- Email: [email protected]
197 oral Modelling the beam characteristics using GEANT4 for an integrated MRI system and radiotherapy accelerator
Proffered papers
backscattering material, was also used to compare the measured values.
Results: Our results present agreement with previous studies for on axis measurements. No references for off axis measurements have been reported. The correction near the exit surface was found larger for lower energies, larger fields and larger depths. For example, for the energy of 6MV, differences between doses at l c m from the exit surface and at the same point in an infinite phantom can be up to 5% but at the exit surface where electronic equilibrium is not established it can reach 10%. Conclusion: According to our results, a correction factor must be applied to correct the basic data measured under full backscatter conditions in order to obtain the actual dose at the exit surface, especially when low energies and large fields are used. This correction factor is important for comparison with invivo dosimetry measurements. Furthermore, if part of the tumor is near the exit surface, a bolus with appropriate thickness can be used to restore the build-down effect and provide sufficient dose to the tumor. 197 oral Modelling the beam characteristics using GEANT4 for an integrated MRI system and radiotherapy accelerator B.W. Raaymakers, A.J.E. Raaijmakers, A.N.T.J. Kotte, J.J.W. Lagendijk UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Within a collaboration between the UMC Utrecht, Philips Research, Hamburg and Elekta, we are investigating the integration of a MRI scanner with a radiotherapy accelerator. This combination aims for on-line, soft-tissue based position verification and treatment monitoring. The study into the technical feasibility of a MRI accelerator led to a preliminary design in which the accelerator is positioned in a ring around the slightly modified MRI system, thus irradiating the patient from outside the MRI system. This design has several consequences for the beam characteristics, which can be simulated with GEANT4. GEANT4 is a freely available Monte Carlo toolkit to simulate the passage of particles through matter. It offers flexible geometry handling and the impact of a magnetic field can be included. GEANT4 was originally developed for high energy purposes but currently there is a low energy extension available, suitable for medical physics applications. The impact of the magnetic field on the dose deposition has been simulated. An increase of the penumbra of approximately 1 mm was found, together with a decrease of the build-up distance of 5 mm. Furthermore fields shifted for 0.7 mm in the direction perpendicular to the magnetic field. Another topic studied is the scatter induction by the introduction of mass in the beam. This will induce secondary electrons, decreasing the build-up in the patient. The impact on the build-up is basically determined by the distance between the scatter mass and the patient together with the field size. However in the case of a MRI accelerator, the permanent 1.5 T transverse magnetic field poses a opposing effect: the magnetic field will bend the trajectories of the secondary electrons, which will decrease the electron contamination of the beam at the surface of the patient and thus restore the original, unperturbed build-up. GEANT4 simulations will be used to investigate the scatter induction and resulting impact on the build-up with and without the presence of a transverse magnetic field. The scatter induction of a pencil beam passing through a homogeneous slab of mass will be simulated. A convolution method will be applied to determine the scatter induction of various field sizes. The scatter induction will be scored for various distances to the slab in a homogeneous water phantom and the impact on the build-up will be determined.
