Biomedical subjects

J J Lagendijk

Publications and source records attributed to J J Lagendijk.

At least 19 recordsLinked to original sources

PubMed2001

The potential impact of treatment variations on the results of radiotherapy of the internal mammary lymph node chain: a quality-assurance report on the dummy run of EORTC Phase III randomized trial 22922/10925 in Stage I--III breast cancer(1).

PURPOSE: To present the results of the dummy run of the European Organization for Research and Treatment of Cancer (EORTC) trial investigating the role of adjuvant internal mammary and medial supraclavicular (IM-MS) irradiation in Stage I--III breast cancer. METHODS AND MATERIALS: All participating institutions were asked to produce a treatment plan without (Arm 1) and with (Arm 2) simultaneous IM-MS irradiation of 1 patient after mastectomy and of 1 patient after lumpectomy. Thirty-two dummy runs have been evaluated for compliance to protocol guidelines, with respect to treatment technique and dose prescription. RESULTS: A number of more or less important deviations in treatment setup and prescription have been found. The dose in the IM-MS region deviated significantly from the prescribed dose in 10% of the cases for Arm 1, and in 21% for Arm 2. Assuming a true 5% 10-year survival benefit from optimal IM-MS irradiation, an increase of only 3.8% will be found due to this suboptimal dose distribution. CONCLUSION: In the dummy run, a number of potential systematic protocol deviations that might lead to false-negative results were detected. By providing recommendations to the participating institutions, we expect to improve the interinstitutional consistency and to promote a high quality irradiation in all institutions participating in the trial.

Breast Neoplasms↗

PubMed2001

An improved breast irradiation technique using three-dimensional geometrical information and intensity modulation.

BACKGROUND AND PURPOSE: In spite of the complex geometry of the breast, treatment planning for tangential breast irradiation is conventionally performed using two-dimensional patient anatomy information. The purpose of this work was to develop a new technique which takes the three-dimensional (3D) patient geometry into account. MATERIALS AND METHODS: An intensity-modulated radiotherapy (IMRT) technique was developed based on the division of the tangential fields in four multi-leaf collimator (MLC) shaped segments. The shape of these segments was obtained from an equivalent path length map of the irradiated volume. Approximately 88% of the dose was delivered by two open fields covering the whole treated volume. Dose calculations for the IMRT technique and the conventional technique were performed for five patients, using computer tomography (CT) data and a 3D calculation algorithm. A planning target volume (PTV) and ipsilateral lung volume were delineated in these CT data. RESULTS: All patients showed similar equivalent path length patterns. Analysis of the dose distribution showed an improved dose distribution using the IMRT technique. The dose inhomogeneity in the PTV was 9.0% (range 6.4-11.4%) for the conventional and 7.6% (range 6.5-10.3%) for the IMRT technique. The mean lung dose was reduced for the IMRT technique by approximately 10% compared with the conventional technique. CONCLUSION: A new breast irradiation technique has been developed which improves the dose homogeneity within the planning target volume and reduces the dose to the lung. Furthermore, the IMRT technique creates the possibility to improve the field matching in case of multiple field irradiations of the breast and lymph nodes.

Breast↗

PubMed2001

High-resolution SAR modelling for regional hyperthermia: testing quasistatic zooming at 10 MHz.

Present-day regional hyperthermia treatment planning systems are limited to centimetre resolution. To obtain CT-resolution SAR distributions, a method called quasistatic zooming has been developed: using the centimetre-resolution E-field distribution and the CT-resolution tomogram, the CT-resolution SAR distribution is obtained. For a low frequency of 10 MHz this method has been validated sucessfully using CT-resolution SAR computations. It appears that these CT-resolution SAR distributions are completely different from centimetre-resolution SAR distributions, indicating the necessity for high-resolution SAR modelling. Using the presented zooming technique, reliable CT-resolution SAR modelling is now possible with relatively short computation times. So far, the zooming method has only been validated for low frequencies, but clinically relevant frequencies appear to be possible.

Algorithms↗

PubMed2001

Quasistatic zooming for regional hyperthermia treatment planning.

