$8 20 The Use of Alanine in Conformal Radiotherapy Dosimetry. B. Behaeken(1), D. Verellen(2), L. Bellekens(3), M. Cogghe(3), D. Van den Weyngaert(1). (1)A.Z. Middelheim, Lindendreef 1, B-2020, Antwerpen, Belgium. (2)A.Z. VUB, Laarbeeklaan 101, B-1090 Brussel, Belgium (3)St. Vincentius Ziekenhuis, St. Vincentiusstraat 20, Antweq~en,Belgium. Dosimetry with alerting is based on the relative and non destructive measurement of stable free radicals produced by radiation. Absorbed dose to the detector is measured as the signal intensity of the central line o1 the eleclron paramagnetic resonance (EPR) powder spectrum o1 alanine. The following experiments prove that solid alanine detectors can be used as reliable transfer dosimeters to check complicated treatment setups in tote for radiothearpy. Experiments have been set-up as a blind test in which neigther the details of irradiation (choice of beam ballistic, beam energy, position of the alanine detector in the phantom, the number of isocenters...) nor the dose administered were previously reported to the read out center. The detector volume (4.8mm diameter, 10 mm length) was outlined on the CT images as target volume and checked with the physical volume of the detector. After irradiation the detectors were returned tot read out using the calibration for Co-60. A) stereotactic treatment: two tests were performed using a water filled head phantom immobilized into a Leksell frame. In a first experiment a single isocenter treatment plan with 5 arcs was performed using a 6 MV photon beam with collimator aperture oi 15 mm and 12 mm resulting in a calculated dose to the detector of 32.5Gy alter evalualing the dose- volume histogram; measured dose wets 32.15 Gy. The experiment was repeated using 8 arcs and a collimator aperture of 40 mm resulting in a more homogeneous dose distribution around the detector: the ratio of specified- to measured dose was 0,995. B) conlormal treatment: again two experiments had been carried out using the Peacock system ® for intensity modulation of the radiation beam (6 MV ) during treatment. In a first experiment one alanine detector was placed into an Alderson head phantom immobilized with a thermoplast mask and irradiated to 20 Gy yielding a ratio between specified and measured dose of 1.01. A second experiment simulaled a brain stem irradiation: one detector was placed at the location of the brain stem and two more bilaterally in the target volume.The measured dose corresponded within 0.5% with the specified dose at the three locations. 22 MR] POLYMER GEL DOSIMETRY PHANTOM FOR EXTERNAL BEAM RADIOTHERAPY OF THE BREAST C.Baldock*, C Hasler*, S.FKeevil**, AGGreener***, **** ***** * N C Billmgham , ILBurford Royal Sussex Coonty Hospital, UK; **United and Medical Dental Schools, London, UK, ***Guy's & St.Thomas' Hospital, London, UK; **** University of Sussex, Brighton, UK;; ***** Umversity of New South Wales, Australia Baekgroond Magneuc resonance tmagmg (MRI) may be used to measure dose distributions of ionising radiataons in tissue equivalent gels In one such technique polymerisation is induced in gelatin gels mixed with aerylamlde and N,N'-methylene-bis-acrylan-ude (bis), commonly known as BANG. These gels can be used in anatomically accurate phantoms to simulate dose distnbataons in radiation therapy and hence to verify computer generated treatment plens. In this work, a phantom was designed to mvesUgate radiation dose distributions from external beam radiotherapy of the breast. Materials and Methods The phantom 'shell" was constructed from 3ram sheets of Barex (BP Chermcals Ltd.), a co-polymer of acrylonitrile and methyl acrylate unpact modified with mtrile rubber. The sheets were moulded over an ART breast phantom (Alderson Research Laboratories Inc.) using a Pamavac forming device (Pamall & Sons Ltd). The shell was filled with BANG gel, as described previously [1] and imaged on a Philips CX/Q CT scanner. The measured Hotmsfield CT number of the gelwas -31.6 + 83 The radiotherapy was planned on a HELAX TMS treatment pluming system. The