Tuesday, October 26, 2004 S89
IMRT (joint) 198 oral
The importance of the low-dose part (D smaller than 95%) of the PTV-DVH in head and neck IMRT
W.J.M. de Kruiif, B.J.M. Heijmen, P. Voet, H. van der Est, P.C. Levendag Erasmus MC - Daniel den Hoed Cancer Center, Department of Radiation Oncology, Rotterdam, The Netherlands In IMRT studies the minimum PTV dose is often lower than 95%, which is in conflict with the ICRU recommendation. Instead, the optimization criterium might be that 95% (instead of 100%) of the PTV-volume has to receive 95% of the dose. In head and neck tumours the sparing of the parotids strongly depends on the local dose distribution in the PTV. The low-dose part of the PTV-DVH strongly influences the amount of sparing that can be obtained. For a set of patients with tonsillar fossa and/or soft palata tumours the CTV of the node-negative neck is determined according to the Rotterdam/Brussels consensus protocol [1]. The amount of sparing is quantified against the low-dose part of the PTV-DVH. The volume receiving a dose less than or equal to a given dose is plotted as a function of the dose, resulting in a kind of inverse DVH. When plotted on a logarithmic scale, it gives a g o o d insight into the low-dose PTVDVH part. A set of straight lines bounding these curves is proposed to prescribe the low-dose part of the PTV-DVH. It is shown that this provides a practical, and much better prescription of the PTV dose distribution than just prescribing a bounding point on the DVH-curve. It would enable comparison between results reported in the literature, which are often obscured by the missing information on the low- and high-dose (D>107%) parts of the PTV-DVH, for which differences are indistinguishable on a linear scale. A radiobiological model has been made which is based on a uniform distribution of the logarithm of the number of tumour cells in patients. This results in a linear dose-response relationship for subclinical disease, as reported in the literature [2]. The model shows that the low-dose part of the PTV-DVH has a small effect on the control probability of the subclinical disease, whereas significant parotid sparing can be achieved by allowing heterogeneous irradiation of the PTV. 1. Gregoire, V., Levendag, P., Ang, K.K., Bernier, J., Braaksma, M., Budach, V , Chao,C., Coche, E., Cooper, J.S., and Cosnard, G., "CT-based delineation of lymph node levels and related CTVs in the node-negative neck: DAHANCA, EORTC, GORTEC, NCIC, RTOG consensus guidelines", Radiotherapy and Oncology, 69, 227-236 (2003). 2. Withers, H.R., Peters, L.J., and Taylor, J.M.G., "Doseresponse relationship for radiation therapy of subclinical disease", International Journal of Radiation Oncology* Biology* Physics, 31,353-359 (1995). 199 oral Acute toxicity of IMRT for head and neck cancer: first clinical experience C. Weltens, B. Bussels, S. Nuyts, B. Vanstraelen, A. Nulens, W. Van den Bogaert UH Gasthuisberg, Radiotherapy, Leuven, Belgium Purpose: To prospectively evaluate the acute toxicity of intensity modulated radiotherapy (IMRT) for head and neck cancer in two patients groups: patients with paranasal sinus tumors and patients with oral cavitY/oropharyngeal tumors. Materials and Methods: Since January 2001, 18 patients were treated with IMRT of the head and neck region. Nine patients were treated after functional endoscopic sinus surgery (debulking) for a paranasal sinus tumor (group 1), 9 patients
backscattering material, was also used to compare the measured values.
Results: Our results present agreement with previous studies for on axis measurements. No references for off axis measurements have been reported. The correction near the exit surface was found larger for lower energies, larger fields and larger depths. For example, for the energy of 6MV, differences between doses at l c m from the exit surface and at the same point in an infinite phantom can be up to 5% but at the exit surface where electronic equilibrium is not established it can reach 10%. Conclusion: According to our results, a correction factor must be applied to correct the basic data measured under full backscatter conditions in order to obtain the actual dose at the exit surface, especially when low energies and large fields are used. This correction factor is important for comparison with invivo dosimetry measurements. Furthermore, if part of the tumor is near the exit surface, a bolus with appropriate thickness can be used to restore the build-down effect and provide sufficient dose to the tumor. 197 oral Modelling the beam characteristics using GEANT4 for an integrated MRI system and radiotherapy accelerator B.W. Raaymakers, A.J.E. Raaijmakers, A.N.T.J. Kotte, J.J.W. Lagendijk UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Within a collaboration between the UMC Utrecht, Philips Research, Hamburg and Elekta, we are investigating the integration of a MRI scanner with a radiotherapy accelerator. This combination aims for on-line, soft-tissue based position verification and treatment monitoring. The study into the technical feasibility of a MRI accelerator led to a preliminary design in which the accelerator is positioned in a ring around the slightly modified MRI system, thus irradiating the patient from outside the MRI system. This design has several consequences for the beam characteristics, which can be simulated with GEANT4. GEANT4 is a freely available Monte Carlo toolkit to simulate the passage of particles through matter. It offers flexible geometry handling and the impact of a magnetic field can be included. GEANT4 was originally developed for high energy purposes but currently there is a low energy extension available, suitable for medical physics applications. The impact of the magnetic field on the dose deposition has been simulated. An increase of the penumbra of approximately 1 mm was found, together with a decrease of the build-up distance of 5 mm. Furthermore fields shifted for 0.7 mm in the direction perpendicular to the magnetic field. Another topic studied is the scatter induction by the introduction of mass in the beam. This will induce secondary electrons, decreasing the build-up in the patient. The impact on the build-up is basically determined by the distance between the scatter mass and the patient together with the field size. However in the case of a MRI accelerator, the permanent 1.5 T transverse magnetic field poses a opposing effect: the magnetic field will bend the trajectories of the secondary electrons, which will decrease the electron contamination of the beam at the surface of the patient and thus restore the original, unperturbed build-up. GEANT4 simulations will be used to investigate the scatter induction and resulting impact on the build-up with and without the presence of a transverse magnetic field. The scatter induction of a pencil beam passing through a homogeneous slab of mass will be simulated. A convolution method will be applied to determine the scatter induction of various field sizes. The scatter induction will be scored for various distances to the slab in a homogeneous water phantom and the impact on the build-up will be determined.
