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Dose distribution for a convergent X- ray field

3er Congreso de Radioquimioterapia y Braquiterapia 6ª Jornada de Física Médica. Rodolfo Figueroa 1 y Mauro Valente 2 1 Departamento de Ciencias Físicas, Universidad de La Frontera, Temuco, Chile. figueror@ufro.cl

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Dose distribution for a convergent X- ray field

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  1. 3er Congreso de Radioquimioterapia y Braquiterapia 6ª Jornada de Física Médica. Rodolfo Figueroa1 y Mauro Valente2 1 Departamento de Ciencias Físicas, Universidad de La Frontera, Temuco, Chile. figueror@ufro.cl 2 Fa.M.A.F Universidad Nacional de Córdoba, Córdoba, Argentina. valente@famaf.unc.udu.ar Dosedistributionfor a convergent X-rayfield

  2. Introduction • Conventional radiation facilities and techniques for external radiotherapy use to employ divergent beams. Therefore, beam intensity becomes dependent (reduces) when increasing the distance from the source.. • Nowadays, there are different mechanisms for generating an X-ray beam. Each one of them offers specific advantages/disadvantages, but they exhibit intrinsical divergences, disregarding the implemented collimation system.

  3. Current situation and implementation • Themaingoal of radiotherapyconsistsonapplyinghighdoselevelstothe target whilekeepingsurroundinghealthtissues at lowdoselevels. • Modernirradiationtechniques: IMRT, IGRT, Arctherapy, Tomotherapy. • Radiosurgery uses severalhighcollimatedradiationfields in ordertoachievedoseconcentration at the target. • Drawbacks : • Target volumereductionwithoutintensityincreasing. • Intensityreduction as a function of thesourcedistance.

  4. … Let consider this … • What may come out from an hypotetic convergent photon beam? • Should it be interesting to try it? • Is it possible to assess a convergent photon beam for clinical purposes?

  5. HINT & MOTIVATIONS • Duetotheconvergence, in-depthdosedepositionmayincreaseswhengeometriccontributionovercomes material attenuation. • Itshouldbeexpectedtoobtainhighdoseconcentrarionaround “focal spot” (focus-peak, similar tohadronbeams).

  6. … Let consider this proposal … • External radiotherapy by means of a single convergent field.

  7. … Whatshouldweexpect as themaindifference? …

  8. The resulting convergent radiation flux • Letcononsideran “ideal convergentphoton flux”, withoutattenuationeffects.

  9. Requiring radiation flux continuity • Consideringaninfinitesinalcone • Primary (non scattered) component flux continuity: • therefore dA0 h dAh

  10. Infinitesimal cone • Wehave: • Then: • Thereforeitresults: • Itisalsorequired: d0 h r0 dh

  11. Resulting Intensity Therefore Whereas, theattenuation contributionis: dA0 h dAh

  12. Intensity contribuitions • Geometry : • Attenuation:

  13. Intensity component factors • In firstapproximation, one has: • and • Therefore:

  14. Intensity components behaivour • Geometry : • Attenuation:

  15. IntensityFactors • Geometric Resultingdoseprofile • Attenuation

  16. Calculation for different attenuation coefficients

  17. Calculation at three different focus positions

  18. … on linear scale …

  19. Example for typical situation • Energy 1.25 MeV • Material: Water • => Attenuationcoefficient = 0,0612 cm-1 • Focus • 15 cm in-depth

  20. Monte Carlo simulations • Incidentphotonbeamisgeneratedonthesurface of a sphericalshell. • Incidentphoton travelling directtofocus (similar toisocenter). • Photonshowersinteractwithphantom (water). • Thedeveloped Monte Carlo codeperformsdepth-dosecalculation as well as 3D assessmentbymeans of suitablevoxelizationtechniques.

  21. Monte Carlo Simulationfirtorder

  22. Monte Carlo simulations* vs analyticalmodel

  23. 45-times higher dose level at entrance

  24. MCS profile low energy • Energy 400 keV • Aperturecone 40° • Focus 5cm

  25. Slice convergent beam MC • Water phantom 400 keV

  26. … How could we design this kind of device?

  27. Sketch of convergent X-ray beam generation device

  28. Convergent X-ray beam generation device

  29. convergent X-ray beam generation device • International Patent in progress

  30. RTC in a LINAC

  31. Convergence of several paralell beams

  32. Dose within transversal plane

  33. .. Haw could we assess larger beams?

  34. Dose volume

  35. Volume rotation

  36. Preliminar comparison between RTC and current radiotherapy techniques RTC

  37. Conclussions • Theobtainedresultssuggestthefeasibilityfortheproposed RTC method. • It has beenpreliminaryobtainedthatsuitabledepth-doseprofilemaybeachievedbymeans of theproposedmethod. • Thedoseprofileis similar tohadrontherapytechnique.

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