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Theoretical modelling of γ-H2AX foci kinetics in human lymphocytes after exposure to fast neutrons
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Abstracts/Physica Medica 31 (2015) S1–S14
of IMRT. Conformity indexes ranged from 0.98 to 1.21 for VMAT and from 0.99 to 1.23 for IMRT. Homogeneity indexes ranged from 5.6% to 11.9% for VMAT and from 4.9% to 10.8% for IMRT. Conclusion: AAPM 119 planning strategies for establishment of VMAT compared well with IMRT. Keywords: VMAT, IMRT, AAPM119 O.12 DETERMINATION OF THE INFLUENCE OF RESPIRATORY MOTION ON THE SUV ACCURACY IN PULMONARY LESIONS P.D. du Toit *,a, H. du Raan b, W.I.D. Rae b. a Private Practice, Bloemfontein, South Africa; b Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction and Aim: Respiratory motion results in reduction of qualitative and quantitative accuracy in PET imaging of the heart, muscles of the diaphragm, liver, spleen, pancreas and imaging of the lung area. It causes blurring of the images and thus a loss of sensitivity in lesion detection. The aim was to determine the importance of respiratory motion on SUV accuracy in pulmonary lesions through simulation of respiratory motion. Materials and Methods: The XCAT Phantom was used to assess the influence of breathing on SUV quantification in a human-like simulation model. Six spherical lesions were introduced into the lungs of the model, three in the left lung and three in the right lung. Activity concentrations assigned to the lesions were varied to create a range of different lesions: background ratios, thus creating a range of contrasts. The simulations were repeated for various lesion sizes. GATE software was used to perform Monte Carlo simulations during breathing. True-, scattered-, random- and total coincidences were extracted from the simulated data and reconstructed using OPL-EM and corrected for attenuation. Results: The % trues-to-total coincidences generally followed a downward trend with a decrease in contrast. Scattered coincidences had a more significant contribution at low contrast and when the lesion was closer to the diaphragm. This was greater in 3D simulations than in 2D simulations. Random coincidences were relatively greater in the apical region than in the other lung regions, particularly at low contrast large lesions. This increased for 3D compared to 2D acquisitions. The fraction of true coincidences-to-total coincidences increased when the lesions became smaller, but the relative amount of scattered events also increased. Conclusion: Breathing plays a significant role when determining the SUV of pulmonary lesions after accounting for the size, location, and the amount of activity within the simulated lesions compared to the background activity. Keywords: PET, SUV, XCAT, Respiratory motion, True coincidences, Scattered coincidences, Random coincidences O.13 EVALUATING THE EFFECT OF REDUCED ENTRANCE SURFACE DOSE ON NEONATAL CHEST IMAGING USING SUBJECTIVE IMAGE QUALITY EVALUATION A. Erasmus *, C.P. Herbst. Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction: Hospitalised neonates often require numerous x-rays. Neonates are very radiosensitive and therefore each institution should investigate the optimal beam parameter combinations in order to deliver images of diagnostic acceptable quality at the lowest possible entrance surface dose (ESD). The aim of this study was to evaluate the effect of different beam parameter combinations on image quality using a subjective approach. Materials and Methods: Five sedated rabbits were used to simulate the neonatal chest. The ESD was reduced using a variation of voltage, tube current and aluminium filtration. Digital x-ray images of the phantom were obtained. Eight radiology registrars, blinded to the dose parameter information, ranked the images from best to worst using a variation of the multiple rank-order-method. T-tests were used to compare the average values for each anatomical area. These average values were obtained from the scores that each of the observers assigned to the images acquired at different ESD. The intraclass correlation coefficient (ICC) was calculated to assess observer agreement.