Due to current computer limitations, specific absorption rate (SAR) distributions in regional hyperthermia treatment planning (HTP) are limited to centimetre resolution. However, since patient anatomy is highly structured on a millimetre scale, millimetre-resolution SAR modelling is required. A method called quasistatic zooming has been developed to obtain a high-resolution SAR distribution within a volume of interest (VOI): using the low-resolution E-field distribution and the high-resolution patient anatomy, the high-resolution SAR distribution is computed within a small zoom volume Q (small compared with the wavelength in water (lambda(w))). Repeating this procedure yields the zoomed-resolution SAR distribution in an arbitrary VOI. To validate this method for a VOI that is not small compared with lambda(w), high-resolution finite-difference time-domain (FDTD) modelling is needed. Since this is impractical for a clinical applicator, a computer model of a small applicator has been created. A partial patient anatomy is inserted into the applicator and both high- and low-resolution SAR distributions are computed for this geometry. For the same geometry, zoomed-resolution SAR distributions are computed with different sizes of Q. To compare the low- and zoomed-resolution SAR distributions with the high-resolution one, the correlation and averaged absolute difference are computed. These numbers are improved considerably using zooming (correlation 58% to 92%; averaged absolute difference 43% to 20%). These results appear to be independent of the size of Q, up to 0.3 lambda(w). Quasistatic zooming is a valuable tool in high-resolution regional HTP.

Computer Simulation↗

PubMed2001

Feasibility of automatic marker detection with an a-Si flat-panel imager.

Here we study automatic detection of implanted gold markers relative to the field boundary in portal images for on-line position verification. Portal images containing 1-2 MU were taken with an amorphous silicon flat-panel imager. The images were obtained with lateral field at 18 MV. Both the detection success rate and the localization accuracy of markers of 1.0 and 1.2 mm diameter were determined with the help of a marker detection method based on a marker extraction kernel. A method for determining a fiducial reference point related to the field boundary was developed. Detection success rates were 0.99, 0.90 and 0.95 for markers of 1.2 mm diameter and 5 mm length, 1.0 mm diameter and 5 mm length and 1.0 mm diameter and 10 mm length respectively. The localization accuracy appeared to be better than 0.3 mm. The reference point could be reproduced with an accuracy equal to 1 pixel (0.5 mm at isocentre) within one fraction. During the first few seconds of a treatment fraction the field edge was not stable, which appeared to be an effect of the motion of the radiation source. Thanks to the use an a-Si flat-panel imager, on-line position verification using implanted gold markers becomes clinically feasible. We can use a clinically acceptable marker diameter as small as 1.0 mm. These markers can be automatically detected in portal images obtained with 1-2 MU relative to a stable reference point related to the field boundary.

Algorithms↗

PubMed2001

Quasistatic zooming of FDTD E-field computations: the impact of down-scaling techniques.

Due to current computer limitations, regional hyperthermia treatment planning (HTP) is practically limited to a resolution of 1 cm, whereas a millimetre resolution is desired. Using the centimetre resolution E-field distribution, computed with, for example, the finite-difference time-domain (FDTD) method and the millimetre resolution patient anatomy it is possible to obtain a millimetre resolution SAR distribution in a volume of interest (VOI) by means of quasistatic zooming. To compute the required low-resolution E-field distribution, a low-resolution dielectric geometry is needed which is constructed by down-scaling the millimetre resolution dielectric geometry. In this study we have investigated which down-scaling technique results in a dielectric geometry that yields the best low-resolution E-field distribution as input for quasistatic zooming. A segmented 2 mm resolution CT data set of a patient has been down-scaled to 1 cm resolution using three different techniques: 'winner-takes-all', 'volumetric averaging' and 'anisotropic volumetric averaging'. The E-field distributions computed for those low-resolution dielectric geometries have been used as input for quasistatic zooming. The resulting zoomed-resolution SAR distributions were compared with a reference: the 2 mm resolution SAR distribution computed with the FDTD method. The E-field distribution for both a simple phantom and the complex partial patient geometry down-scaled using 'anisotropic volumetric averaging' resulted in zoomed-resolution SAR distributions that best approximate the corresponding high-resolution SAR distribution (correlation 97, 96% and absolute averaged difference 6, 14% respectively).

Anisotropy↗

PubMed2001

Regional hyperthermia applicator design using FDTD modelling.

