Recombination factor dependancy of high and low dose pulsed accelerators for electron beam energies

Transcription

1 Recombination factor dependancy of high and low dose plsed accelerators for electron beam energies Giseppe Felici Sordina IORT Technologies S.p.A.

2 Talk Overview Dose per plse what is the so called high dose per plse? Where does it comes from? Why IORT Linacs have a higher dose per plse? Dosimetric formalism and framework : IAEA TRS 398 The physics modelization of ion recombination A different modelization for k s Calcls and examples

3 EBRT LINAC Low and high dose per plse IORT LINAC Dose Rate (DR) 0Gy min Dose Rate (DR) [4Gy min, 0 Gy min] PlseRepetition Freqency (PRF) [00 Hz, 400 Hz] PlseRepetition Freqency (PRF) [5 Hz, 40Hz] DR[ Gy min] Dose per plse ( Dose per plse ( d p ) 0. 05cGy p PRF [ Hz] 60 d p ) [0.4 cgy p, 5cGy p] The dose per plse generated by IORT linacs is p to 00 times higher respect to the standard 3

4 Where does the difference come from? EBRT LINAC IORT LINAC 4

5 LIAC e-beam transport system is realized with low Z material 5

6 Beam Optic Transfer efficiency is higher for IORT linacs Transfer efficiency varies between 50 Me and 6 Me 6 XI Ulsal Medikal Fizik Kongresi

7 IORT Linacs design specs. Electron ONLY linac. Maximm mobility, able to move inside standard hospital space (elevators, doors ); 3. Not isocentric bt capable of performing easily and safely docking process; 4. As small and light as possible; 5. Different energies, giving the possibility of irradiating a PT with thickness ranging p to 3.5 cm inside the 90% isodse; 6. It has to prodce the minimm X rays possible, for radioprotection isses; High dose per plse Low stray radiatio n

8 Absorbed Dose to water according to IAEA TRS 398 formalism Absorbed dose to water at the reference depth z ref in water for a reference beam of qality Q 0 D = M N k w w, Q Q D, Q Q 0 Q, 0 M Q = M ' ki = M ' kt, P i k H k pol k s M measred signal k Q,Q0 Factor to correct for the difference between the response of an ionization chamber in the reference beam qality Qo sed for calibrating the chamber and in the actal ser beam qality, Q. k s Factor to correct the response of an ionization chamber for the lack of complete charge collection (de to ion recombination). 8

9 k s modelization : from the definition of the problem to the soltion The satration loss for plane parallel ionization chambers at high dose-per-plse vale A. Piermattei, S. Delle Canne, L. Azario, A. Rsso, A. Fidanzio, R. Miceli, A. Soriani, A. Orvieto, and M. Fantini

10 Laitano s paper allows TA (Two oltage Analysis) for determining k s, eliminating the need of an external chemical dosimeter and greatly simplifying the dosimetric characterization Chemical dosimeter can be sed for external adit D w = ΔA N w Πk i 0

11 k s the standard approach according to IAEA TRS 398 k s = f k s ln ( + ) + f = ln + ( ) = µ d d(m) distance between chamber plates μ (m/c) a constant depending on the gas in the cavity chamber r (C/m3) the charge liberated in air per nit volme and per plse (olt) the voltage spply of the chamber. This model is applicable only if the fraction of electrons escaping attachment to oxygen molecles and reaching the collecting electrodes is negligible. The high-energy plsed electron beams of interest in clinical dosimetry have sally a dose-per-plse vale of abot 0.05 cgy/ plse. This means that r and, conseqently, are small for the sally employed planeparallel ionization chambers. Ths can be expanded to first order, giving If k s <.03 r

12 This model fails for high dose per plse!

13 k s : Laitano s theory f = ln + ( ) f ''' = λ + e ln + λ λ ( λ ) λ = p µ d r = p is the free electron fraction d d interelectrodic distance wτ = w p e τ d w free electrons migration velocity inside the air cavity Boag, 987 τ electron mean life before recombination w,τ can be calclated vs electric field intensity Free electron fraction is independent from the dose per plse and is always different from zero. Its effect becomes relevant when the dose per plse is high, that means when is increasing. lim 0 + f ''' = f 3

14 w electron drift velocity w= a + b ne ce ce d c e ne ( e ) + n n c Hochhaser et al, 997 parametres m d (cm) () E (/cm) w (cm/s) a 5,83535E+04cm/s 0, ,0,36E+06 b,480e+07cm/s () c,56809e-04cm/ ,0,33E+06 d 3,86396E-03cm/ e,03039e-03cm/ w(e) n 4,89435E-03cm/ 3,5E+06 3,0E+06 cm/s,5e+06,0e+06,5e+06,0e+06 5,0E+05 /cm 0,0E

15 Electron Mean lifetime before attachment be de ( e ) + c( e ) τ = a Hochhaser et al, 997 parametres m d (cm) () E (/cm) ta (s) a 6,6950E-08 s 0, ,35E-08 b,8679e-04 cm/ () c 6,4440E-08 s ,0,E-08 d,8e-04 cm/ ta(e) 8,E-08 7,E-08 s 6,E-08 5,E-08 4,E-08 3,E-08,E-08,E-08 0,E+00 /cm National Medical Physics Conference

16 TA :k s determination 6 After w, ta have been determined p can be calclated ; now it is possible to calclate ks experimentally sing TA techniqe ( ) ( ) ( ) = ln ln ' ' e e Q Q λ λ λ λ λ λ λ λ = ( ) =,, ' ' p f p f Q Q ( ) ( ), p f k sat λ = r d µ = ' ' f f Q Q = It is resolved nmerically respect to Once is determined f is calclated with the corresponding voltage

17 A specific calcls software has been developed by Sordina 7

18 k s : nmerical examples chamber type interelectrodic spacing (cm) polarization () PTW Roos 0, 00 PTW Adv. Marks 0, 400 IBA PPC 05 0, ROO S PPC05 Adv. Marks = k d µ d s p r > 0,4,,,08,06,04,0 Advanced PPC d p (cgy/p)

19 Cylindical ionization chambers It is possible to calclate ks also for cylindrical chambers sing the Boag formla / b internal radis (mm) a + b ln( a / b) d cyl = a external radis (mm) ( a b) a b d cyl eqivalent electrode separation (mm) Semiflex Pin Point 9

20 Thanks! 0