Geri Dön

Soğurulan doz ölçümü için paralel elektrotlu suya eşdeğer iyon odasının geliştirilmesi

Başlık çevirisi mevcut değil.

  1. Tez No: 24062
  2. Yazar: HASAN ALKAN
  3. Danışmanlar: PROF. DR. ALİ NEZİHİ BİLGE
  4. Tez Türü: Doktora
  5. Konular: Nükleer Mühendislik, Nuclear Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1992
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 128

Özet

ÖZET Yüksek enerjili foton ve elektron radyasyon demetleri üreten yüksek enerjili tanecik hızlandırıcıları günümüzde radyoterapi alanında oldukça yaygın olarak kullanılmaktadır. Ancak, bu cihazların suda soğurulan doz olarak kalibrasyonlarında halen sorunlar bulunmaktadır. Radyoterapide başarı veya başarısızlık tedavi için belirlenen dozun, tümör hacmine doğru olarak verilmesine bağlıdır. ICRU soğurulan dozun tümör hacmına en fazla ± % 5 hata ile verilmesi gerektiği sonucuna varmıştır. Radyoterapide kullanılan yüksek enerjili foton ve elektron demetlerinin günlük kalibrasyonlarında doğrudan mutlak doz ölçümü yapan Faraday kapları, Kalorimetreler ve Fricke dozimetreleri düşük duyarlıkta olmaları, karma şık yapı ve ölçüm teknikleri nedeniyle kullanılmamaktadır. Bunların yerine daha duyarlı ve pratik olan iyonimetrik dozimetri yöntemlerinin kullanılması dozimetri protokollarında önerilmektedir. iyonometrik doz ölçümü Bragg-Gray kavite teorisine dayandığından bu amaçla kullanılacak iyon odaları ile ölçüm fantomlarının teoride öngörülen koşulları içermesi gerekmektedir. Yüksek enerjili radyasyonlar için önerilen silindirik tipteki iyon odalarının duvar, elektrot ve ölçüm fantomları değişik maddelerden yapılmış olduğundan Bragg-Gray koşullarını taşımamakta ve böylece doz ölçümü için belirsizlikler içeren pek çok düzeltme faktörünün kullanılmasına gerek duyulmaktadır. Düşük enerjili elektronlar için paralel elktrotlu iyon odaları önerilmişsede şimdiye kadar yapılmış bu türdeki iyon odaları ile suda soğurulan doz, doğrudan su ortamında ölçümüne olanak vermektedir. Bu çalışmada, klinik olarak kullanılan her enerjide ki elektron ve yüksek enerjili foton demetlerinde doğrudan suda soğurulan doz ölçümüne olanak veren, Bragg-Gray teorisine uygun, ölçüm fantomu dahil bütün elemanları su ya eşdeğer maddeden oluşan paralel elektrotlu katı su iyon odası tasarlanarak yapımı gerçekleştirilmiştir. Katı su iyon odasının ikincil standart dozimetri ve kalibrasyon laboratuvarlarında Nd, Nk, ve Nx kalibrasyonları yapılmış ve yapısal kpp parametresi tayin edilerek yeni dozimetri protokolları ile soğurulan doz tayinine olanak sağlanmıştır. Paralel elektrotlu katı su iyon odasında uygun genişlikte bir koruyucu elektrot sistemi ile elektron ve foton pertürbasyonundan, ölçümlerin etkilenmesi önlendiği, gibi, homojen yapısı nedeniyle silindirik ve küresel iyon odalarında karşılaşılan pek çok sorun ve hata kaynağı ortadan kaldırılmıştır. vi

Özet (Çeviri)