Results: Large variation amongst the observers was noticed and confirmed by an ICC value of less than 0.5. It could not be concluded with a 95% confidence that different ESD values, resulting in a 50%–77% dose reduction compared to current practice, would result in different overall image quality. This was observed for all three anatomical areas and indicated that no preference existed towards an image acquired with a specific beam parameter combination. Conclusion: The large variation in observers’ opinion of diagnostically acceptable image quality emphasizes the importance of subjective image quality assessment in the clinical environment. The multiple rank order method and the rabbit phantom were considered appropriate for subjective image quality evaluation. The results showed that a dose reduction of 64% can be achieved by altering the beam parameters. Keywords: Neonatal, Chest x-rays, Dose, Rabbit phantom, Image quality O.14 THEORETICAL MODELLING OF γ-H2AX FOCI KINETICS IN HUMAN LYMPHOCYTES AFTER EXPOSURE TO FAST NEUTRONS H. Fourie *,a, O.V. Belov b, M.S. Panina b, J.P. Slabbert a. a Department of Radiation Biophysics, iThemba LABS-NRF, Somerset West, South Africa; b Department of Radiobiology, JINR, Dubna, Russia Introduction: We present a theoretical model which describes the repair pathways of DNA double-strand breaks (DSBs) in mammalian and human cells, which has been validated for ionizing radiations with a wide range of LETs (0.2–236 keV/μm). It considers the homologous recombination, nonhomologous end-joining (NHEJ), single-strand annealing, microhomologymediated end-joining and alternative-NHEJ repair pathways. This model provides the proposed mathematical descriptions for each pathway which satisfies the kinetics of DSB rejoining for the considered types of exposure, and reproduces the time-courses of radiation-induced foci. The validity of the model is assessed by comparing the γ-H2AX foci kinetics predicted by the model to experimental curves observed for 0.5 Gy neutrons (20 keV/ μm) and 60Co γ-ray exposures. Materials and Methods: The model consists of a large set of dependent differential equations which simultaneously describes the above mentioned time-based repair pathways. The fraction of non-repairable DSBs, the fraction of clustered DSBs, the dose and the LET of the particle being investigated are used as input parameters for the model. Results: The maximum number of foci per cell, predicted by the model, should occur 39 min and 43 min after γ- or neutron exposures, which fall within the experimentally observed time frame. The neutron induced foci disappeared more slowly than those induced by γ-rays, reaching approximately 22% of the maximum number for neutrons and 8% for γ-rays after 24 hours. This indicates that a larger number of DSBs remain unrepaired or are mis-repaired after neutron irradiation than after low-LET γ-irradiation. The same trend was observed experimentally, the corresponding values being 25% and 12% respectively. Conclusion: Despite the deviations from experimental observations, the quantitative model has been shown to accurately describe and predict the kinetics of γ-H2AX foci in human lymphocytes after exposure to fast neutrons and 60Co γ-rays. Keywords: DNA DSB repair, Fast neutrons, Theoretical radiobiology O.15 DERIVING A TIME DELIVERY MODEL FOR A SIEMENS ARTISTE LINAC N.S. Fourie *,a, O.A. Ali a,b, W.I.D. Rae b. a Department of Medical Physics, Equra Health, Cape Town, South Africa; b Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction: With IMRT delivery time is significantly longer than with 3DCRT; therefore accurate dose delivery becomes difficult due to intrafractional patient movement. To our knowledge there is as yet no time delivery model proposed for a Siemens Artiste Linear Accelerator (Linac). Our aim is to propose such a model. Materials and Methods: Using the principles of the time delivery model created by Li and Xing for a Varian Linac, a formula was proposed that determines time delivery on a Siemens Artiste Linac. We added the Radio
Abstracts/Physica Medica 31 (2015) S1–S14
of IMRT. Conformity indexes ranged from 0.98 to 1.21 for VMAT and from 0.99 to 1.23 for IMRT. Homogeneity indexes ranged from 5.6% to 11.9% for VMAT and from 4.9% to 10.8% for IMRT. Conclusion: AAPM 119 planning strategies for establishment of VMAT compared well with IMRT. Keywords: VMAT, IMRT, AAPM119 O.12 DETERMINATION OF THE INFLUENCE OF RESPIRATORY MOTION ON THE SUV ACCURACY IN PULMONARY LESIONS P.D. du Toit *,a, H. du Raan b, W.I.D. Rae b. a Private Practice, Bloemfontein, South Africa; b Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction and Aim: Respiratory motion results in reduction of qualitative and quantitative accuracy in PET imaging of the heart, muscles of the diaphragm, liver, spleen, pancreas and imaging of the lung area. It causes blurring of the images and thus a loss of sensitivity in lesion detection. The aim was to determine the importance of respiratory motion on SUV accuracy in pulmonary lesions through simulation of respiratory motion. Materials and Methods: The XCAT Phantom was used to assess the influence of breathing on SUV quantification in a human-like simulation model. Six spherical lesions were introduced into the lungs of the model, three in the left lung and three in the right lung. Activity concentrations assigned to the lesions were varied to create a range of different lesions: background ratios, thus creating a range of contrasts. The simulations were repeated for various lesion sizes. GATE software was used to perform Monte Carlo simulations during breathing. True-, scattered-, random- and total coincidences were extracted from the simulated data and reconstructed using OPL-EM and corrected for attenuation. Results: The % trues-to-total coincidences generally followed a downward trend with a decrease in contrast. Scattered coincidences had a more significant contribution at low contrast and when the lesion was closer to the diaphragm. This was greater in 3D simulations than in 2D simulations. Random coincidences were relatively greater in the apical region than in the other lung regions, particularly at low contrast large lesions. This increased for 3D compared to 2D acquisitions. The fraction of true coincidences-to-total coincidences increased when the lesions became smaller, but the relative amount of scattered events also increased. Conclusion: Breathing plays a significant role when determining the SUV of pulmonary lesions after accounting for the size, location, and the amount of activity within the simulated lesions compared to the background activity. Keywords: PET, SUV, XCAT, Respiratory motion, True coincidences, Scattered coincidences, Random coincidences O.13 EVALUATING THE EFFECT OF REDUCED ENTRANCE SURFACE DOSE ON NEONATAL CHEST IMAGING USING SUBJECTIVE IMAGE QUALITY EVALUATION A. Erasmus *, C.P. Herbst. Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction: Hospitalised neonates often require numerous x-rays. Neonates are very radiosensitive and therefore each institution should investigate the optimal beam parameter combinations in order to deliver images of diagnostic acceptable quality at the lowest possible entrance surface dose (ESD). The aim of this study was to evaluate the effect of different beam parameter combinations on image quality using a subjective approach. Materials and Methods: Five sedated rabbits were used to simulate the neonatal chest. The ESD was reduced using a variation of voltage, tube current and aluminium filtration. Digital x-ray images of the phantom were obtained. Eight radiology registrars, blinded to the dose parameter information, ranked the images from best to worst using a variation of the multiple rank-order-method. T-tests were used to compare the average values for each anatomical area. These average values were obtained from the scores that each of the observers assigned to the images acquired at different ESD. The intraclass correlation coefficient (ICC) was calculated to assess observer agreement.
