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94 An optimized distance-strength relationship for planning Ir-192 double-plane implants
93 CVD diamond as a thermoluminescent dosimeter for brachytherapy: measurement of the dose build-up in HDR microSelectron Ir-192 source calibrations. P Hopwood and Colin H Jones, Joint Departmentof Physics, The Royal Marsden NHS Trust and The Institute of Cancer Research,Fulham Road, London SW3 655,UK In the course of evaluating Chemical Vapow Depositimr(CVD) syn&fic diamondas an alternative to LiF for radiotherapy dosimetry, we have investigated its use as a TL dosimeter for various problems in brachytherapy. One of the advantagesof CVD is that it can be produced in thin (loo-200~) films so it can be used to study build-up dosesat interfaces Diamond is well suited to making clinical measurements.It is non-toxic and can be sterilised: it has a low Atomic No (2=6). We have used a Harshaw 3500 TLD reader run from a Dell 486 computer: the read out cycle consisted of a pre-heat of 240°C for 16 s followed by signal acquisition for 30 s during which the T was raised at the rate of 10°C / s Using an HDR 370 GBq microselectron Ir- 192 source, “I we have found that CVD gives 011 reproducible results: the S.D I, over five measurementsof a 111 fixed dose was 1% linearity ~IL ovzr 0 - 1 Gy was excellent ’ b,ti~-d~,,,’ IQ ” (R = 1.000). We have studied the build-up requirementsfor in-air calibration measurements of lr-192 sources using 100~ thick CVD dosimeters.Goetsch et al (1991) have drawn attention to (I) the need td correct for secondary electrons from the source capsuleand (2) the need to use a chamberof adequatewall thickness.The figure shows in-air measurementsat IOOmmthat have beenmadewith thin CVD and illustrate the presenceof a high surface dose (due to electrons) and a small build-up of dose within l-2 mm depth.
94 An optimized dlstancwtrength relationship for planning k-192 double-plane implants B. Roth, V. Bourel, S. Caneva Universidad de Buenos Aires, Institute de Oncologia Angel H. Roffo Departamentode Radioterapia Av. San Martin 5481, CP(1417). Cap. Fed. Bs. As. Argentina Introduction : In the Paris bracbytberapy system, to deliver a prescribed dose to the isodose contour associatedwith tbe 85% of the basal dose and equally-spaced array of ribbons or wires of tbe same strength is commonly used. Implant parameters such as uniformity and tre.ated volume outside the treatment isodose contour. deoend critically on the distance between ribbons and planes and tbe seed strength. We want to know the distance that optimizes implant parametersfor any seed strength between 0.4 and 2 mCi. Materials and Methods : The distribution of volume as function of dose rate of a configuration of three ribbons per plane of five seeds each, was analysed varying both distance from 0.2 to 2.5cm and strength from 0.4 to 2mCi. Volume-dose data were obtained from Monte-Carlo calculations, considering point isotropic sources and a third order polynomial to describe attenuation in water. In order to evaluate implant parameters, the corresponding Anderson’s volumedose histograms were performed. Those configurations with higher uniformity and minimum peripheral treated volume were chosen as optimum. Results : Distance-strength data corresponding to those optimized configurations were plotted and a fifth order polynomial resulted from curve fitting. Conclusion : For any seed strength between 0.5 and 2.5 mCi, optimum distance can be exaapolated from the curve. However, for lower values than 0.5 mCi no ootimum distance has been found. A comparison between implant paiameters corresponding to optimized configurations show that increasing strength results on lower values of uniformity and treated volume outside the treatment isodose contour.
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96
OPTIMIZATION OF SOURCE SEPARATION FOR CONSTANT REFERENCE DOSE RATE OF A PARIS SYSTEM
SIMULATION. DESCRIPTION AND INTERPRETATION OF THE ENERGY RELEASE OF I-125 SEEDS FOR BRAIN TUMOURS BRACHYTHERAPY. St&m AGOSTEO’,ArmandoFOGLIO PARA’, Enrico BORTOLAZZI’“. RobettoFORONI”. Arman& PASOLI ‘Dipartlmmtodi lngegneriaNudeare,Politecnxo di Milano. Italy. “Dipanimmto di Neurochirurgta,Ospedaledi BorgoTrmto.Verona.Italy
T. Lahtinen and M. Juntunen,Department of Radiotherapy and Oncology, Kuopio University Hospital, Kuopio, Finland Due to a radioactive decay of ‘“Ir, low dose rate iridium wires with varying specific activity are available for interstitial brachytherapy at the departmentsof radiotherapy. To maintain a constant dose rate and thus a radiobiological effect for a dose specification point, the source separation of an implant for the available specific acticity and wire lengths has to be determined. We have calculated the source separation for a Paris reference dose rate of 10 Gy/day using several wire lengths and specific activities. First an arbitrary basal dose point from one of the middle triangles is chosen to form a dose rate equation. The equation is then solved numerically by Nelder-Mead algorithm. An example of a typical implant and calculated source separations (cf) for specific activities 1 = 0.5 - 1.2 mCi/cm and wire lengths L = 2 - 10 cm is shown below. The separation of the planesis &/2xd=OS7xd.