Recently published results confirm the positive effect of regional hyperthermia combined with external radiotherapy on pelvic tumours. Several studies have been published on the improvement of RF annular array applicator systems with dipoles and a closed water bolus. This study investigates the performance of a next-generation applicator system for regional hyperthermia with a multi-ring annular array of antennas and an open water bolus. A cavity slot antenna is introduced to enhance the directivity and reduce mutual coupling between the antennas. Several design parameters, i.e. dimensions, number of antennas and operating frequency, have been evaluated using several patient models. Performance indices have been defined to evaluate the effect of parameter variation on the specific absorption rate (SAR) distribution. The performance of the new applicator type is compared with the Coaxial TEM. Operating frequency appears to be the main parameter with a positive influence on the performance. A SAR increase in tumour of 1.7 relative to the Coaxial TEM system can be obtained with a three-ring, six-antenna per ring cavity slot applicator operating at 150 MHz.

Female↗

PubMed2000

SAR and temperature changes in the leg due to an RF decoupling coil at frequencies between 64 and 213 MHz.

Specific absorption rate (SAR) due to a butterfly surface coil in a realistic model of the leg was calculated for frequencies 64 < or = v < or = 213 MHz. The resulting temperature distribution and temperature changes (deltaT) were found using the bioheat transfer equation. To compare results at different frequencies, the minimum B-field within the coil's footprint in the plane parallel to the coil but displaced 50 mm from it was kept constant. To achieve the same minimum B-field as that associated with operation at 64 MHz that was compliant with safety guidelines (peak SAR in 1 cm3 of tissue of 2.3 W/kg), it was predicted that SAR would exceed recommended levels when v > or = 149 MHz. The corresponding maximum deltaT at 64, 128, and 213 MHz were 0.3, 1.3, and 5.0 degrees C, respectively.

Absorption↗

PubMed2000

How to apply a discrete vessel model in thermal simulations when only incomplete vessel data are available.

For accurate predictions of the temperature distribution during hyperthermia treatment a thermal model should incorporate the individual impact of discrete vessels. In clinical practice not all vessels can be reconstructed individually. This paper investigates five possible strategies to model the thermal impact of these missing vessels. A tissue volume with a detailed, realistic, counter-current discrete vasculature is heated and the steady-state temperature distribution is calculated using our Discrete Vasculature (DIVA) thermal model. To mimic incomplete discrete vasculatures the full tree is gradually stripped, that is, the number of discretely described vessels is reduced in four steps until no discrete vessels are left. At each strip level the steady state temperature distribution is calculated for five different strategies to model the missing vessels. The strategies all use a local or global heat sink model in addition to the discrete vasculature. The resulting temperature distributions are compared with the full tree simulation. With increasing strip level the correspondence with the full tree simulation deteriorated for all strategies. An optimal strategy was found to model the missing vessels depending on the available angiographic data. It was also found that simulations with a decreased number of discrete vessels, or no vessels at all, yield temperatures which are too high. Theoretically this can be compensated by increasing the thermal conductivity; finding the optimal value is done empirically.

Blood↗

PubMed2000

Modelling individual temperature profiles from an isolated perfused bovine tongue.

To predict the temperature distribution during hyperthermia treatments a thermal model that accounts for the thermal effect of blood flow is mandatory. The DIscrete VAsculature (DIVA) thermal model developed at our department is able to do so; geometrically described vessels are handled individually and the remaining vasculature is modelled collectively. The goal of this paper is to experimentally validate the DIVA model by comparing measured with modelled temperature profiles on an individual basis. Temperature profiles in an isolated bovine tongue heated with three hot water tubes were measured at three controlled perfusion levels, 0, 6 and 24 ml (100 g)(-1) min(-1). The geometries of the tongue, the hot water tubes, thermocouples and discrete vasculature down to 0.5 mm diameter were reconstructed by using cryo-microtome slices at 0.1 mm cubic resolution. This reconstruction of the experimental set-up is used for the modelling of individual profiles. In a no-flow agar-agar phantom, DIVA showed nearly perfect correspondence between measurements and simulations. In the isolated bovine tongue the correspondence at no flow was slightly disturbed due to geometrical distortion in the reconstruction of the experimental set-up. Measurements showed decreasing temperature profiles with increasing perfusion. DIVA correctly predicted this decrease in temperature as well as the thermal impact of a large vessel close to a thermocouple. Blood flow was modelled using discrete vasculature and using a heat sink model. Although at 24 ml (100 g)(-1) min(-1) correspondence between heat sink simulations and measurements was reasonable, modelling discrete vasculature yielded the best correspondence at both 6 and 24 ml (100 g)(-1) min(-1). The results strongly suggest that with accurate data acquisition DIVA can predict temperature profiles on an individual basis. For this kind of patient-specific treatment planning in the clinic, geometrical reconstruction of the anatomy, vasculature and the heating implant is necessary. MRI is capable of providing these data. Further research will be done on thermal simulations of actual clinical hyperthermia treatments.

