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OC-0380: Tissue dentification by dual energy computed tomography for brachytherapy
S147
ESTRO 33, 2014
less than 2 % for all inserts inside 6 cm when the optimal atomic composition is homogeneous. However for the latter, fluctuations of Zeff and ρe in the image due to noise alter the agreement and lead to an increase of the RMSE. Conclusions: Tissue assignment is well performed by calculating the Mahalanobis distance between the measured and reference Zeff and ρe for the Gammex RMI 467 phantom. Tissue reduced to a three element model also shows good agreement when the distribution is homogeneous. However, noise alters the radial dose function. In summary, two medium assignment schemes to input data from DECT scans were verified in a MC treatment planning system for brachytherapy. Optimal tissue assignment is obtained with Mahalanobis distance. OC-0381 Effect of metal CT artifact with model-based dose calculation algorithms B. Reniers1, F. Verhaegen1 1 MAASTRO Clinic, Radiotherapy, Maastricht, The Netherlands Results: The combined standard uncertainty on the Dw value was 1.4%, lower than that traditionally obtained from RAKR measurements. From experimental measurements of Dw and RAKR, a value of 1.113·104 ± 1.8% was derived for the 192Ir dose rateconstant, Λ. This is in excellent agreement with the Λ values of 1.108·104 (recommended in the ESTRO database) and 1.109·104 calculated by Daskalov et al. (1998) and Taylor et al. (2008), respectively, and that measured by Sarfehnia et al. (2010), 1.092·104 ± 1.9% (see table).
Conclusions: The realization of an absorbed dose standard allows the absorbed dose to water to be used as the reference quantity for dosimetry in brachytherapy. In fact, the availability of a primary standard for the characterization of the HDR brachytherapy sources in terms of Dw allows the calibration of hospital instrumentation directly in terms of this quantity, improving the accuracy of clinical dosimetry in brachytherapy. OC-0380 Tissue dentification by dual energy computed tomography for brachytherapy F. Verhaegen1, M. Gaudreault1, G. Landry2, L. Beaulieu3 1 MAASTRO Clinic, Medical physics, Maastricht, The Netherlands 2 Ludwig-Maximilians-Universität München, Medical Physics, Munich, Germany 3 Département de Radio-oncologie et Centre de Recherche du CHU de Québec, Medical Physics, Québec, Canada
Purpose/Objective: Modern treatment planning systems (TPS) for brachytherapy are now available that are based on model-based dose calculation algorithms (MBDCA). They allow performing heterogeneity corrections for brachytherapy. In many case, imaging artifacts are present in CT images used for brachytherapy due to the presence of the applicator, dummies or shielding. We studied the effect of those artifacts on the dose calculations when MBDCA are used. Materials and Methods: A phantom was designed using a cylinder of stainless steel to simulate a shielding and stainless steel fletcher applicator. The metallic component were placed between 2 slabs of Super Stuff® bolus (Radiation Products Design) to minimize the amount of air. The phantom was scanned and the images were used in BrachyVision™ (BV) (Varian Medical Systems, Inc., Palo Alto, USA) for the calculation of the plans using ACUROS™. We compared the results of the calculation using the tissue assignment scheme from BV based on CT images and calculations where the materials were manually assigned to the different tissues to remove the effect of the artifacts (see figure). Gafchromic® EBT3 films were placed in the phantom to measure the dose distribution at different depths. They were analyzed using the triple channel procedure[1]. [1] Micke,Lewis, and Yu, Med. Phys. 38 (5), May 2011. Results:
Purpose/Objective: Dual Energy CT imaging (DECT) is a known technology in diagnostic radiology. The tissue-differentiating capabilities of DECT for radiotherapy have only recently been explored. DECT provides the distribution of the electron densities ρe and the atomic numbers Zeff of the geometry. This technology is thus expected to improve tissue segmentation, as recommended by the AAPM TG186 for brachytherapy. Materials and Methods: Tissue assignment is performed on DECT scans reconstructed using sinogram affirmed iterative reconstruction (SAFIRE, Siemens Healthcare, Forchheim, Germany) with strength 5 of the Gammex RMI 467 phantom. Two assignment schemes are compared. The first one determines atissue type based on the calculation of the Mahalanobis distance between the measured Zeff and ρe for each voxel. The assignment scheme is tested by calculating the radial dose function for an I-125 source obtained from Monte Carlo (MC) simulations. The performance of the method is assessed with respect to the root mean squared error (RMSE) calculated from the two radial dose functions. The other assignation scheme involves reduction to the atomic composition of each insert to a three elements model, (H,C, O) for inserts having Zeff < 8 and (H, C, Ca) otherwise. A relationship between the concentration of H and Zeff is obtained based on the optimal composition that leads to the minimal RMSE. This relation is used to assign an atomic composition to each voxel in the MC simulations. Results: The radial dose function calculated from the assignation scheme based on the Mahalanobis distance shows an excellent agreement with respect to the reference insert. The calculated RMSE is less than 1 % at distances up to 6 cm from the source for all inserts considered. The agreement is also good with the three elements model, with an RMSE
Figure A and B shows the comparison between the dose profiles beyond the stainless steel cylinder calculated using Acuros for the original Hounsfield Units and for the tissue assigned materials. The artifacts from the CT scanner produce amongst other effects black streaks due to photons starvation induced by high density regions. The air inside the stainless steel cylinder also disappears in the CTscan due to artifacts. The effect of artifacts on the dose is negligible below the cylinder (figure B). This is confirmed by the film dosimetry (not shown). That difference can however reach 40% above the cylinder (figure A). The analysis of the film that was placed above the cylinder (figure C and D)
ESTRO 33, 2014
less than 2 % for all inserts inside 6 cm when the optimal atomic composition is homogeneous. However for the latter, fluctuations of Zeff and ρe in the image due to noise alter the agreement and lead to an increase of the RMSE. Conclusions: Tissue assignment is well performed by calculating the Mahalanobis distance between the measured and reference Zeff and ρe for the Gammex RMI 467 phantom. Tissue reduced to a three element model also shows good agreement when the distribution is homogeneous. However, noise alters the radial dose function. In summary, two medium assignment schemes to input data from DECT scans were verified in a MC treatment planning system for brachytherapy. Optimal tissue assignment is obtained with Mahalanobis distance. OC-0381 Effect of metal CT artifact with model-based dose calculation algorithms B. Reniers1, F. Verhaegen1 1 MAASTRO Clinic, Radiotherapy, Maastricht, The Netherlands Results: The combined standard uncertainty on the Dw value was 1.4%, lower than that traditionally obtained from RAKR measurements. From experimental measurements of Dw and RAKR, a value of 1.113·104 ± 1.8% was derived for the 192Ir dose rateconstant, Λ. This is in excellent agreement with the Λ values of 1.108·104 (recommended in the ESTRO database) and 1.109·104 calculated by Daskalov et al. (1998) and Taylor et al. (2008), respectively, and that measured by Sarfehnia et al. (2010), 1.092·104 ± 1.9% (see table).
