| 1. |
- Daşu, Alexandru, et al.
(författare)
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Theoretical simulation of tumour oxygenation and results from acute and chronic hypoxia
- 2003
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 48:17, s. 2829-2842
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Tidskriftsartikel (refereegranskat)abstract
- The tumour microenvironment is considered to be responsible for the outcome of cancer treatment and therefore it is extremely important to characterize and quantify it. Unfortunately, most of the experimental techniques available now are invasive and generally it is not known how this influences the results. Non-invasive methods on the other hand have a geometrical resolution that is not always suited for the modelling of the tumour response. Theoretical simulation of the microenvironment may be an alternative method that can provide quantitative data for accurately describing tumour tissues. This paper presents a computerized model that allows the simulation of the tumour oxygenation. The model simulates numerically the fundamental physical processes of oxygen diffusion and consumption in a two-dimensional geometry in order to study the influence of the different parameters describing the tissue geometry. The paper also presents a novel method to simulate the effects of diffusion-limited (chronic) hypoxia and perfusion-limited (acute) hypoxia. The results show that all the parameters describing tissue vasculature are important for describing tissue oxygenation. Assuming that vascular structure is described by a distribution of inter-vessel distances, both the average and the width of the distribution are needed in order to fully characterize the tissue oxygenation. Incomplete data, such as distributions measured in a non-representative region of the tissue, may not give relevant tissue oxygenation. Theoretical modelling of tumour oxygenation also allows the separation between acutely and chronically hypoxic cells, a distinction that cannot always be seen with other methods. It was observed that the fraction of acutely hypoxic cells depends not only on the fraction of collapsed blood vessels at any particular moment, but also on the distribution of vessels in space as well. All these suggest that theoretical modelling of tissue oxygenation starting from the basic principles is a robust method that can be used to quantify the tissue oxygenation and to provide input parameters for other simulations.
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| 2. |
- Toma-Dasu, Iuliana, et al.
(författare)
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The relationship between temporal variation of hypoxia, polarographic measurements and predictions of tumour response to radiation
- 2004
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 49:19, s. 4463-4475
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Tidskriftsartikel (refereegranskat)abstract
- The polarographic oxygen sensor is one of the most used devices for in vivo measurements of oxygen and many other measurement techniques for measuring tumour hypoxia are correlated with electrode measurements. Little is known however about the relationship between electrode measurements and the real tissue oxygenation. This paper investigates the influence of the temporal change of the hypoxic pattern on the electrode measurements and the tumour response. Electrode measurements and tumour response were simulated using a computer program that allows both the calculation of the tissue oxygenation with respect to the two types of hypoxia that might arise in tumours and the virtual insertion of the electrode into the tissue. It was therefore possible to control the amount of each type of hypoxia in order to investigate their influence on the measurement results. Tissues with several vascular architectures ranging from well oxygenated to poorly oxygenated were taken into consideration as might be seen in practice. The influence of the electrode measurements on the treatment outcome was estimated by calculating the tumour control probability for the tumours characterized either by the real or by the measured tumour oxygenation. We have simulated electrode oxygen measurements in different types of tissues, covering a wide range of tumour oxygenations. The results of the simulations showed that the measured distribution depends on the details of the vascular network and not on the type of hypoxia. We have also simulated the effects of the temporal change of the acute hypoxic pattern due to the opening and the closure of different blood vessels during a full fractionated treatment. The results of this simulation suggested that the temporal variation of the hypoxic pattern does not lead to significantly different results for the electrode measurements or the predicted tumour control probabilities. In conclusion, it was found that the averaging effect of the electrode leads to a systematic deviation between the actual oxygen distribution and the measured distribution. However, as the electrode reflects the general trends of the tissue oxygenation it has the potential of being used for the general characterization of tumour hypoxia even if the actual type of hypoxia measured by the electrode cannot be determined. Indeed, the change in time of the acute hypoxic region does not compensate for the lack of oxygenation at a specific moment and therefore does not influence the polarographic oxygen measurements.
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| 3. |
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| 4. |
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| 5. |
- Blomquist, M, et al.