Agar↗

PubMed2000

Temperature simulations in tissue with a realistic computer generated vessel network.

The practical use of a discrete vessel thermal model for hyperthermia treatment planning requires a number of choices with respect to the unknown part of the patient's vasculature. This work presents a study of the thermal effects of blood flow in a simple tissue geometry with a detailed artificial vessel network. The simulations presented here demonstrate that an incomplete discrete description of the detailed network results in a better prediction of the temperature distribution than is obtained using the conventional bio-heatsink equation. Therefore, efforts to obtain information on the positions of the large vessels in an individual hyperthermia patient will be rewarded with a more accurate prediction of the temperature distribution.

Arteries↗

PubMed2000

Hyperthermia treatment planning.

The development of hyperthermia, the treatment of tumours with elevated temperatures in the range of 40-44 degrees C with treatment times over 30 min, greatly benefits from the development of hyperthermia treatment planning. This review briefly describes the state of the art in hyperthermia technology, followed by an overview of the developments in hyperthermia treatment planning. It particularly highlights the significant problems encountered with heating realistic tissue volumes and shows how treatment planning can help in designing better heating technology. Hyperthermia treatment planning will ultimately provide information about the actual temperature distributions obtained and thus the tumour control probabilities to be expected. This will improve our understanding of the present clinical results of thermoradiotherapy and thermochemotherapy, and will greatly help both in optimizing clinical heating technology and in designing optimal clinical trials.

Biophysical Phenomena↗

PubMed2000

Numerical modeling of temperature distributions within the neonatal head.

Introduction of hypothermia therapy as a neuroprotection therapy after hypoxia-ischemia in newborn infants requires appraisal of cooling methods. In this numerical study thermal simulations were performed to test the hypothesis that cooling of the surface of the cranium by the application of a cooling bonnet significantly reduces deep brain temperature and produces a temperature differential between the deep brain and the body core. A realistic three-dimensional (3-D) computer model of infant head anatomy was used, derived from magnetic resonance data from a newborn infant. Temperature distributions were calculated using the Pennes heatsink model. The cooling bonnet was at a constant temperature of 10 degrees C. When modeling head cooling only, a constant body core temperature of 37 degrees C was imposed. The computed result showed no significant cooling of the deep brain regions, only the very superficial regions of the brain are cooled to temperatures of 33-34 degrees C. Poor efficacy of head cooling was still found after a considerable increase in the modeled thermal conductivities of the skin and skull, or after a decrease in perfusion. The results for the heatsink thermal model of the infant head were confirmed by comparison of results computed for a scaled down adult head, using both the heatsink description and a discrete vessel thermal model with both anatomy and vasculature obtained from MR data. The results indicate that significant reduction in brain temperature will only be achieved if the infant's core temperature is lowered.

Adult↗

PubMed1999

Electromagnetic and thermal modeling of SAR and temperature fields in tissue due to an RF decoupling coil.

The finite difference time domain method is used to calculate the specific absorption rate (SAR) due to a butterfly surface coil in a realistic tissue model of the leg. The resulting temperature distribution and temperature changes are found using a finite difference solution to the bioheat transfer equation. Reasonable agreement is found between predicted temperature changes and those measured in vivo provided that the resulting hyperthermia does not induce noticeable changes in perfusion. The method is applicable to radiofrequency dosimetry problems associated with high Bo field magnetic resonance systems and where knowledge of spatial variation in SAR is important in assessing the safety of new magnetic resonance procedures.

Absorption↗

PubMed1999

Modelling the thermal impact of a discrete vessel tree.

