824

85

Parameterization of theheadscatter correctionfactor Sc for rectangular photon beamsSymmetrical andasymmetrical. open and wedged fields

J1M vanGasteren I) andJLMVenselwr)I) ART!, Anthem. 2) Dr B Verbeeten Instituut, Tilburg, TheNetherlands

In lbecalculation ofmonitor units and tteatment times of MYphotoobeamsthebeadscatter factor S, accounts forthe change in pboton nuence when lbecollimator setting andthus lbefield sizeis changed. S, can bemeasured witlla narrow beam-coaxial phantom [I) asa fimctioo of the settings of tileX andYcollimator blocks. Allboogh S, isexpected to have a symmetrical nature S,turns outto beanasymmetrical fimClioo of these settings. Eloogated fields inX andYdirections can haveSevalues which differseveral percent. Inm1erto obtain sufficient accuracy usually Se is measured for a matrix of fieldsizes of for instance X=2up to 40an andY=2up to 40an . Thereason ofthisasymmetry originates from the fact thatthe X and Y blocks are locatedindifferent planes. So the apenure - loolring upstreem from the measurementpointto lbeblocks - is anasymmetrical'function of lbecollimator setting.Our approach is to correct lbe asymmetry in S, by considering S, as afunction of lbe corrected field dimensions either CrX and Y or X and Cry.Thefactor Crhasa valuebetween 0.0 and 1.0depending onlbe beam energyand the accelerator design. An iteration algorithm based OIl the Clarksonapproach is used to calculate a bestfit Crvalue from Sevalues of square andsome elongated fields. Effectively the Se matrix is reduced to a onedimensional fimClion Se(r). with r the average radius of fields wilbdimensions X=~ and Y=aor X=aandY=~. The algorithm is applied tocalculate Se of rectangular shaped fields . The accuracy of the method.based on one single . a priori • unknown. parameter Cr is well withinmeasurement accuracy (0.5 %). Besides. a substantial reduction of thenumber of Se measurements. necessary to covertheclinically used fields. isachieved thisway.Themethod is validated for symmetrical beams with energies ranging from6 to 25 MY of accelerators of several manufacturers and also forasymmetrical wedged fields of the 6 MY beams of a Philips SU5 and aSallIme-41 machine.

[I) Van Gasterenet aI. Radiotherapy & Oncology, 20,250-257,1991.

87

MICRODOSIMETRIC DEPTII MEASUREMENTS IN llffi NICEFAST NEUTRONTHERAPEUTICBEAM

P.COLAUTIl' . N.BRASSARTA, V.CONTE'. A. COURDIA, j,HERAULT\G.TORNIELU', P.CHAlNELA

'INFN-LNL viaRomea 4,1·35020 LEGNARO· ITAI.Yo INFN-P.dova. vi. Marwlo8, 1·35100 PADOVA -ITALY"Centre Antoine-Lacassagne, Cyclotron Biomedical. F·ll6200 NICE· FRANCE

In Nice biomedicalcyclotron, proton beamsof 65 MeV of energy areused since June 1991 to treat ocular tumours (mainly uvealmelanomas); up today 550 patients have been treated. Anotherapplicationof the facility is tumour treatment by using fast neutronbeams. 'The neutron beams are produced by bombarding berylliumtargets with 60 MeV protons.The treatment unit consist of a verticalbeam completed with a multileafCOllimator. The maximum apertureof the collimator at the treatment distance, 20 em from the end ofcollimator. is 23x24.5 em', The depth dose characteristics arc similarto those ones of a 8 MeV Linac. For lOx10 cm2 field the maximumdose is at 2 cm and the 50% isodose at 17em. The penumbra(80% ­20%) at 5 em depth for this field is about 8 mm.

Neutronsinteractwith biologicaltargetsgiving rise to charge particlesecondary spectra of protons, deuterons, alfa particlesand light ions.Since neutron energy spectrum changes with depth, charge particleenergy spectra is expected to change as well as their LETspectra. Onthe other words, the quality of the neutron beam is expected tochange with the depth. Dosimetric measurements performed withiorusauon chambersare not able to measure the variation of radiationquality. Microdosimetry is a well known scientific tool able todescribethe radiation quality and 10 measureits variation.

We have performed microdosimetric measurements in the Nice fastneulron. beam at different depths with a spherical tissue-equivalentproporuonalcounter which simulates211m of thickness.E~perim~n~ da~ will be compared with the dose depth curve andwith radiobiological experimentdata, Results and comparisons willbe discussed.

86

MICRODOSIMETRIC MEASUREMENTS INTHEWGH ENERGYPROTON CANCER THERAPY BEAMS AT CLATIERBRIDGEANDORSAY.

V.P.Cosgrove, S. Green. M.e.Scon, A. Kacperek and A. Mazal.The University of Birmingham, Edgbaston, Birmingham, England.

Microdosimetric measurements have been made in the 62MeVproton beam in Clatterbridge, UK, and in the 73MeV beam inOrsay, France. Both beams are currently used for the treatment ofocular melanoma (eye tumours). Measurementswere made using aplanar, wall-less proportional counter. designed specifically for usein proton beams. Measured lineal energy spectra indicated a shiftto higher lineal energies with depth of beam penetration, with themost significant increases occurring near the end of each beamsrange. This trend was observed in both unmodulated and rangemodulated beams. The measured spectra were used to calculatemean quality factors.Q.using the latest ICRPrelationshipsbetweenQand LET•. For the Clatterbridge and Orsay full-range modulatedbeams, 15 was i.07±O.06 and i.26±OJ, respectively. at beamentrance. These values increased to 5.33±O.4 and 4.42±O.4.respectively, at the end of the beams range. with the mostsignificant increase in~ occurring in the final 2-3mmof range. Theeffect the increases in<:! will have on dose distribution with depthof penetration of the proton beams will be discussed, as too theimplicationsthis will have regardingthe planning of proton therapy.

88

Quality assurance in 3-D treatment planning systems

LA.D. Bruinvis(Amsterdam. The Netherlands)

Abstractnot received