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191 oral Sequential versus simultaneous integrated boost IMRT for treatment of head and neck cancers
$74 Wednesday, 17 September 2003
189
oral
What is the optimum frequency of quality control tests in radiotherapy? A. McKenzie Bristol Oncology Centre, Medical Physics, Bristol, United Kingdom A multidisciplinary Working Group has been set up to see how to optimise the limited time available on linear accelerators by looking at the balance of risk in using different test schedules. The Working Group aims to offer guidance in producing the evidence that may form the basis of local initiatives to optimise the number and frequency of quality control tests with a view to maximising the time available for patient treatment. In order to do this, the Working Group will seek evidence to enable them to derive relations between test interval and the probability and magnitude of dosimetric or geometric error for different tests in different circumstances. These results will be used to determine a relation between a change in the intervals between quality control tests and the probability of a clinically significant radiation incident (ie, normal tissue complication, local recurrence or metastatic spread) involving single or multiple patients. This will in turn be used by the multidisciplinary team in developing quantitative guidance on optimising quality control schedules. Essentially, we are saying - if we do the tests less frequently, how likely is it that there will be a dose error, what is the clinical effect of that dose error, and can we accept that risk? 190
oral
Quality control of treatment planning systems (TPS) using the quality index (QI) methodology : accuracy of dose computation in presence of inhomogeneities with and without perturbation of electron transport J.C. Rosenwald 1, S. Zefkifi 1, M. Tsiakalos 1,2, S. Caneva 1, A. Bouzidi 1, K. Theodorou2, C. Kappas 2 l lnstitut Curie, Medical Physics, Paris, France 2University of Larissa, Medical Physics, Larissa, Greece Introduction: Checking the accuracy of TPS photon dose computation in presence of inhomogeneities is uneasy. It is generally done through benchmarking for various phantom configurations where it is necessary, either to measure the dose distribution for complex setup, or to perform a time consuming TPS parameterization in order to match the beam characteristics used for calculation with those used in published results. The QI methodology(1 ) relieves from such constraints. It consists in using published benchmark data where perturbation coefficients in presence of inhomogeneities were plotted as a function of beam QI and found to be independent of the other beam characteristics. Three different benchmark situations have been investigated and tested using several TPS. Methods and materials: The investigated situations were the following : 1. correction factor within and below a lung equivalent slab 2. build-up after an air gap 3. penumbra enlargement in lung equivalent material For all these situations, measurements were performed for a range of photon energy between 4 MV and 20 MV, with and without perturbation, using parallelepipedic polystyrene phantoms as reference. Ionization chamber at specific points were used for situations 1 and 2. Films were used for situation 3. A correction factor (CF) was defined for each situation and plotted as a function of QI for various field sizes. Some of the measurements were also complemented by Monte-Carlo simulations. The results were used to test the ability of several TPS (i.e. Cadplan, Isis, Pinnacle, Plato) to restitute the correct CF. Results: In spite of the use of different types of accelerator, a linear relationship was generally found between the CF and the QI of the beams. Such a relationship made the test of TPS calculations straightforward. Most of the tested systems gave acceptable results within and below the inhomogeneity but were unable to account for the electron transport perturbation. Conclusions: Provided that appropriate benchmark data are made available, the QI methodology provides an easy mean to test the accuracy of TPS dose computation in presence of inhomogeneities. As the sophistication of TPS algorithms evolves, the same tests are still valid and could be repeated to assess the improvements of dose computation methods. (I) S. Caneva, J.C. Rosenwald, S. Zefkili A method to check the accuracy of dose computation using quality index, Med Phys., 27, 1018-1024 (2000)