SUMMARY The widespread use of high energy accelerators ca pable of production high energy electron and photon beams has emphasized, the problems encountered with the cali bration of these beams for application in radiation the rapy. It has been demonstrated that the success or fail ure of radiation treatment depends on the absorbed dose delivered to the tumor volume and that this should not vary by more than few percent from the prescribed values. Thus, the International Commission on Units and Measure ments (ICRU) concludes that there is need for an accuracy of ± % 5 in delivery of absorbed dose to the tumor volume [1]. Recent investigations indicate that an even better accuracy is needed for some treatment situations [2]. Therefore calibration of radiation beam used for the radiation therapy becomes very important. It is clear that the Faraday cap, calorimeter and Fricke dosimeters, which* provides absolute absorbed dose measurement are not suitable because of beeing unsensi- tivity at lower doses, bulky and complicated for routine calibration of high energy radiation therapy machines. Therefore instead of using this kind of complicated abso lute dosimeters, it has been recommended to use more sensitive and practical dosimeter by dosimetry protocols. While these protocols prtovide radiological physicist with an accurate methods for determining the absorbed dose to water from high energy photon and electron beams. These protocols recommended to use ionometric methods of dosi metry using commercially available ionization chambers for calibration of therapy machines. All ionometric dose measurements for high energy radiations are based on Bragg-Gray cavity theory. The ionization chamber that is used clearly must achieve close approximation to ideal Bragg-Gray conditions. The introduction of an air-filled cavity into a medium irra diated by electrons gives rise to a perturbation of the electron fluence present in the undisturbed medium [18]. This perturbation effect increases with a decreases in electron energy due to the large scattering electrons at low energies. The commonly used cylindrical ionization chamber recommended for the absorbed dose measurement in electron and in high energy photons beams gives a pertur bation of several percent below Ez= 10 MeV. Due to the uncertainty in the perturbation correction factors for cylindrical ionization chambers, plane-parallel chambers are recommended for electron energies Eo below 15 MeV. viiIn order to make an accurate absorbed dose measure ments, the design and construction of the ionization chambers must satisfy a number of criteria such as: (i) Homogeneous construction: chamber walls and phantom of same material, (ii) Negligible perturbation of particle and photon f luence, (iii) well-defined collection volume: collection elec trode both guarded and electrostatically shield ed, (iv) Wel}.-def ined point of measurement, (v) High collection efficiency in pulsed radiation beams, (vi) Short and long term stability, (vii) Permit accurate conversion of cavity ionization to the absorbed dose in water. Commonly used heterogeneous cylindrical chambers constructed of different materials which do not match the phantoms subject to many uncertainties concerning the origin of cavity ionization and the subsequent conversion of ionization to dose. Most of the commercially avail able plane-parallel ionization chambers, which are con structed more homogeneous are not suitable for low energy electron beams. Also none of the plane-parallel chambers are not adequate to measure the absorbed dose directly to water. In this work a water equivalent plane-parallel ion ization chamber has been developed for determination of absorbed dose in all range of energy of electron and high energy photon beams. All components of the ionization chambers including the measuring phantom have been made from solid water. The collecting electrode is effective ly guarded and shielded electrostatically in all direc tions by covering the external surface of a lower plate with a layer of conducting graphite which is also connec ted to the guard electrode. This type of homogeneous construction satisfies in close approximation the Bragg- Gray cavity theory and fulfills criteria listed above. The absorbed dose to air calibration Nd.pp of the plane-parallel water equivalent chamber has been deter mined by comparison with a cylindrical ionization chamber with a known Nd in high energy electrons and saCo photon viiibeams. Also air kerma calibration factor (Nk) has been carried out in accredited secondary standard dosimetry laboratory (SSDL). By using these two factors (Nd and Nk) the characteristic chamber parameter kpp has been deter mined experimentally. Also various physical parameters l which are required to be considered for an ionization chamber used for the calibration of high energy radio therapy machines have been investigated in actual working conditions and the following results have been obtained: (i) Homogenity: All the components of the ionization chamber including the measuring phantom have been made from solid water. For this reason composition of the measuring system (chamber and phantom) contains only two types of material which are solid water and air. This type of construction satisfies the homogenity condition of the Bragg-Gray theory. Avoids to use the phantom/water conversion factor (hm), which con verts absorbed dose to water. (ii) Shape and volume: The shape and volume of the plane-parallel chamber are chosen in such a way that (coin shape and less than 1 cm3) they do not disturb the electron fluence and fluence density in irradiated medium. (iii) Perturbation effect: The width of the guard electrode surrounding the collecting electrode is about 5 mm. This range of guard is large enough to prevent the in-scattered electrons to reach in the collecting volume. The measurement of the electron perturbation (in-scattering) from side-walls of coin-shaped cavity were car ried out at Eo=6, 9, 13, 17 and 21 MeV electron beams by irradiating films in the cavity at the dose maximum in solid water phantom. It has been found that the perturbation of the electron fluence and hence absorbed dose does not extend into sensitive region of the air volume. So the problem of perturbation is overcomed and there is no need to use correction factor (Pu) at any range of electron energy. (iv) Polarity effect: A plane-parallel chamber must be designed in such a way that it does not have large polarity effect. For a given irradiation the absolute value of the collected charge should be independent of polarity on the high voltage electrode of the chamber. The polarity ixeffect depends on the depth of the chamber in the phantom and may be of different sign at small and large depths. The acceptable value of polarity effect for an ionization chamber is: OQ-I-IQ-I )/(lQ-i+IQ-l ) < 0.005 In order to reduce the polarity effect to the acceptable level for plane-parallel solid water chamber, the mass of insulating material under the collecting electrode has been kept as small as possible and electrostatically shielded in all directions. The effect of reversing of the voltage polarity applied to an ionization cham ber (polarity effect) has been investigated in electron and photon beams for several irradi-. ation conditions and has been found less than 0.005 in all depths of maximum dose and energies using in radiotherapy. However, in all cases, the avarage of readings with both polarities is a very good representation of the ionization in air of the chamber cavity. (v) Effective point of measurement: The effective point of measurement for plane-parallel solid water ionization chamber is at inner conducting surface of upper electrode, which is very easy to set up the calibration distance for output measurements. (vi) Ion collection efficiency: The ion collection efficiency of an ionization chamber is the ratio of the rate of charge collection to the rate of charge production within the effective volume; it depends, among other things, on the volume rate of charge formation, the electron and ion mobilities, the recombination rates, and the electronic field strength. The collection volt age, between the the electrodes of the chamber should not be so large as to cause avalanches or breakdown of the insulation, but should be suf ficient to insure that most of the charges re leased in the chamber are collected. The ion collection efficiency of plane-parallel solid water chamber has been measured for ac- cordig to the methods described in AAPM and IAEA protocols. The collection efficiency with a polarization potential + 300 V, for 8°Co conti nuous radiations at rate of 0.5 Gy/minute, for high energy pulsed x-ray beams at 2.5 Gy/minuteand for magnetically scanning pulsed electron beams at 3 Gy/minute has been found 100, 99.7 and 96.2 percent respectively. The loss of ion collection efficiency in con tinuous radiation and high energy pulsed x-ray beams is not a serious problem, but this is not true for magnetically scanned pulsed electron beams. (vii) Depth resolution: The spatial depth resolution has been found less than 1 mm. (viii) Short and long term stability: The short and long term stability found within ± 0.02 percent. (ix) Leakage current: Pre irradiation leakage has been found less than 7 femtoamper. (x) Calibration and absorbed dose determination: Two alternative metods performed for derivation of the absorbed dose to air calibration factor, Nd,pp for plane-parallel solid water chamber: (i) by comparison with NE.2571 cylindrical ion ization chamber with known Nn in a high energy electron beam in clinical condition, and (ii) by comparison with a NPL secondary standard cylin drical chamber in BOCo radiation beam at ac credited secondary standard dosimetry labora-, tory. The plane parallel solid water ionization cham ber has been also calibrated against secondary standard dosimeter which is traceable to the primary standards in terms of air kerma, (Nk) and structural parameter kpp has determined experimentally. After determination of these two parameters (Nk, kPP) ionization chamber has been made available for absorbed dose measure ments which are suitable for the methods in new dosimetry protocols. It has been shown that when the homogeneous con structed, plane-parallel water equivalent ionization chamber to used in a solid water phantom for calibration of the high energy radiation therapy machines, it exhib its very close Bragg-Gray cavity behaviour, and avoids the use of many conversion and correction factors which are subject to uncertainties. The water equivalent 3d.plane-parallel chamber provides an accurate, reliable and simple method for absorbed dose determination in electron and high energy photon beams. xii