The precalculated tables for several geometries are usehI when source configuration with flexible implants is planned. Using a correction for a source strength the tables can also be used for HDR or PDR treatments if a constant dose rate in these treatments is considered necessary
The dose distribution around a brachytherapy seed in which a I- 125 source is inserted has been described by several authors following different mathematical approaches and providing di&reut approximation formulae. lu the present work the problem has beeu revisited by means df a detailed simulation which utijizes the MCNP code iu the photon-electron mode and which takes into account the complex structure of the seedsin common use for brain tumour treatments and the iutoractious of the emitted radiations with the seed materials. The considered brain tissue-equivalent material is water. The geometrical description was defined so as to optimise the statistical precision of the data. As a result, au accurate description of the dose distriiution has been obtained, particularly iu the zones around the seed. Moreover, the energy release in the seed itself has been accurately evaluated. The detailed data allowed to define simple and accurate interpolation formulae for the radii1 and angular dose distributions around the considered seed structures up to distances of 5 cm The goodness of the obtained fits is clearly demonstrated by +e improvement of the cl&square parsnwters by at least of an order of magnitude with respect to other formulae usually adopted. In addition, the interpolations enable to precisely quantify the total energy release both in the therapy zone and iu the seed and to check their correspondence with the emitted energy resulting from the nuclear data. The dierent formulae cau therefore be validated in a simple and direct way and the apparent source intensities cau be stated on physical grounds. The adopted interpolation formulae can also be easily integrated in larger zones around the seeds, to provide simple figures of merit of the dose distributions.
94 An optimized dlstancwtrength relationship for planning k-192 double-plane implants B. Roth, V. Bourel, S. Caneva Universidad de Buenos Aires, Institute de Oncologia Angel H. Roffo Departamentode Radioterapia Av. San Martin 5481, CP(1417). Cap. Fed. Bs. As. Argentina Introduction : In the Paris bracbytberapy system, to deliver a prescribed dose to the isodose contour associatedwith tbe 85% of the basal dose and equally-spaced array of ribbons or wires of tbe same strength is commonly used. Implant parameters such as uniformity and tre.ated volume outside the treatment isodose contour. deoend critically on the distance between ribbons and planes and tbe seed strength. We want to know the distance that optimizes implant parametersfor any seed strength between 0.4 and 2 mCi. Materials and Methods : The distribution of volume as function of dose rate of a configuration of three ribbons per plane of five seeds each, was analysed varying both distance from 0.2 to 2.5cm and strength from 0.4 to 2mCi. Volume-dose data were obtained from Monte-Carlo calculations, considering point isotropic sources and a third order polynomial to describe attenuation in water. In order to evaluate implant parameters, the corresponding Anderson’s volumedose histograms were performed. Those configurations with higher uniformity and minimum peripheral treated volume were chosen as optimum. Results : Distance-strength data corresponding to those optimized configurations were plotted and a fifth order polynomial resulted from curve fitting. Conclusion : For any seed strength between 0.5 and 2.5 mCi, optimum distance can be exaapolated from the curve. However, for lower values than 0.5 mCi no ootimum distance has been found. A comparison between implant paiameters corresponding to optimized configurations show that increasing strength results on lower values of uniformity and treated volume outside the treatment isodose contour.
95
96
OPTIMIZATION OF SOURCE SEPARATION FOR CONSTANT REFERENCE DOSE RATE OF A PARIS SYSTEM
SIMULATION. DESCRIPTION AND INTERPRETATION OF THE ENERGY RELEASE OF I-125 SEEDS FOR BRAIN TUMOURS BRACHYTHERAPY. St&m AGOSTEO’,ArmandoFOGLIO PARA’, Enrico BORTOLAZZI’“. RobettoFORONI”. Arman& PASOLI ‘Dipartlmmtodi lngegneriaNudeare,Politecnxo di Milano. Italy. “Dipanimmto di Neurochirurgta,Ospedaledi BorgoTrmto.Verona.Italy
T. Lahtinen and M. Juntunen,Department of Radiotherapy and Oncology, Kuopio University Hospital, Kuopio, Finland Due to a radioactive decay of ‘“Ir, low dose rate iridium wires with varying specific activity are available for interstitial brachytherapy at the departmentsof radiotherapy. To maintain a constant dose rate and thus a radiobiological effect for a dose specification point, the source separation of an implant for the available specific acticity and wire lengths has to be determined. We have calculated the source separation for a Paris reference dose rate of 10 Gy/day using several wire lengths and specific activities. First an arbitrary basal dose point from one of the middle triangles is chosen to form a dose rate equation. The equation is then solved numerically by Nelder-Mead algorithm. An example of a typical implant and calculated source separations (cf) for specific activities 1 = 0.5 - 1.2 mCi/cm and wire lengths L = 2 - 10 cm is shown below. The separation of the planesis &/2xd=OS7xd.
The precalculated tables for several geometries are usehI when source configuration with flexible implants is planned. Using a correction for a source strength the tables can also be used for HDR or PDR treatments if a constant dose rate in these treatments is considered necessary
The dose distribution around a brachytherapy seed in which a I- 125 source is inserted has been described by several authors following different mathematical approaches and providing di&reut approximation formulae. lu the present work the problem has beeu revisited by means df a detailed simulation which utijizes the MCNP code iu the photon-electron mode and which takes into account the complex structure of the seedsin common use for brain tumour treatments and the iutoractious of the emitted radiations with the seed materials. The considered brain tissue-equivalent material is water. The geometrical description was defined so as to optimise the statistical precision of the data. As a result, au accurate description of the dose distriiution has been obtained, particularly iu the zones around the seed. Moreover, the energy release in the seed itself has been accurately evaluated. The detailed data allowed to define simple and accurate interpolation formulae for the radii1 and angular dose distributions around the considered seed structures up to distances of 5 cm The goodness of the obtained fits is clearly demonstrated by +e improvement of the cl&square parsnwters by at least of an order of magnitude with respect to other formulae usually adopted. In addition, the interpolations enable to precisely quantify the total energy release both in the therapy zone and iu the seed and to check their correspondence with the emitted energy resulting from the nuclear data. The dierent formulae cau therefore be validated in a simple and direct way and the apparent source intensities cau be stated on physical grounds. The adopted interpolation formulae can also be easily integrated in larger zones around the seeds, to provide simple figures of merit of the dose distributions.