Dosimetric system Chapter 10 The Interaction of Single Beams of X and Gamma Rays with a Scattering Medium

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1 Chpter 10 The Interction of Single Bes of n G Rys with Scttering Meiu Rition osietry I Text: H.E Johns n J.R. Cunningh, The physics of riology, 4 th e. osietric syste Estblishe proceure for clculting ose t point bse on the esureent Now nee to be ble to clculte the ose t ny point bse on the known ose t the reference point A set of functions ws evelope to enble these clcultions Preters for clcultion of bsorbe ose Fiel with W istnce fro the source F epth in phnto epth of the xiu ose in phnto ose eposite t certin point,, etc. ose is obtine uner conition of electronic equilibriu (for ir: enough phnto-like teril surrouning the point) Functions use in ose clcultion Tissue-ir rtio T W / Bcksctter fctor B W (, T (, W / Percentge epth ose Inverse squre lw F I( F,, ) F P W, F, 100 / Reltionship between functions ose t one point cn be clculte bse on the known ose t ifferent point using function long the corresponing rrow: B P I T Reltionship between functions More thn one function cn be involve in relting oses t ifferent points: B P I T Reltionship between functions: 100T I P(, W, F) B T, W F 100 BW F

2 Tissue-phnto rtio Tissue-ir rtio phnto tissue For high-energy bes soli phnto is usully use for osietry Tissue-phnto rtio: T, W / p r It cn be relte to T : T p T W T (, W r Introuce to siplify clcultions for rottionl therpy with tuor locte t the rottionl xis In such rrngeent the source-to-xis istnce is fixe For istnces lrger thn 50 c T is inepenent of the istnce to the source (first eterine experientlly) Tissue-ir rtio Tissue-ir rtio Co-60 priry only Co-60 T W T vlues re tbulte for ifferent fiel sizes, epths, n energies (Tble B-5) ecreses lost exponentilly with epth (Co-60 source is lost ono-energetic) Increses continuously with fiel size t ll epth Bcksctter fctor B( W, T (, W epens on the fiel size n qulity of rition Position of the xiu ose epens on the fiel size n qulity of rition In generl is not the se s the epth where electronic equilibriu occurs: for lrge fiel sizes the sctter contribution eterines the position of Bcksctter fctor The epenence on the rition qulity is non-onotonicl, epens on fiel size For higher energies ost of the sctter is forwr-peke: the ount of bcksctter ecreses

3 Percentge epth ose Percentge epth ose P (, W, F, hv ) 100 For higher energy bes there is buil-up region ue to electrons scttere in forwr irection After reching its xiu ose eposition follows exponentil ttenution After reching its xiu ose eposition follows lost exponentil ecrese A useful quntity to use s n inex of the penetrtion is the epth t which the ose flls to 50% of its pek vlue Percentge epth ose Exple 1 All of the following re true regring percentge epth ose (P) except: For sller file sizes sctter contribution is sll For high energy bes sctter contribution is sll, therefore P is less epenent on the fiel size A. Increses with incresing energy B. epens on fiel size C. Is the ose t epth expresse s percentge of the ose t. ecreses with incresing SS E. ecreses s epth increses ISL input ecreses s istnce increses through increse in SS Equivlent fiel size 1 T A) n Rectngulr fiel cn be ivie into segents of circulr fiels Functions (T or P) cn be clculte for irregulr fiel sizes Sller ngulr segents increse the ccurcy (10 o is typiclly enough) n T r ) i1 A re of the fiel t epth r rius of the circulr be i Equivlent fiel size A rectngulr fiel gives sller epth ose n tissue-ir rtio thn oes circulr or squre fiel of the se re Rule of thub etho: Squre fiels n rectngulr fiels re equivlent if the rtios fore by iviing re by perieter re the se (/p) Tbles 10.3 n 10.4 show rii of circulr fiel n sie length of squre fiel corresponing to equivlent rectngulr fiels

4 Equivlent fiel size Exple A fiel esuring 5 x 5 c t SS hs n "equivlent squre" fiel of sie c. In prctice the output of the chine nees to be crefully esure for rectngulr fiel sizes, ue to the sctter fro ccelertor he A. 5.0 B. 8.3 C E. 5.0 b c ( b) 4c b 55 c 8.3 b 5 5 ose profile ose profile For fiel size (be with) t epth, W, geoetricl penubr with p, the be profile cn be escribe by function: Finite source size n be shping evices introuce geoetric penubr for priry rition profile Sctter n flttening filter soften the eges of the be Preters 1,, n t re eterine by best fit of esure profiles ose profile Even in the bsence of sctter fro be shping evices (priry collitor, jws, etc.) the bsorbe ose profile hs significnt penubr region Loss of lterl electronic equilibriu between ker (energy lost by photons) n bsorbe ose (energy lost by electrons) escription of the rition be Priry n sctter coponents: zero-fiel tissue-ir rtio n sctter-ir rtio T r T 0, S(, r For high-energy bes tissue-ir rtio is replce with tissue-phnto rtio T r Tp r, r T ( r, r ue to reltively sll ount of scttere rition t high energies, T ( r,r, ~1

5 Isoose curves A p of the ose pttern in one plne Exple 3 Copre with 6 MeV electrons, superficil x-rys: No skin spring, the ose flls continuously 5% t 10 c epth Shrp be eges Lrge ount of sie sctter (beyon the be) Skin spring, the xiu t 5 5% t 10 c epth Penubr Low sie sctter Skin spring, the xiu t 4 c 83% t 10 c epth Penubr Low sie sctter A. Hve lower skin ose B. eliver less ose to unerlying tissues C. Require thicker shieling. Hve shrper penubr Exple 4 In irregulr fiel clcultions, the increse in MU setting to ccount for blocking is gretest for: A. 18 MV photons, 1 c epth B. 18 MV photons, C. 6 MV photons, 1 c epth. 6 MV photons, Where the sctter contribution is the gretest: lower energy, greter epth Rition be chrcteriztion TG-106 report on ccelertor be coissioning: t iniu, the following t shoul be collecte uring coissioning: For photon bes percent epth ose P n profiles in-plne n/or cross-plne t vrious epths for open n wege fiels, t relte to ultilef collitor MLC such s inter- n intrlef lekge, penubr, tongue n grove effect, etc., he collitor sctter, totl sctter, try, n wege fctors I. J. s, T. C. Zhu, et l. Accelertor be t coissioning equipent n proceures: Report of the TG-106 of the Therpy Physics Coittee of the AAPM, Me. Phys. 35, 4186, 008. Rition be chrcteriztion TG-1 report protocol on clinicl reference osietry, bse on ir ker (obsolete) TG-51 report protocol on clinicl reference osietry, bse on ose to wter TG-40 report - protocol on coprehensive QA for eicl liner ccelertors TG-14 report - the ost upte protocol on coprehensive QA for Rition therpy, inclues QA for on-bor iging n S(B)RT P.R. Alon et l., AAPM s TG-51 protocol for clinicl reference osietry of high-energy photon n electron bes, Me. Phys. 6, , G. J. Kutcher et l., Coprehensive QA for rition oncology: Report of AAPM rition therpy coittee tsk group 40, Me. Phys. 1, , E.E. Klein, et l. Tsk Group 14 report: Qulity ssurnce of eicl ccelertors, Me. Phys. 36, , 009.