Proffered papers
IMRT: D E L I V E R Y T E C H N I Q U E S 191
oral
Sequential versus simultaneous integrated boost IMRT for treatment of head and neck cancers N. Dogan, S. King, B. Emami Loyola University Chicago Medical Center, Radiation Oncology Department, Maywood, IL, U.S.A. In conventional 3-D CRT, the different dose levels for each treatment site is delivered in several phases. The field sizes are reduced in stages to limit the dose to microscopic and sub-clinical disease in order to protect critical structures. This kind of fractionation approach requires the creation of different treatment plans for each phase of the treatment. These fractionation schemes used in 3-D CRT can also be used in IMRT. For example, the initial target volume may be treated with IMRT followed by a sequential IMFIT boost to the gross tumor volume. Alternatively, the treatment can be delivered using the simultaneous integrated boost(SIB-IMRT) fractionation scheme in which the doses for initial and boost fields are delivered in the same number of fractions. The purpose of this work was to assess the use of sequential- and SIB-IMRT techniques in terms of target coverage and normal tissue sparing. Ten patients with H&N cancer were selected for this study. The target volumes consisted of CTV1 and CTV2. The prescription doses to CTV1 and CTV2 were 46Gy and 66Gy respectively. The critical structures included spinal cord, parotid glands, and brainstem. For all patients, two IMRT plans were created: 1)IMRT to CTV1 followed by sequential IMRT boost to CTV2 and 2)Simultaneous Integrated IMRT boost to both CTV1 and CTV2 (SIB-IMRT). The plans were compared using the target volumes receiving the 95% of the prescription dose (D95%), maximum and mean structure doses (Dmax, Dmean). The average D95°/o for CTVl and CTV2 were 100% and 99% respectively for both techniques. The mean parotid doses were 24.4Gy and 22.5Gy using sequential- and SIB-IMRT respectively. The mean spinal cord dose was 20.6Gy and 21.4Gy using sequential- and SIB-IMRT. The mean brainstem dose was 124Gy and 14.7Gy using sequential- and SIB-IMRT techniques respectively. Both techniques provided adequate target coverage. Although both sequentialIMRT and SIB-IMRT provided the desired sparing of the spinal cord and parotids, the dose distributions obtained with SIB-IMRT were much more conformal. However, one must consider biological, medical, and sometimes logistic reasons in deciding whether to use SIB-IMRT versus sequentialIMRT delivery techniques. Compared to the sequential-IMRT, SIB-IMRT may be easier to use because the same plan is used for the entire course of treatment. 192
oral
Radiation treatment of the targets with respiratory motion using Breathing Synchronized Delivery (BSD) - A feasibility study T. Zhang 1, H. Keller 1, R. Jeraj 1, R. Mannon2, J. Welsh2, R. Patel2, M. Mehta2, R. Mackie 1.2, B. Paliwal 1,2 1Unversity of Wisconsin, Medical Physics, Madison, U.S.A. 2Unversity of Wisconsin, Radiation Oncology, Madison, U.S.A Purpose: To develop a new technique for IMRT/tomotherapy lung cancer treatment. Material and Methods: Patient breathing is synchronized with treatment delivery. Target motion is studied using a 3D deformable model and is included in the treatment planning optimization. The procedure of breathing synchronized delivery (BSD) can be summarized by the following steps: 1. Treatment planning CT scans and guiding cycle acquisition. 2. 3D lung motion study using finite element method (FEM). 3. Generation of 4D CT series using FEM result. 4. Pencil beam calculation and dose mapping. 5. Optimization of treatment including motion. 6. Patient coaching and treatment delivery. Results: The lung surfaces from two breathing phases were reconstructed from CT scans and a finite element model was created. The FEM model gave a displacement vector field which reflects the displacement of every pixel inside the lung. A lung image sequence was created using the FEM results and correlated to the spirometer signal by calculating lung volume changes. A special laser/spirometer combined respiratory motion tracking system has been developed to provide a reproducible drift-free lung volume breathing signal. To simulate the BSD tomotherapy situation, a water phantom with a "W" shape target inside was created and deformed according to a given displacement field. Its deformation was synchronized with gantry rotation. Pencil beams were calculated and mapped into deformed pencil