(författare)
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Comparison between a conventional treatment energy and 50 MV photons for the treatment of lung tumours
- 2002
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Ingår i: Physics in Medicine and Biology. - 0031-9155 .- 1361-6560. ; 47:6, s. 889-897
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Tidskriftsartikel (refereegranskat)abstract
- Radiation therapy in the thoracic region is difficult due to the presence of many dose-limiting structures and the large density differences that affect the dose distribution. Conventional irradiation techniques use low-energy photon beams to avoid build-up effects superficially in the tumour and increased lateral scattering of the beams. For deep-seated tumours higher beam energies could have lung-sparing properties that would enable dose escalation. A comparison was made for a conventional low photon energy (6 MV) and 50 MV photons for the treatment of a lung tumour. A representative patient geometry was selected, consisting of a small tumour semi-enclosed in lung tissue. Treatment plans were designed using a commercial 3D-pencil beam treatment planning system. The treatment beams designed in the TPS were simulated with the Monte Carlo code EGS4/BEAM and the dose distribution in the phantom created from the patients CT-data was calculated using MCDOSE with identical beam geometry for both energies. The intrinsic difference between the two photon energies implies a sparing effect of lung that can be utilized for dose escalation. For a treatment with two beams the mean total dose to the tumour could be increased by 5.3% for 50 MV, corresponding to 3.2 Gy for a prescription dose of 60 Gy, with the same complication probability for the treated lung as for 6 MV. In conclusion, high-energy beams have qualities that can be taken advantage of for irradiation of lung tumours. Optimum solutions would probably require the use of both high- and low-energy beams.
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| 6. |
- Blomquist, M, et al.
(författare)
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Multileaf collimation of electrons-clinical effects on electron energy modulation and mixed beam therapy depending on treatment head design
- 2002
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Ingår i: Physics in Medicine and Biology. - 0031-9155 .- 1361-6560. ; 47:7, s. 1013-1024
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Tidskriftsartikel (refereegranskat)abstract
- The aim Of this Study was to explore the possibilities of using multileaf-collimated electron beams for advanced radiation therapy with conventional scattering foil flattened beams. Monte Carlo simulations were performed with the aim to improve electron beam characteristics and enable isocentric multileaf collimation. The scattering foil positions, monitor chamber thickness, the MLC location and the amount of He in the treatment head were optimized for three common commercial accelerators. The performance of the three optimized treatment head designs was compared for different SSDs in air. at treatment depth in water and for some clinical cases. The effects of electron/photon beam matching including generalized random and static errors using Gaussian one-dimensional (1D) error distributions, and also electron energy modulation, were studied at treatment depth in beater, The modification of the treatment heads improved the electron beam characteristics and enabled the use of multileaf collimation in isocentric delivery of both electron and photon beams in a mixed beam IMRT procedure.
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| 7. |
- Georg, Dietmar, et al.
(författare)
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A practical method to calculate head scatter factors in wedged rectangular and irregular MLC shaped beams for external and internal wedges
- 2004
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 49:20, s. 4689-4700
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Tidskriftsartikel (refereegranskat)abstract
- Factor based methods for absorbed dose or monitor unit calculations are often based on separate data sets for open and wedged beams. The determination of basic beam parameters can be rather time consuming, unless equivalent square methods are applied. When considering irregular wedged beams shaped with a multileaf collimator, parametrization methods for dosimetric quantities, e.g. output ratios or wedge factors as a function of field size and shape, become even more important. A practical method is presented to derive wedged output ratios in air (S-c,S-w) for any rectangular field and for any irregular MLC shaped beam. This method was based on open field output ratios in air (Sc) for a field with the same collimator setting, and a relation f(w) between S-c,S-w and S-c. The relation f(w) can be determined from measured output ratios in air for a few open and wedged fields including the maximum wedged field size. The function fw and its parametrization were dependent on wedge angle and treatment head design, i.e. they were different for internal and external wedges. The proposed method was tested for rectangular wedged fields on three accelerators with internal wedges (GE, Elekta, BBC) and two accelerators with external wedges (Varian). For symmetric regular beams the average deviation between calculated and measured S-c,S-w/S-c ratios was 0.3% for external wedges and about 0.6% for internal wedges. Maximum deviations of 1.8% were obtained for elongated rectangular fields on the GE and ELEKTA linacs with an internal wedge. The same accuracy was achieved for irregular MLC shaped wedged beams on the accelerators with MLC and internal wedges (GE and Elekta), with an average deviation < 1 % for the fields tested. The proposed method to determine output ratios in air for wedged beams from output ratios of open beams, combined with equivalent square approaches, can be easily integrated in empirical or semi-empirical methods for monitor unit calculations.
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| 8. |
- Georg, Dietmar, et al.