phantom was then "treated' using a ABB LA6 linear accelerator of nominal energy 6 MV. Imaging was undertaken using a Philips Gyroscan 0.5T (22 IvlHz) MILl scanner and interleaved inversion recovery / spin echo ('mixed') sequences. Relaxation tmae maps were ~ enerated on a ptxel-by-pixel basis using manufacturer's software amples of gel were calibrated separately to enable production of 3- dimensional tsodose contour plots from the relaxation tame maps. These were compared wath those generated on the HELAX txeatment plm'ming system. Results and Conclusions This work has usefully demonstrated how MRI polymer gel dosimetry may be used to generate a continuous 3-dimensional dose distribution, enablingthe treatment planning process to be validated. I C.Baldock et el. Prec. Intemat. Soc Magn Reson Med. 1996:1594 21 COMPARISON BETWEEN POLYMER-GEL AND FILM DOS1METRY IN ANTHROPOMORPHIC PHANTOMS C. De Wagter, Y. De Deene, B. Mersseman, and W. De Neve. Dept. of Radiotherapy and Nuclear Medicine, University Hospital, Gent. BELGIUM basic mechanism of polymer-gel dosimeU'y is that radiation induces a polymerization process which, in torn, enhances the magnetic resonance (MR) rdaxivity parame=r R2 = l/T2. This allows, in principle, a three-dimensional (3D) dose acquisition using a T2-imaging procedure on a clinical MR-scanner. Wc are investigating the potentials of the method to measure complex dose distributions as being envisaged by conformal radiotherapy. Film dosimeu'y is done in parallel. A first comparative experiment involved the conformal t~eamaent of a concave target that partially surrounded the spinal cord at the lower neck. A cast modelled according to the RANDO phantom was filled with the gel while, for film dosimeu'y, radiographic films wcra inserted between subsequent sections of the actual RANDO phantom. The comparison of the film-measured dose distributions with the corresponding gel- measured dose images d emo~ an overall root mean square deviation ( R M S D ) of I0 %. This RMSD value is about the double of what we obtained when comparing gel and ionisafion chamber dosimetry of depth-dose curves and cross-section profiles of standard beams. A first source of discrepancy is that film is basically a 2D method, while MR meusums a slice of finite thickness. A potential problem with film dosimelay is the dependence of sensitometric curve with respect to film orientation. Gel, in contrast, is water equivalent and responds practically independently of beam quality. Issues under investigation regarding gel dosimetry are : spatial variations in dnse-respon,~, oxygen residues that initially inhibit the polymerization process, magnetic- susceptibility effects and radiofrequency inhomogeneities during MR-scanning. This work is supported by grant # 01 IV1395 of the University of Gent and the prize "Rimaux-Bartier". 23 APPLICATION OF MOSFETS IN RADIE}THERAPY DOSIMETRY P Frmccecon t . [~. 5 c a l c h l t . $. Core J , A.Teeto[in -" . R. Gt.~ch~ ; , M . Balfi ~ I Scrvifio di}'isic= Smfiuaia, ULSS n° 6,Vicmza.llaly 2 Divi.ion¢ di Rad~o~rapia. D'L.SS n*6, Viccnza. Italy Metal-Oxide-Field-Effect Transistors (MOSFETs) can be used m Radiotherapy as radiation detectors. Their marn features are: - inherent small size; - dose-response usually highly reproducible, with an order of 2.5 % {2c0 both tbr single dosimeter and for a batch of dosimeters; temperature and dose-rate independence; - munediate response unlike other dosimeters (e.g. TLD). The latter two make MOSFETs usefid for patient dosunetry tbr standard Radiotherapy and special techniques as Total Body Irradiation ~TBI). In our clinicalpractice w e use MOSFET for a quality assurance prod'am of the treatments involving thorax, head and neck irradiation and tbr the es6.mation of the dose distribution in TBI by in-rive dosimeWy. In this paper the results of a comparison between the dose at different points in the midline plane of a targe~ volume, calculated by a treatment planning and estimated by in-v~vo dosimetry, are presented. View publication stats View publication stats