189
oral
What is the optimum frequency of quality control tests in radiotherapy? A. McKenzie Bristol Oncology Centre, Medical Physics, Bristol, United Kingdom A multidisciplinary Working Group has been set up to see how to optimise the limited time available on linear accelerators by looking at the balance of risk in using different test schedules. The Working Group aims to offer guidance in producing the evidence that may form the basis of local initiatives to optimise the number and frequency of quality control tests with a view to maximising the time available for patient treatment. In order to do this, the Working Group will seek evidence to enable them to derive relations between test interval and the probability and magnitude of dosimetric or geometric error for different tests in different circumstances. These results will be used to determine a relation between a change in the intervals between quality control tests and the probability of a clinically significant radiation incident (ie, normal tissue complication, local recurrence or metastatic spread) involving single or multiple patients. This will in turn be used by the multidisciplinary team in developing quantitative guidance on optimising quality control schedules. Essentially, we are saying - if we do the tests less frequently, how likely is it that there will be a dose error, what is the clinical effect of that dose error, and can we accept that risk? 190
oral
Quality control of treatment planning systems (TPS) using the quality index (QI) methodology : accuracy of dose computation in presence of inhomogeneities with and without perturbation of electron transport J.C. Rosenwald 1, S. Zefkifi 1, M. Tsiakalos 1,2, S. Caneva 1, A. Bouzidi 1, K. Theodorou2, C. Kappas 2 l lnstitut Curie, Medical Physics, Paris, France 2University of Larissa, Medical Physics, Larissa, Greece Introduction: Checking the accuracy of TPS photon dose computation in presence of inhomogeneities is uneasy. It is generally done through benchmarking for various phantom configurations where it is necessary, either to measure the dose distribution for complex setup, or to perform a time consuming TPS parameterization in order to match the beam characteristics used for calculation with those used in published results. The QI methodology(1 ) relieves from such constraints. It consists in using published benchmark data where perturbation coefficients in presence of inhomogeneities were plotted as a function of beam QI and found to be independent of the other beam characteristics. Three different benchmark situations have been investigated and tested using several TPS. Methods and materials: The investigated situations were the following : 1. correction factor within and below a lung equivalent slab 2. build-up after an air gap 3. penumbra enlargement in lung equivalent material For all these situations, measurements were performed for a range of photon energy between 4 MV and 20 MV, with and without perturbation, using parallelepipedic polystyrene phantoms as reference. Ionization chamber at specific points were used for situations 1 and 2. Films were used for situation 3. A correction factor (CF) was defined for each situation and plotted as a function of QI for various field sizes. Some of the measurements were also complemented by Monte-Carlo simulations. The results were used to test the ability of several TPS (i.e. Cadplan, Isis, Pinnacle, Plato) to restitute the correct CF. Results: In spite of the use of different types of accelerator, a linear relationship was generally found between the CF and the QI of the beams. Such a relationship made the test of TPS calculations straightforward. Most of the tested systems gave acceptable results within and below the inhomogeneity but were unable to account for the electron transport perturbation. Conclusions: Provided that appropriate benchmark data are made available, the QI methodology provides an easy mean to test the accuracy of TPS dose computation in presence of inhomogeneities. As the sophistication of TPS algorithms evolves, the same tests are still valid and could be repeated to assess the improvements of dose computation methods. (I) S. Caneva, J.C. Rosenwald, S. Zefkili A method to check the accuracy of dose computation using quality index, Med Phys., 27, 1018-1024 (2000)
Proffered papers