Tuesday, October 26, 2004 S89
IMRT (joint) 198 oral
The importance of the low-dose part (D smaller than 95%) of the PTV-DVH in head and neck IMRT
W.J.M. de Kruiif, B.J.M. Heijmen, P. Voet, H. van der Est, P.C. Levendag Erasmus MC - Daniel den Hoed Cancer Center, Department of Radiation Oncology, Rotterdam, The Netherlands In IMRT studies the minimum PTV dose is often lower than 95%, which is in conflict with the ICRU recommendation. Instead, the optimization criterium might be that 95% (instead of 100%) of the PTV-volume has to receive 95% of the dose. In head and neck tumours the sparing of the parotids strongly depends on the local dose distribution in the PTV. The low-dose part of the PTV-DVH strongly influences the amount of sparing that can be obtained. For a set of patients with tonsillar fossa and/or soft palata tumours the CTV of the node-negative neck is determined according to the Rotterdam/Brussels consensus protocol [1]. The amount of sparing is quantified against the low-dose part of the PTV-DVH. The volume receiving a dose less than or equal to a given dose is plotted as a function of the dose, resulting in a kind of inverse DVH. When plotted on a logarithmic scale, it gives a g o o d insight into the low-dose PTVDVH part. A set of straight lines bounding these curves is proposed to prescribe the low-dose part of the PTV-DVH. It is shown that this provides a practical, and much better prescription of the PTV dose distribution than just prescribing a bounding point on the DVH-curve. It would enable comparison between results reported in the literature, which are often obscured by the missing information on the low- and high-dose (D>107%) parts of the PTV-DVH, for which differences are indistinguishable on a linear scale. A radiobiological model has been made which is based on a uniform distribution of the logarithm of the number of tumour cells in patients. This results in a linear dose-response relationship for subclinical disease, as reported in the literature [2]. The model shows that the low-dose part of the PTV-DVH has a small effect on the control probability of the subclinical disease, whereas significant parotid sparing can be achieved by allowing heterogeneous irradiation of the PTV. 1. Gregoire, V., Levendag, P., Ang, K.K., Bernier, J., Braaksma, M., Budach, V , Chao,C., Coche, E., Cooper, J.S., and Cosnard, G., "CT-based delineation of lymph node levels and related CTVs in the node-negative neck: DAHANCA, EORTC, GORTEC, NCIC, RTOG consensus guidelines", Radiotherapy and Oncology, 69, 227-236 (2003). 2. Withers, H.R., Peters, L.J., and Taylor, J.M.G., "Doseresponse relationship for radiation therapy of subclinical disease", International Journal of Radiation Oncology* Biology* Physics, 31,353-359 (1995). 199 oral Acute toxicity of IMRT for head and neck cancer: first clinical experience C. Weltens, B. Bussels, S. Nuyts, B. Vanstraelen, A. Nulens, W. Van den Bogaert UH Gasthuisberg, Radiotherapy, Leuven, Belgium Purpose: To prospectively evaluate the acute toxicity of intensity modulated radiotherapy (IMRT) for head and neck cancer in two patients groups: patients with paranasal sinus tumors and patients with oral cavitY/oropharyngeal tumors. Materials and Methods: Since January 2001, 18 patients were treated with IMRT of the head and neck region. Nine patients were treated after functional endoscopic sinus surgery (debulking) for a paranasal sinus tumor (group 1), 9 patients