Benzer Tezler

  1. Tez No

    142096

    UAEA'nın TRS277, TRS381 ve TRS398 no.lu soğurulan doz protokollerinin farklı foton ve elektron enerjilerine göre karşılaştırılması

    Comparison of IAEA absorbe dose protocols TRS277, TRS381 and TRS398 for different photon and electron energies

    HİLAL ACAR

    Yüksek Lisans

    Türkçe

    Türkçe

    2004

    Radyoloji ve Nükleer TıpHacettepe Üniversitesi

    Radyoterapi Fiziği Bilim Dalı

    PROF. DR. FADIL AKYOL

  2. Tez No

    271458

    Ametist'in termolüminesans ve optiksel özelliklerinin araştırılması

    Searching thermoluminescence and optical properties of amethyst

    SERA İFLAZOĞLU

    Yüksek Lisans

    Türkçe

    Türkçe

    2010

    Fizik ve Fizik MühendisliğiNiğde Üniversitesi

    Fizik Ana Bilim Dalı

    YRD. DOÇ. DR. ADİL CANIMOĞLU

  3. Tez No

    34180

    ESR tekniği ile doz ölçümü ve bunun endüstriyel-sağlık alanlarındaki uygulamaları

    Dose measurement by ESR technigue and the applications of these in industrial-healt areas

    TALAT AYDIN

    Yüksek Lisans

    Türkçe

    Türkçe

    1994

    Fizik ve Fizik MühendisliğiHacettepe Üniversitesi

    Fizik Ana Bilim Dalı

    PROF. DR. MUSTAFA KORKMAZ

  4. Tez No

    540575

    Siverek ilçesi toprak, su ve kaya numunelerindeki doğal radyoaktivitenin ölçümü.

    Determination of natural radioactivity concnetrations in Siverek town soil, water and stone samples

    MEHMET ALTINTAS

    Yüksek Lisans

    Türkçe

    Türkçe

    2019

    Fizik ve Fizik MühendisliğiHarran Üniversitesi

    Fizik Ana Bilim Dalı

    DR. ÖĞR. ÜYESİ MEHMET KOŞAL

  5. Tez No

    373782

    Uşak ili ve çevresinin doğal radyoaktivitesinin ve yıllık etkin doz eşdeğerinin belirlenmesi

    Determination of natural radioactivity and annual effective equivalent dose in Uşak province in Turkey

    ÖNDER AKAR

    Doktora

    Türkçe

    Türkçe

    2014

    Fizik ve Fizik MühendisliğiCelal Bayar Üniversitesi

    Fizik Ana Bilim Dalı

    PROF. DR. İSMAİL MARAŞ

Geri Dön