Işınlayıcı Dizileriyle Doku-içi Hipertermide Enerji Yoğunlaştırılması

thumbnail.default.placeholder
Tarih
1996
Yazarlar
Ferikoğlu, Abdullah
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Özet
Despite the large amount of efforts in recent years, cancer continues to be the number two causes of death. At present, only about 50 percent of the patients who have cancer will survive for more than five years, so there is still a need for better methods of diagnosis and treatment. A therapeutic technique that has received increasing attention in recent years is a procedure in which tumor temperatures are elevated to the range of 43 to 50 °C. This type of therapy is called hyperthermia. After the advent of cell culture methods in 1950's, experiments performed on tumor bearing animals and human patients have clearly shown that hyperthermia affords preferential killing of malignant cells. Besides this inherent sensitivity of tumor cells to hyperthermia, it has been shown to enhance the cytotoxic effects of many anticancer drugs and to potentiate the cell-killing ability of ionising irradiation. At present hyperthermia shows greatest promise when combined with radiotherapy. Recent clinical reports show overall response rates up to 90 percent for combined modality against 55 percent with radiotherapy. It appears that hyperthermia inhibits DNA repair at more modest doses of irradiation, thus inhibiting cell division process, than conventional therapy alone. It should be noted that despite the increasing uses in the clinic, hyperthermia treatment is at an early stage of development. The important target of this development is the production of adequate thermal field distribution in superficial, accessible and deep-seated tumors. During the last two decades a large number of antennas and applicators have been designed to produce therapeutic heating of tumors of different volumes in a variety of anatomic sites. Noninvasive techniques devised recently for heating deep-seated tumors include capacitive plates, helical coils and multiapplicator arrays. Invasive methods for inducing hyperthermia have been developed as an alternative when noninvasive systems are inadequate for producing therapeutic temperatures throughout the entire tumor volume without overheating the surrounding healthy tissue. Interstitial methods presently under investigation are ferromagnetic seeds, low frequency electrodes and coaxial microwave antennas. The first technique uses Eddy currents, which are induced by the magnetic field of a circular coil placed around the body, in intratumorally implanted metal rods. An important advantage of ferromagnetic seeds is that this metal could be combined with an alloy containing iodine- 125. Xll These seeds could then be implanted within a tumor and left in place permanently, allowing the delivery of therapeutic doses of radiation through the natural isotopic decay of iodine- 125. Interstitial heating with low frequency electrodes is produced by conductive currents between electrically connected arrays of needles Interest in interstitial microwave antennas as a means for localised hyperthermia for both superficial and deep-seated tumors arose in the late 1970's and much recent work is devoted to develope different antenna designs and arrays. The microwave power source can operate at a variety of frequencies between 433 and 2450 MHz. The power source is followed by a power divider and a set of power controllers that deliver microwave energy to each antenna. Antennas are normally inserted into catheters implanted in the tumor site by the physician using standard interstitial brachytherapy techniques. Thus, hyperthermia is entirely compatible with the brachytherapy. The temperature throughout the tumor volume is measured by a thermometry system using a set of sensors. Using the information from this thermometry system a feedback control is provided so that the microwave power be deposited to the required location at the needed levels. The key element in an interstitial microwave antenna array hyperthermia is the antenna, which is basically a coaxial cable with an extension of inner conductor at one end. The 1st Section of this thesis work is devoted to a review of the historical development of hyperthermia and explaining the rationale of hyperthermia as an adjuvant modality to radiotherapy and chemotherapy. The types of applicators and radiators devised for clinical use in connection with a classification of hyperthermia as superficial, regional and whole-body procedures are also given in this section. In the 2n(* Section, solutions to Maxwell equations having source terms at the right-hand side are presented. Here, the sources consist of electric current density and magnetic current density, which is introduced to.the problem for the sake of convrnience. First, the helping vector potentials A and B are found in terms of electric and magnetic current sources, respectively, using Love's equivalent surface currents. Then the expressions for the electric and magnetic fields are obtained via superposition principle as a sum of two terms including these surface currents. In Section 3, a cross-switch coaxial radiator is selected for hyperthermia due to its advantage that an extended length of surface contributes to the radiation. The radiator is considered to be inserted in a catheter, inside which a cooling liquid (water) circulates. A total of 10 cm of the coaxial makes radiation: the bare inner conductor of 5 cm and another length of 5 cm, at the connection of which the inner and outer conductors are cross-connected. The feeding part is assumed to be 0.5 m with a current source at the end. The constants of the radiator are 8!= e0; Uı;o* ı for the conductor, s2; \x.%, o2 for the cooling liquid, sc; Uciov^ for the dielectric of the coaxial, s3; u^; a3 for the catheter, and e4; U4; a4 for the surrounding medium (muscle tissue), inside which the power density deposited by the radiator W(x;y;z) = a |E4(x;y;z)| is aimed to be calculated, m me vicinity of the radiators, the near field which is of interest is given in TM modes independent of the azimuthal variable. This field has ty>, ep and ez components, general expressions of which are given in terms of cylindrical functions, namely Bessel, Hankel and Neumann functions. The axial wave xi 11 number kz appearing in these expressions is found from the characteristic equation which is constructed by applying the boundary conditions for the conductor-liquid, liquid-catheter, and catheter-medium interfaces. Then these field components are expressed in terms of conductor currents, which in tura is obtained as a fünction of the axial variable z. in the last step Love's equivalent surfaces currents are obtained evaluating the near field expressions on the