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Granero, D., Perez-Calatayud, J., Vijande, J., Ballester, F., & Rivard, M. J. (2014). Limitations of the TG-43 formalism for skin high-dose-rate brachytherapy dose calculations. Med. Phys., 41(2), 021703–8pp.
Abstract: Purpose: In skin high-dose-rate (HDR) brachytherapy, sources are located outside, in contact with, or implanted at some depth below the skin surface. Most treatment planning systems use the TG-43 formalism, which is based on single-source dose superposition within an infinite water medium without accounting for the true geometry in which conditions for scattered radiation are altered by the presence of air. The purpose of this study is to evaluate the dosimetric limitations of the TG-43 formalism in HDR skin brachytherapy and the potential clinical impact. Methods: Dose rate distributions of typical configurations used in skin brachytherapy were obtained: a 5 cm x 5 cm superficial mould; a source inside a catheter located at the skin surface with and without backscatter bolus; and a typical interstitial implant consisting of an HDR source in a catheter located at a depth of 0.5 cm. Commercially available HDR Co-60 and Ir-192 sources and a hypothetical Yb-169 source were considered. The Geant4Monte Carlo radiation transport code was used to estimate dose rate distributions for the configurations considered. These results were then compared to those obtained with the TG-43 dose calculation formalism. In particular, the influence of adding bolus material over the implant was studied. Results: For a 5 cm x 5 cm Ir-192 superficial mould and 0.5 cm prescription depth, dose differences in comparison to the TG-43 method were about -3%. When the source was positioned at the skin surface, dose differences were smaller than -1% for Co-60 and Ir-192, yet -3% for Yb-169. For the interstitial implant, dose differences at the skin surface were -7% for Co-60, -0.6% for Ir-192, and -2.5% for Yb-169. Conclusions: This study indicates the following: (i) for the superficial mould, no bolus is needed; (ii) when the source is in contact with the skin surface, no bolus is needed for either Co-60 and Ir-192. For lower energy radionuclides like Yb-169, bolus may be needed; and (iii) for the interstitial case, at least a 0.1 cm bolus is advised for Co-60 to avoid underdosing superficial target layers. For Ir-192 and Yb-169, no bolus is needed. For those cases where no bolus is needed, its use might be detrimental as the lack of radiation scatter may be beneficial to the patient, although the 2% tolerance for dose calculation accuracy recommended in the AAPM TG-56 report is not fulfilled.
Keywords: HDR; brachytherapy; skin; Monte Carlo; Geant4; Co-60; Ir-192; Yb-169
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Aguiar, P., Rafecas, M., Ortuño, J. E., Kontaxakis, G., Santos, A., Pavia, J., et al. (2010). Geometrical and Monte Carlo projectors in 3D PET reconstruction. Med. Phys., 37(11), 5691–5702.
Abstract: Purpose: In the present work, the authors compare geometrical and Monte Carlo projectors in detail. The geometrical projectors considered were the conventional geometrical Siddon ray-tracer (S-RT) and the orthogonal distance-based ray-tracer (OD-RT), based on computing the orthogonal distance from the center of image voxel to the line-of-response. A comparison of these geometrical projectors was performed using different point spread function (PSF) models. The Monte Carlo-based method under consideration involves an extensive model of the system response matrix based on Monte Carlo simulations and is computed off-line and stored on disk. Methods: Comparisons were performed using simulated and experimental data of the commercial small animal PET scanner rPET. Results: The results demonstrate that the orthogonal distance-based ray-tracer and Siddon ray-tracer using PSF image-space convolutions yield better images in terms of contrast and spatial resolution than those obtained after using the conventional method and the multiray-based S-RT. Furthermore, the Monte Carlo-based method yields slight improvements in terms of contrast and spatial resolution with respect to these geometrical projectors. Conclusions: The orthogonal distance-based ray-tracer and Siddon ray-tracer using PSF image-space convolutions represent satisfactory alternatives to factorizing the system matrix or to the conventional on-the-fly ray-tracing methods for list-mode reconstruction, where an extensive modeling based on Monte Carlo simulations is unfeasible.
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Albiol, A., Corbi, A., & Albiol, F. (2017). Automatic intensity windowing of mammographic images based on a perceptual metric. Med. Phys., 44(4), 1369–1378.
Abstract: Purpose: Initial auto-adjustment of the window level WL and width WW applied to mammographic images. The proposed intensity windowing (IW) method is based on the maximization of the mutual information (MI) between a perceptual decomposition of the original 12-bit sources and their screen displayed 8-bit version. Besides zoom, color inversion and panning operations, IW is the most commonly performed task in daily screening and has a direct impact on diagnosis and the time involved in the process. Methods: The authors present a human visual system and perception-based algorithm named GRAIL (Gabor-relying adjustment of image levels). GRAIL initially measures a mammogram's quality based on the MI between the original instance and its Gabor-filtered derivations. From this point on, the algorithm performs an automatic intensity windowing process that outputs the WL/WW that best displays each mammogram for screening. GRAIL starts with the default, high contrast, wide dynamic range 12-bit data, and then maximizes the graphical information presented in ordinary 8-bit displays. Tests have been carried out with several mammogram databases. They comprise correlations and an ANOVA analysis with the manual IW levels established by a group of radiologists. A complete MATLAB implementation of GRAIL is available at . Results: Auto-leveled images show superior quality both perceptually and objectively compared to their full intensity range and compared to the application of other common methods like global contrast stretching (GCS). The correlations between the human determined intensity values and the ones estimated by our method surpass that of GCS. The ANOVA analysis with the upper intensity thresholds also reveals a similar outcome. GRAIL has also proven to specially perform better with images that contain micro-calcifications and/or foreign X-ray-opaque elements and with healthy BI-RADS A-type mammograms. It can also speed up the initial screening time by a mean of 4.5 s per image. Conclusions: A novel methodology is introduced that enables a quality-driven balancing of the WL/WW of mammographic images. This correction seeks the representation that maximizes the amount of graphical information contained in each image. The presented technique can contribute to the diagnosis and the overall efficiency of the breast screening session by suggesting, at the beginning, an optimal and customized windowing setting for each mammogram.