(författare)
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On empirical methods to determine scatter factors for irregular MLC shaped beams
- 2004
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 31:8, s. 2222-2229
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Tidskriftsartikel (refereegranskat)abstract
- Multileaf collimators (MLCs) are in clinical use for more than a decade and are a well accepted tool in radiotherapy. For almost each MLC design different empirical or semianalytical methods have been presented for calculating output ratios in air for irregularly shaped beams. However, until now no clear recommendations have been given on how to handle irregular fields shaped by multileaf collimators for independent monitor unit (MU) verification. The present article compares different empirical methods, which have been proposed for independent MU verification, to determine (1) output ratios in air (S-P) and (2) phantom scatter factors (Sp) for irregular MLC shaped fields. Ten dedicated field shapes were applied to five different types of MLCs (Elekta, Siemens, Varian, Scanditronix, General Electric). All calculations based on empirical relations were compared with measurements and with calculations performed by a treatment planning system with a fluence based algorithm. For most irregular MLC shaped beams output ratios in air could be adequately modeled with an accuracy of about 1%-1.5% applying a method based on the open field aperture defined by the leaf and jaw setting combined with the equivalent square formula suggested by Vadash and Bjarngard [P. Vadash and B. E. Bjarngard, Med. Phys. 20, 733-734 (1993)]. The accuracy of this approach strongly depends on the inherent head scatter characteristics of the accelerator in use and on the irregular field under consideration. Deviations of up to 3% were obtained for fields where leaves obscure central parts of the flattening filter. Simple equivalent square methods for S-P calculations in irregular fields did not provide acceptable results (deviations mostly >3%). S-P values derived from Clarkson integration, based on published tables of phantom scatter correction factors, showed the same accuracy level as calculations performed using a pencil beam algorithm of a treatment planning system (in a homogeneous media). The separation of head scatter and phantom scatter contributions is strongly recommended for irregular MLC shaped beams as both contributions have different factors of influence. With rather simple methods S, and SP can be determined for independent MU calculation with an accuracy better than 1.5% for most clinical situations encountered in conformal radiotherapy. (C) 2004 American Association of Physicists in Medicine.
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| 9. |
- Karlsson, M G, et al.
(författare)
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Electron beam collimation with focused and curved leaf end MLCs - Experimental verification of Monte Carlo optimized designs
- 2002
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 29:4, s. 631-637
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Tidskriftsartikel (refereegranskat)abstract
- In general, electron beams, from conventional accelerators using applicators with lead alloy inserts are not suitable for advanced conformal radiation therapy. However, interesting electron treatments have been demonstrated on a few advanced accelerators. These accelerators have been equipped with helium filled treatment heads and computer controlled MLCs that produce clinically useful energy modulated electron beams or mixed photon electron beams in an automated sequence. This study analyzes the characteristics of different MLC designs, curved and focused leaf ends in helium filled treatment heads, with respect to their effect on electron beams. In addition, this study analyzes the effects that different treatment head designs have on the output factor due to collimator scattering and shielding of secondary sources during treatment. The investigation of the different treatment head designs was performed with the Monte Carlo package BEAM and was verified by experimental methods. The results show that the difference between curved leaf ends and focused ends is negligible in most practical cases. The results also show the importance of scattering foil optimization in the optimization of parameters such as penumbra. virtual source position, and in the reduction of the output variation. In all cases, the experimental data verifies the calculations. (C) 2002 American Association of Physicists in Medicine.
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| 10. |
- Mu, Xiangkui, et al.
(författare)
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Can photon IMRT be improved by combination with mixed electron and photon techniques?
- 2004
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Ingår i: Acta Oncologica. - : Informa UK Limited. - 0284-186X .- 1651-226X. ; 43:8, s. 727-735
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Tidskriftsartikel (refereegranskat)abstract
- Conformal radiotherapy or intensity modulated radiotherapy (IMRT) commonly leads to a large integral dose in the patient. Electrons would reduce the integral dose but are not suitable for treating deep-seated tumours, owing to their limited penetration. By combining electron and photon beams, the dose distributions may be improved. In this study, the possibility is explored of using a mixture of electron and photon beams for a deep-seated target volume in the head and neck region. Treatment plans were made for five simulated head and neck cancer cases. Mixed electron and photon beam plans (MB) were constructed using a manual iterative procedure. Photon IMRT plans were optimized automatically. Both electron and photon beams were collimated by a computer controlled multi-leaf collimator (MLC). Both methods were able to produce clinically acceptable plans. Criteria for the target dose were met similarly by both as were the criteria for critical organs. The integral dose outside the planning target volume (PTV) showed a tendency to be lower with MB plans compared with photon IMRT plans. A mixed electron and photon technique has the potential to treat deep-seated tumours. It is reasonable to expect that if computerized optimization tools were coupled with the mixed electron and photon beam technique, treatment goals would be more readily achieved than if using solely pure photon IMRT.
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