Results: Large variation amongst the observers was noticed and confirmed by an ICC value of less than 0.5. It could not be concluded with a 95% confidence that different ESD values, resulting in a 50%–77% dose reduction compared to current practice, would result in different overall image quality. This was observed for all three anatomical areas and indicated that no preference existed towards an image acquired with a specific beam parameter combination. Conclusion: The large variation in observers’ opinion of diagnostically acceptable image quality emphasizes the importance of subjective image quality assessment in the clinical environment. The multiple rank order method and the rabbit phantom were considered appropriate for subjective image quality evaluation. The results showed that a dose reduction of 64% can be achieved by altering the beam parameters. Keywords: Neonatal, Chest x-rays, Dose, Rabbit phantom, Image quality O.14 THEORETICAL MODELLING OF γ-H2AX FOCI KINETICS IN HUMAN LYMPHOCYTES AFTER EXPOSURE TO FAST NEUTRONS H. Fourie *,a, O.V. Belov b, M.S. Panina b, J.P. Slabbert a. a Department of Radiation Biophysics, iThemba LABS-NRF, Somerset West, South Africa; b Department of Radiobiology, JINR, Dubna, Russia Introduction: We present a theoretical model which describes the repair pathways of DNA double-strand breaks (DSBs) in mammalian and human cells, which has been validated for ionizing radiations with a wide range of LETs (0.2–236 keV/μm). It considers the homologous recombination, nonhomologous end-joining (NHEJ), single-strand annealing, microhomologymediated end-joining and alternative-NHEJ repair pathways. This model provides the proposed mathematical descriptions for each pathway which satisfies the kinetics of DSB rejoining for the considered types of exposure, and reproduces the time-courses of radiation-induced foci. The validity of the model is assessed by comparing the γ-H2AX foci kinetics predicted by the model to experimental curves observed for 0.5 Gy neutrons (20 keV/ μm) and 60Co γ-ray exposures. Materials and Methods: The model consists of a large set of dependent differential equations which simultaneously describes the above mentioned time-based repair pathways. The fraction of non-repairable DSBs, the fraction of clustered DSBs, the dose and the LET of the particle being investigated are used as input parameters for the model. Results: The maximum number of foci per cell, predicted by the model, should occur 39 min and 43 min after γ- or neutron exposures, which fall within the experimentally observed time frame. The neutron induced foci disappeared more slowly than those induced by γ-rays, reaching approximately 22% of the maximum number for neutrons and 8% for γ-rays after 24 hours. This indicates that a larger number of DSBs remain unrepaired or are mis-repaired after neutron irradiation than after low-LET γ-irradiation. The same trend was observed experimentally, the corresponding values being 25% and 12% respectively. Conclusion: Despite the deviations from experimental observations, the quantitative model has been shown to accurately describe and predict the kinetics of γ-H2AX foci in human lymphocytes after exposure to fast neutrons and 60Co γ-rays. Keywords: DNA DSB repair, Fast neutrons, Theoretical radiobiology O.15 DERIVING A TIME DELIVERY MODEL FOR A SIEMENS ARTISTE LINAC N.S. Fourie *,a, O.A. Ali a,b, W.I.D. Rae b. a Department of Medical Physics, Equra Health, Cape Town, South Africa; b Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction: With IMRT delivery time is significantly longer than with 3DCRT; therefore accurate dose delivery becomes difficult due to intrafractional patient movement. To our knowledge there is as yet no time delivery model proposed for a Siemens Artiste Linear Accelerator (Linac). Our aim is to propose such a model. Materials and Methods: Using the principles of the time delivery model created by Li and Xing for a Varian Linac, a formula was proposed that determines time delivery on a Siemens Artiste Linac. We added the Radio