conductor surfaces, and the field in the surrounding lossy medium is found through the electric and magnetic vector potentials applying the superposition principle. in the practical solution of the problem an approximation is made by taking the axial wave number as kz2 = 0^4(0)84 - ja4) = \rf, thus letting an error of nearly ten per cent, which is acceptable from the standpoint of the main interest of the following investigation. The power density distribution in the muscle tissue is numerically calculated on computer, the two-fold integrals in the electric field expression being taken över 100x 100 steps by Simpson method, for four frequency values, namely 13.56, 27.12, 915 and 2450 MHz. The results are shown in contour curves. The input impedances of the antenna are also calculated in this section. in the first part of Section 4 a circular array is considered. The axis parallel to the radiators is taken to be z. The x and y coordinates of the radiators are (0,15), (- 15,0), (O,-15) and (15,0) mm. For tracing the power deposition patterns within the cubic volume of (6 cm x 6 cm x 6 cm) three planes are considered: PLI - the plane which intersects the radiators at their mid points (z = ZQ); PLn - the plane which intersects the radiators close to their tips (z=Za) ; PLDI-the plane which intersects the radiators close to their feed points. For each of the four frequencies, the power density W =<74 |E(P)| at a point P is calculated as W=o4Ek(P) ; Erf(P) = £ E«(P) i=t i=l and the distributions numerically calculated at nearly a thousand points on each of the three planes are plotted in contour curves. From those curves a slower field variation with decreasing frequencies was observed, the highest values being on the mid-point plane Z=ZQ. it is well known that the success of a cünical hyprthermia process is critically dependent on the minimally heated points throughout the tümör volume. Therefore it may be required to increase the heat more at some target points while the process is going on. in the second part of Section 4 this problem is investigated. The actual xiv localisation of these target points can be determined by the readings from the thermometry system during the clinical application. Six target points are chosen for power concentration. Point Pjl lies inside the region enclosed by the array in PLI, P2I is outside the region enclosed by the array in PLI. P^ and P,!!! are the projections of Ptl in PLn and PLÜI, respectively. P2n and P2in are the projections of P2I in PLn and PL1H, respectively. As a first approxünation to the power (correspondingly heat) concentration, changing the phase angles of the feed currents according to the propagation constant in the medium is considered. 915 MHz and 2450 MHz are chosen as operating frequencies. For z = ZQ plane the maximum possible a4|£(p)| at a target point is obtained when the phases are chosen equal to the products of the radial distances of the radiators to the target point and the propagation constant. However, in planes z = za and z = z,, the optimum phases do not obey the above rule, because different current elements on the radiator contribute to the total field in different scales. Therefore this technique remains applicable in a limited region of the turnor mass. The second method, which is developed for phase coherence in the whole tümör region utilise the specific radiation property of the cross-switch radiator: the radial component of the electric field can be ignored beside the axial component, with an absolute error of less than 0.01 dB. in this method individual electric fields of the radiator at a target point is first calculated, and then the phase angles of these complex field terms are determined and assigned to the feed current with a sign reversal, which gives an exact compensation for coherence. The condition for this technique to be used with success is that the axial component be much greater than the radial öne. in the case that the axial component is greater but such an ignorance is not acceptable, then this approach can stili be utilised in order to set a starting point in a phase finding subroutine. The last technique developed for phase coherence uses a computer code and is the most general öne, which can be used with any kind of radiators, regardless of the magnitude of their field components. Here, a 2n phase angle is divided into 40 equal pieces and ali possible combinations of feed current phase angles are tested at a given target point, which yields satisfactorily the proper feed current phases for power maximisation at that point. it is shown that for 915 MHz, at target points P2I, P2n and P2in SAR increases up to 5.1 dB, for 2450 MHz, at target point P:I, PjII and PİÜI SAR increases up to 9.4 dB can be attained. in the third part of Section 4 another technique is investigated to increase the heat stili more: to move the radiators along the z axis in proper directions. A position shift of 2 cm is allowed in both directions with 0.2 cm increments. For this aim the current expressions for the radiators are modified as iy(z) -> i. (z-hj), j = l, 2,..., 10 XV for shifting in the positive direction, and iy(z) -> ijCz+hj), j = l, 2,..., 10 for shifting in the negative direction. With this method SAR increases up to 10 dB compared to equal-phase feeding are obtained. Instead of moving ali the radiators, moving only the nearest radiator to the target point is also examined and SAR increases at the chosen target points up to 8 dB are found. Finally, the phase coherence program is applied in addition to the axial shifting with the result that the total increase in SAR for 2450 MHz operating frequency amounted to 21.2 dB at the target points inside the array circle and to 10.5 dB at the target points outside the array circle, and for 915 MHz operating frequency amounted to 12.4 dB at the target points inside the array circle and to 12.2 dB at the target points outside the array circle, on the z=za and z=zjj planes. After the evaluation of the numerical results obtained on computer it is concluded that each of the methods developed can be used in clinical hyperthermia with an aim of heat concentration. it is here recommended that before the clinical application of hyperthermia the tümör area should be divided into elementary cells by a numerical method and the proper feed current phase angles of radiators for each celi calculated by the above presented techniques should be deposited on computer memory with an eye to access to them when needed during the actual procedure without any time delay.
Açıklama
Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1996
Thesis (Ph.D.) -- İstanbul Technical University, Institute of Science and Technology, 1996
Anahtar kelimeler
Işınlama, Neoplazmlar, Irradiation, Neoplasms
Alıntı