Conclusions: The realization of an absorbed dose standard allows the absorbed dose to water to be used as the reference quantity for dosimetry in brachytherapy. In fact, the availability of a primary standard for the characterization of the HDR brachytherapy sources in terms of Dw allows the calibration of hospital instrumentation directly in terms of this quantity, improving the accuracy of clinical dosimetry in brachytherapy. OC-0380 Tissue dentification by dual energy computed tomography for brachytherapy F. Verhaegen1, M. Gaudreault1, G. Landry2, L. Beaulieu3 1 MAASTRO Clinic, Medical physics, Maastricht, The Netherlands 2 Ludwig-Maximilians-Universität München, Medical Physics, Munich, Germany 3 Département de Radio-oncologie et Centre de Recherche du CHU de Québec, Medical Physics, Québec, Canada
Purpose/Objective: Modern treatment planning systems (TPS) for brachytherapy are now available that are based on model-based dose calculation algorithms (MBDCA). They allow performing heterogeneity corrections for brachytherapy. In many case, imaging artifacts are present in CT images used for brachytherapy due to the presence of the applicator, dummies or shielding. We studied the effect of those artifacts on the dose calculations when MBDCA are used. Materials and Methods: A phantom was designed using a cylinder of stainless steel to simulate a shielding and stainless steel fletcher applicator. The metallic component were placed between 2 slabs of Super Stuff® bolus (Radiation Products Design) to minimize the amount of air. The phantom was scanned and the images were used in BrachyVision™ (BV) (Varian Medical Systems, Inc., Palo Alto, USA) for the calculation of the plans using ACUROS™. We compared the results of the calculation using the tissue assignment scheme from BV based on CT images and calculations where the materials were manually assigned to the different tissues to remove the effect of the artifacts (see figure). Gafchromic® EBT3 films were placed in the phantom to measure the dose distribution at different depths. They were analyzed using the triple channel procedure[1]. [1] Micke,Lewis, and Yu, Med. Phys. 38 (5), May 2011. Results:
Purpose/Objective: Dual Energy CT imaging (DECT) is a known technology in diagnostic radiology. The tissue-differentiating capabilities of DECT for radiotherapy have only recently been explored. DECT provides the distribution of the electron densities ρe and the atomic numbers Zeff of the geometry. This technology is thus expected to improve tissue segmentation, as recommended by the AAPM TG186 for brachytherapy. Materials and Methods: Tissue assignment is performed on DECT scans reconstructed using sinogram affirmed iterative reconstruction (SAFIRE, Siemens Healthcare, Forchheim, Germany) with strength 5 of the Gammex RMI 467 phantom. Two assignment schemes are compared. The first one determines atissue type based on the calculation of the Mahalanobis distance between the measured Zeff and ρe for each voxel. The assignment scheme is tested by calculating the radial dose function for an I-125 source obtained from Monte Carlo (MC) simulations. The performance of the method is assessed with respect to the root mean squared error (RMSE) calculated from the two radial dose functions. The other assignation scheme involves reduction to the atomic composition of each insert to a three elements model, (H,C, O) for inserts having Zeff < 8 and (H, C, Ca) otherwise. A relationship between the concentration of H and Zeff is obtained based on the optimal composition that leads to the minimal RMSE. This relation is used to assign an atomic composition to each voxel in the MC simulations. Results: The radial dose function calculated from the assignation scheme based on the Mahalanobis distance shows an excellent agreement with respect to the reference insert. The calculated RMSE is less than 1 % at distances up to 6 cm from the source for all inserts considered. The agreement is also good with the three elements model, with an RMSE
Figure A and B shows the comparison between the dose profiles beyond the stainless steel cylinder calculated using Acuros for the original Hounsfield Units and for the tissue assigned materials. The artifacts from the CT scanner produce amongst other effects black streaks due to photons starvation induced by high density regions. The air inside the stainless steel cylinder also disappears in the CTscan due to artifacts. The effect of artifacts on the dose is negligible below the cylinder (figure B). This is confirmed by the film dosimetry (not shown). That difference can however reach 40% above the cylinder (figure A). The analysis of the film that was placed above the cylinder (figure C and D)