Based on a modelling technique to calculate the thermal influence of a single vessel segment, a combination of segments representing a vessel tree is presented. At segment junctions the blood temperature is passed with a correction for a single vessel artefact. Blood leaving the modelled arterial vessel network at a junction or at the end of a terminal branch need not be equilibrated with the local surrounding tissue temperature. The thermal effect of this equilibration process can be taken into account using the 'sink set'. Blood entering a venous vessel tree is given an inflow temperature based on the tissue temperatures in the 'sample set'. With these sets we model the thermal impact of the vasculature too small to be taken into account discretely. The formation of the sink/sample sets is subject of current research; to show the capabilities of the presented method we present a minimal simulation collection with cube-shaped sets in combination with limited vasculature. The results of using the entire simulation volume as sink and sample sets for all the terminal branches matches the reference temperature profile best.

Arteries↗

PubMed1999

Calculation of change in brain temperatures due to exposure to a mobile phone.

In this study we evaluated for a realistic head model the 3D temperature rise induced by a mobile phone. This was done numerically with the consecutive use of an FDTD model to predict the absorbed electromagnetic power distribution, and a thermal model describing bioheat transfer both by conduction and by blood flow. We calculated a maximum rise in brain temperature of 0.11 degrees C for an antenna with an average emitted power of 0.25 W, the maximum value in common mobile phones, and indefinite exposure. Maximum temperature rise is at the skin. The power distributions were characterized by a maximum averaged SAR over an arbitrarily shaped 10 g volume of approximately 1.6 W kg(-1). Although these power distributions are not in compliance with all proposed safety standards, temperature rises are far too small to have lasting effects. We verified our simulations by measuring the skin temperature rise experimentally. Our simulation method can be instrumental in further development of safety standards.

Body Temperature↗

PubMed1998

Modelling tissue heating with ferromagnetic seeds.

Interstitial hyperthermia using ferromagnetic seeds demands accurate treatment planning: the seed characteristics and implant geometry must be determined prior to the treatment. A new, finite difference based, seed modelling method is presented. The seed, together with all its surrounding (non-tissue) layers is described as one unit, independent of the tissue grid. The calculation of the seed-tissue interaction is based on the local seed temperature and several tissue temperature samples in the direct vicinity. All the layers between the seed and the surrounding tissue are taken into account in this interaction calculation. The presented implementation describes the analytical solution of the modelled steady-state configurations very accurately. The separation between tissue and seed allows easy assessment of the resulting seed temperature profile which is essential to the optimization of the seed characteristics in treatment planning. The thermal effect due to blood flow in the modelled tissue volume surrounding the seed can be accounted for by inclusion of a heat sink term as well as by inclusion of realistic discrete vasculature.

Algorithms↗

PubMed1998

Dose uniformity of ferromagnetic seed implants in tissue with discrete vasculature: a numerical study on the impact of seed characteristics and implantation techniques.

The results from simulations with a new three-dimensional treatment planning system for interstitial hyperthermia with ferromagnetic seeds are presented in this study. The thermal model incorporates discrete vessel structures as well as a heat sink and enhanced thermal conductivity. Both the discrete vessels and the ferroseeds are described parametrically in separate calculation spaces. This parametric description has the advantage of an arbitrary orientation of the structures within the tissue grid, easy manipulation of the structures and independence from the resolution of the tissue voxels (tissue calculation space). The power absorption of the self-regulating seeds is according to empirical data. The thermal effects of an unlimited number of thin layers surrounding the seed (coatings, catheters) can be modelled. The initial calculations have been performed for an array of 12 identical ferromagnetic seeds in a tissue volume with a computer generated artificial vessel network spanning four vessel generations in both the arterial and venous tree. The heterogeneously distributed large isolated vessels impair the temperature distribution significantly, indicating the limited accuracy of continuum models. Simulations with different types of ferromagnetic seeds have confirmed that the efforts of previous studies to optimize the self-regulating temperature control and the implantation techniques of the ferroseeds will improve the homogeneity of the temperature distribution in the target volume. Multifilament seeds implanted in brachytherapy needles and tubular seeds appear to be the most favourable configurations. The division of long seeds into shorter segments with the appropriate Curie temperature will further improve the homogeneity of the temperature distribution without increasing the average temperature in the volume of interest. Given the proper thermal tissue data, the model presented in this study will prove to be a useful tool in making choices for the implant geometry, seed spacing and Curie temperature.

Biophysical Phenomena↗