IMRT: D E L I V E R Y T E C H N I Q U E S 191
oral
Sequential versus simultaneous integrated boost IMRT for treatment of head and neck cancers N. Dogan, S. King, B. Emami Loyola University Chicago Medical Center, Radiation Oncology Department, Maywood, IL, U.S.A. In conventional 3-D CRT, the different dose levels for each treatment site is delivered in several phases. The field sizes are reduced in stages to limit the dose to microscopic and sub-clinical disease in order to protect critical structures. This kind of fractionation approach requires the creation of different treatment plans for each phase of the treatment. These fractionation schemes used in 3-D CRT can also be used in IMRT. For example, the initial target volume may be treated with IMRT followed by a sequential IMFIT boost to the gross tumor volume. Alternatively, the treatment can be delivered using the simultaneous integrated boost(SIB-IMRT) fractionation scheme in which the doses for initial and boost fields are delivered in the same number of fractions. The purpose of this work was to assess the use of sequential- and SIB-IMRT techniques in terms of target coverage and normal tissue sparing. Ten patients with H&N cancer were selected for this study. The target volumes consisted of CTV1 and CTV2. The prescription doses to CTV1 and CTV2 were 46Gy and 66Gy respectively. The critical structures included spinal cord, parotid glands, and brainstem. For all patients, two IMRT plans were created: 1)IMRT to CTV1 followed by sequential IMRT boost to CTV2 and 2)Simultaneous Integrated IMRT boost to both CTV1 and CTV2 (SIB-IMRT). The plans were compared using the target volumes receiving the 95% of the prescription dose (D95%), maximum and mean structure doses (Dmax, Dmean). The average D95°/o for CTVl and CTV2 were 100% and 99% respectively for both techniques. The mean parotid doses were 24.4Gy and 22.5Gy using sequential- and SIB-IMRT respectively. The mean spinal cord dose was 20.6Gy and 21.4Gy using sequential- and SIB-IMRT. The mean brainstem dose was 124Gy and 14.7Gy using sequential- and SIB-IMRT techniques respectively. Both techniques provided adequate target coverage. Although both sequentialIMRT and SIB-IMRT provided the desired sparing of the spinal cord and parotids, the dose distributions obtained with SIB-IMRT were much more conformal. However, one must consider biological, medical, and sometimes logistic reasons in deciding whether to use SIB-IMRT versus sequentialIMRT delivery techniques. Compared to the sequential-IMRT, SIB-IMRT may be easier to use because the same plan is used for the entire course of treatment. 192
oral
Radiation treatment of the targets with respiratory motion using Breathing Synchronized Delivery (BSD) - A feasibility study T. Zhang 1, H. Keller 1, R. Jeraj 1, R. Mannon2, J. Welsh2, R. Patel2, M. Mehta2, R. Mackie 1.2, B. Paliwal 1,2 1Unversity of Wisconsin, Medical Physics, Madison, U.S.A. 2Unversity of Wisconsin, Radiation Oncology, Madison, U.S.A Purpose: To develop a new technique for IMRT/tomotherapy lung cancer treatment. Material and Methods: Patient breathing is synchronized with treatment delivery. Target motion is studied using a 3D deformable model and is included in the treatment planning optimization. The procedure of breathing synchronized delivery (BSD) can be summarized by the following steps: 1. Treatment planning CT scans and guiding cycle acquisition. 2. 3D lung motion study using finite element method (FEM). 3. Generation of 4D CT series using FEM result. 4. Pencil beam calculation and dose mapping. 5. Optimization of treatment including motion. 6. Patient coaching and treatment delivery. Results: The lung surfaces from two breathing phases were reconstructed from CT scans and a finite element model was created. The FEM model gave a displacement vector field which reflects the displacement of every pixel inside the lung. A lung image sequence was created using the FEM results and correlated to the spirometer signal by calculating lung volume changes. A special laser/spirometer combined respiratory motion tracking system has been developed to provide a reproducible drift-free lung volume breathing signal. To simulate the BSD tomotherapy situation, a water phantom with a "W" shape target inside was created and deformed according to a given displacement field. Its deformation was synchronized with gantry rotation. Pencil beams were calculated and mapped into deformed pencil