Of the use of new EBT3 for dose profile and dose-area product measurements in small fields

Abstract

)coupled through optical fibers to a two-channel photodetection module.The module is basically composed of two silicon photomultipliers syn-chronized together by a FPGA card. The two probes are positioned in aPMMA cylinder, at 18 mm away from its axis and spaced apart by 29 mm.A 2 mm diameter source transfer catheter is inserted into the cylinderaxis.Testing results have shown that, the seed activity can be estimated with aprecision better than 5 % when it is positioned in front of one of the twodosimetric probes. This positioning is chosen to estimate the dose with thehighest measurement SNR and the lowest sensitivity on seed positioningerrors (There is a low dose rate gradient when the seed is facing the probe).With the acquisition of a treatment plan used as a reference, the dwell posi-tion can be estimated with sub-milimetric accuracy over a range of 20 mm,and the dwell time with a mean error of 24 ms. It also integrates measure-ments of thetransit timeduringtheintroduction andremovalof the source.Thus, it is possible to define the dose at any point in space from the activityreferencesourcebyusingthedosecalculationformalismTG-43AAPM-ESTROconsensus [2]. The difference between the dose calculated by the systemsoftware and the TPS is less than 4% for the consideredpoints of interest.Theproposedsystemallowsanentirequalitycheckoftheafterloaderbeforetreatment.Bycomparingmeasuredvalueswithexpectedones,italsoallowsreal-time detection of different types of errors such as bad source activity,treatment protocol error, in-catheter curvature-induced position errors [3].Keywords: Brachytherapy, DoRGaN, Quality assuranceReferences[1]R.Nath,L.L.Anderson,J.A.Meli,A.J.Olch,J.A.Stitt,andJ.F.Williamson,“Code of practice for brachytherapy physics: Reportof the AAPM RadiationTherapy Committee Task Group No. 56,”Med. Phys. Vol 24 (1997),p.1557 e1598.[2] J. Perez-Calatayud, F. Ballester, R.K. Das, L.A. Dewerd, G.S. Ibbott, A.S.Meigooni, Z. Ouhib Z.M.J. Rivard, R.S. Sloboda, J.F. Williamson, “Dosecalculation for photon-emitting brachytherapy sources with average en-ergy higher than 50 keV: full report of the AAPM and ESTRO report of thehigh energy brachytherapy source dosimetry (HEBD) working group,”august 2012, ISBN: 978-1-936366-17-0, ISSN: 0271-7344.[3] A. Palmer, B. Mzenda, “Performance assessment of the BEBIG Multi-Source high dose rate brachytherapy treatment unit, “Phys Med Biol. Vol 54(2009) p. 7417-34.http://dx.doi.org/10.1016/j.ejmp.2014.10.048ESTABLISHMENT OF DOSIMETRIC REFERENCES IN SMALL FIELDSTHANKS TO A NEW QUANTITY OF INTERESTS. Dufreneix, J.M. Bordy, F. Delaunay, J. Daures, J. Gouriou, M. Le Roy, A.Ostrowsky, B. Rapp, L. Sommier CEA, LIST, Laboratoire National HenriBecquerel, 91191 Gif-sur-Yvette, FranceIntroduction: The output factor ratio measurement assumes that thecalibration factor of the ionization chamber in terms of absorbed dose towater is invariant with field size. A new approach was suggested in orderto verify that this hypothesis is true for a parallel-plate chamber whichsensitive volume is larger than the beam’s section. The quantity of interestis then no longer a dose at a point but a dose integrated over a surface.Methods: A graphite calorimeter and a parallel-plate ionization chamberwith the same sensitive volume (3 cm diameter) were built. They werefully characterized in a large 60Co beam and then used in small circularfields of 2, 1 and 0.75 cm diameter. The ratio of calorimetric againstionometric measurements was considered as a good approximation of thecalibration factor and thus studied.Results: The dose rate established in a large 60Co field with the newcalorimeter is in agreement within 0.4% with previous calorimeters. Theionization chamber shows good characteristics except for a 0.06% drift perhourinwater.Insmallfields,statisticaluncertaintyontheratioconsideredincreases with decreasing field size but stays under 0.5%. A difference of1.3% of this ratio was found for the three field sizes of interest.Conclusion: Results presented here highlight the possibility of measuringcalibration factors in small fields using a dose-area product. Monte Carlosimulations are underway in order to precisely determine the calibrationfactor of the parallel plate chamber in the three small fields studied.Keywords: Radiotherapy; Dosimetric reference; Small fields; Dose-areaproducthttp://dx.doi.org/10.1016/j.ejmp.2014.10.049