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Granero, D., Vijande, J., Ballester, F., & Rivard, M. J. (2011). Dosimetry revisited for the HDR Ir-192 brachytherapy source model mHDR-v2. Med. Phys., 38(1), 487–494.
Abstract: Purpose: Recently, the manufacturer of the HDR Ir-192 mHDR-v2 brachytherapy source reported small design changes (referred to herein as mHDR-v2r) that are within the manufacturing tolerances but may alter the existing dosimetric data for this source. This study aimed to (1) check whether these changes affect the existing dosimetric data published for this source; (2) obtain new dosimetric data in close proximity to the source, including the contributions from 192Ir electrons and considering the absence of electronic equilibrium; and (3) obtain scatter dose components for collapsed cone treatment planning system implementation. Methods: Three different Monte Carlo (MC) radiation transport codes were used: MCNP5, PENELOPE2008, and GEANT4. The source was centrally positioned in a 40 cm radius water phantom. Absorbed dose and collision kerma were obtained using 0.1 mm (0.5 mm) thick voxels to provide high-resolution dosimetry near (far from) the source. Dose-rate distributions obtained with the three MC codes were compared. Results: Simulations of mHDR-v2 and mHDR-v2r designs performed with three radiation transport codes showed agreement typically within 0.2% for r >= 0.25 cm. Dosimetric contributions from source electrons were significant for r<0.25 cm. The dose-rate constant and radial dose function were similar to those from previous MC studies of the mHDR-v2 design. The 2D anisotropy function also coincided with that of the mHDR-v2 design for r >= 0.25 cm. Detailed results of dose distributions and scatter components are presented for the modified source design. Conclusions: Comparison of these results to prior MC studies showed agreement typically within 0.5% for r >= 0.25 cm. If dosimetric data for r<0.25 cm are not needed, dosimetric results from the prior MC studies will be adequate. c 2011 American Association of Physicists in Medicine.
Keywords: Ir-192; brachytherapy; dosimetry; TG-43; PSS model; MCNP5; PENELOPE2008; GEANT4
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Candela-Juan, C., Niatsetski, Y., van der Laarse, R., Granero, D., Ballester, F., Perez-Calatayud, J., et al. (2016). Design and characterization of a new high-dose-rate brachytherapy Valencia applicator for larger skin lesions. Med. Phys., 43(4), 1639–1648.
Abstract: Purpose: The aims of this study were (i) to design a new high-dose-rate (HDR) brachytherapy applicator for treating surface lesions with planning target volumes larger than 3 cm in diameter and up to 5 cm in size, using the microSelectron-HDR or Flexitron afterloader (Elekta Brachytherapy) with a Ir-192 source; (ii) to calculate by means of the Monte Carlo (MC) method the dose distribution for the new applicator when it is placed against a water phantom; and (iii) to validate experimentally the dose distributions in water. Methods: The PENELOPE2008 MC code was used to optimize dwell positions and dwell times. Next, the dose distribution in a water phantom and the leakage dose distribution around the applicator were calculated. Finally, MC data were validated experimentally for a 192Ir mHDR-v2 source by measuring (i) dose distributions with radiochromic EBT3 films (ISP); (ii) percentage depth-dose (PDD) curve with the parallel-plate ionization chamber Advanced Markus (PTW); and (iii) absolute dose rate with EBT3 films and the PinPoint T31016 (PTW) ionization chamber. Results: The new applicator is made of tungsten alloy (Densimet) and consists of a set of interchangeable collimators. Three catheters are used to allocate the source at prefixed dwell positions with preset weights to produce a homogenous dose distribution at the typical prescription depth of 3 mm in water. The same plan is used for all available collimators. PDD, absolute dose rate per unit of air kerma strength, and off-axis profiles in a cylindrical water phantom are reported. These data can be used for treatment planning. Leakage around the applicator was also scored. The dose distributions, PDD, and absolute dose rate calculated agree within experimental uncertainties with the doses measured: differences of MC data with chamber measurements are up to 0.8% and with radiochromic films are up to 3.5%. Conclusions: The new applicator and the dosimetric data provided here will be a valuable tool in clinical practice, making treatment of large skin lesions simpler, faster, and safer. Also the dose to surrounding healthy tissues is minimal.
Keywords: skin applicator; Valencia applicator; HDR brachytherapy; dosimetry; Monte Carlo
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