta Ltd., Stockholm, Sweden) 54.…”

Brachytherapy dose‐volume histogram commissioning with multiple planning systems

“…The dose‐rate constant, Λ, was $1.101\hspace{0.17em}\text{cGy}\hspace{0.17em}{\mathrm{normalh}}^{-1}{\mathrm{normalU}}^{-1}$, the geometry function used an effective length of 0.5 cm, and the 2D anisotropy function $false(Ffalse(r,\theta false)false)$ was set to 1.00 for r and θ to simplify the hand calculation. The Model mHDR‐v2 (Nucletron‐Elekta; Stockholm, Sweden) source was also commissioned for use 15 , 16 . As done for the Model VS2000, a source strength of 27,076 U was selected to deliver 100 Gy at the reference position from a 20 min temporary implant.…”