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Radiotherapy Gel Dosimetry

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Radiotherapy Gel Dosimetry IAEA-CN-96-117 XA0203913 RADIOTHERAPY GEL DOSIMETRY C. BALDOCK Radiotherapy Physics and Dosimetry Group Centre for Medical, Health and Enviromnental Physics Queensland University of Technology, Brisbane, Australia In radiotherapy, the primary objective is to deliver a prescribed dose of radiation to a turnour or lesion within a patient while minimising the dose delivered to the surrounding healthy tissue. Traditional radiotherapy treatments usually involve simple external or internal irradiations of a turnour. External irradiations are nornially achieved in the clinic with photon or electron beams produced by high energy linear accelerators. The photon or electron beams are collimated into regular shapes as they emerge from the treatment head of the unit which is supported by a gantry that can be rotated isocentrically to any position. A discrete number of photon or electron beams with different angles of incidence that intersect at the iso-centre are used to produce a region of high dose around the tumour volume (positioned at the sio-centre). Internal irradiations are non-nally achieved in the clinic by implanting radioactive sources in and around the turnour or lesion. Such irradiations are characterised by very high doses local to the turnour. Radioactive sources are also used to prevent post-angioplasty restenosis by inserting sources into arteries. Usually when treating a turnour a compromise is made between tumour control and complications arismig from non-nal tissue damage. One measure of this compromise, the therapeutic ratio, is defined as the radiation dose producing complications 50% of patients divided by the dose providing turnour control in 50% of the patients. The therapeutic ratio depends on the radiobiological characteristics of the cancerous tissue and surrounding healthy tissues and on the radiation dose distribution achieved by the radiotherapy treatment. It is generally believed that the therapeutic ratio can be minimised by optimising the conforination of the radiation dose distribution to the target volume. his is difficult with traditional radiotherapy techniques since they do not produce dose distributions that adequately cover turnour volumes of complex shapes and sizes while sparing normal tissue. The situation is further complicated if the normal tissues are critical organs or are particularly sensitive to radiation. Radiotherapy techniques employed to obtain a closer conformation of the dose distribution to the turnour volume are referred to as confort-nal radiotherapy techniques. The clinical implementation of confon-nal therapy has been delayed by limitations in the verification of conformal dose distributions calculated by treatment planning systems prior to the irradiation of the patient and the verification of complex treatments during its delivery to the patient. There are several aspects of conformal therapy that complicate dose verification. To achieve the dose distributions conforming to complex 31) volumes, high dose gradients arise in the treatment volume. Further, overdose or underdose regions can exist when separate radiation fields are used to deliver additional radiation. These aspects require that practical dose measurement (dosimetry) techniques be able to integrate dose over time and easily measure dose distributions in 31) with high spatial resolution. Traditional dosimeters, such as ion chambers, then-noluminescent dosimeters and radiographic fil do not fulfil these requirements. Novel gel dosimetry techniques are being developed in wch dose distributions can potentially be determined vitro in 3D using anthropomorphic phantoms to simulate a clinically iradiated situation. As long ago as the 1950's, radiation-miduced colour change in dyes was used to investigate radiation doses in gels [1]. It was subsequently shown that radiation induced changes in nuclear magnetic resonance (NMR) relaxation properties of gels infused with conventional Fcke dosimetry solutions [2] could be measured using magnetic resonance imaging (MRI 3 In Fricke gels, Fe 21 ions in ferrous sulphate solutions are usually dispersed throughout a gelatin, agarose or PVA matrix. Radiation-induced changes the dosimeters are considered to be either through direct absorption of ionising radiation or via intermediate water free radicals. Fe 2+ions are converted to Fe'+ ions with a 222 IA]EA-CN-96-117 corresponding change in paramagnetic properties that may be quantified using NNIR relaxation measurements or optical techniques. Due to predominantly diff-usion-related limitations 4 (and references therein), alternative polymer gel dosimeters were subsequently suggested [5]. In polymer gels, monomers such as acrylan-dde and NN'-methylene-bis-acrylamide are usually dispersed in a gelatin o agarose matrix. Monomers undergo a polymerisation reaction as a function of absorbed dose resulting in a D polymer gel matrix. The radiation-induced formation of polymer influences NMR relaxation properties and results in other physical changes that may be used to quantify absorbed radiation dose (Fig. 1). As well as NIRI, other quantitative techniques for measuring dose distributions include X-ray computer tomography, vibrational spectroscopy and ultrasound. Clinical applications of these radiologically tissue equivalent gel dosimeters have been reported n the literature. For further infortnation of gel dosimetry and specifically clinical applications the proceedings of the Td international onference on Radiotherapy Gel Dosimetry 6 and references therein should be consulted. 4.5 -7 T_ 4.0 - M (on15 3.5 4)W .Z .7 3.0 C X Cy)M 2.5 co 75 E a 2.0 - Z 1.5 1.0 L10 40 50 Absoted Dose (Gy) Fig. 1. Change in nuclear magnetic resonance transverse relaxation rate as a fnction f absorbed dose REFERENCES [11 Day M J and Stein G, 1950. Chemical effects of ionizing radiation in some gels. Nature 166 146-7. [2] Fricke H and Morse SI, 1927. The chemical action of roentgen rays on dilute ferrous sulfate solutions as a measure of radiation dose. Am. J RoentgenoL Radium Therapy NucL Med. 18 430-2. [3] Gore J C Kang Y and Schulz R J, 1984. Measurement of radiation dose distributions by nuclear magnetic resonance (NMR) imaging. Phys. Med. Biol. 29 1189-97. [4] Baldock C Harris P J, Piercy A R and Healy B, 2001a. Experimental determination of the diffusion coefficient in two-dimensions in ferrous sulphate gels using the fite element method. Aust. Phys. Eng. Sci. Med. 24 19-30. [5] Maryanski M J, Gore C Kerman R P and Schulz R J, 1993. NMR relaxation enhancement in gels polymerized and cross-linked by ionizing radiation: a new approach to 3D dosimetry by MRI. Magn. Reson. Imag. 11 253-8. [6] DOSGEL 2001. Proceedings of te Td International Conference on Radiotherapy Gel Dosimetry. Eds. C. Baldock and Y. De Deene (Queensland University of Technology, Brisbane). 223.
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