Basic of medical linac

the basic structure of medical linac

1. Med i cal Linear Accelera t or (LIN A C) Augnas Johnson M.Sc R a diat i on P h ys i cs KIMS THIRUVANANTHAPURAM

2. Med i cal Lin e ar Accelera t or (LINAC) • The l i near ac c e l e r at or i s a d e v i ce th a t u s es hig h -f r e q uency e l ect r omagn e tic w a v es to accel e rat e ch a r g ed part i cles such as e l ec t r ons t o high ene r gies th r ou g h a l i near tu b e. • E l e ct r on t r a j ec t or i e s a r e l i near in the ac c e l e r at or t u be hen c e the name ‘ L I N E A R A C CE LER A T OR ’ . • The hi g h-ene r g y e l ect r on b e am i t se l f c an be used f or t r e a t i ng superficial t umo r s, or i t ca n b e made t o s tr i k e a t a r g e t t o p r o d u ce x - r a y s f or t r e a ting dee p -se a t ed tumo r s .

3. Hi st o r y of the medi c al linear ac c ele r a t or: 1 95 2 : Henry K aplan and E d w a r d G i nz t on be g in b u i l ding a medi c al l inear ac c e l e r at o r . 1 95 6 : T he fi r s t m e di c al l i n e ar ac c e l e r a t or i n t he W e s t e rn Hem i sp h e r e is in st al l ed a t S t a n f o r d Hospi t al i n San F r an c i s c o . 1 95 9 : S t a n f o r d m e di c al sch o o l and h ospi t al m o v e t o the P alo Al t o c am p us, b r i n ging the m e di c al l i near ac c e l e r at o r . 1 96 2 : K aplan and S a ul R osenbe r g be g in tr i als using the l i near ac c e l e r at or • • • • w i th c h e m o t he r a p y to t r e a t Hod g ki n 's dis e a s e, an a pp r oach t h a t d r am a ti c a l ly i m p r o v es p a ti e n t su r v i v a l . 1 99 4 : Fi r s t use of t he C y b er K ni f e, i n v e n t ed a t S t a n f o r d, which uses sop h i s ti c at ed c ompu t er i z ed i m aging t o aim a nar r o w X - r a y beam p r eci s e l y . 1 99 7 : S t a n f o r d pionee r s the use of i nt ensi t y -m o d u l at ed r adi a t ion the r a p y , which c omb i n es p r ec i se im agi n g w i th l i near ac c ele r a t o r s th a t de l i v e r h u n d r eds of thin bea m s of r adi a ti o n f r om a n y angle. 2 00 4 : I m ple m e n ta t ion of f o ur-di m ens i onal r adiothe r a p y , whi c h a cc o u n t s f or the mot i on of b r e a thing d u r i ng i m aging and r adi a tion del i v e r y . • • • • M edi c al l i ne a r ac c e l e r a t o r s h a v e b ec o m e the ba c k b o ne of r a di a t i on t h e r apy f or c an c er w or l dwide.

4. Hi s t o r y of L I N A C A 2 - y ea r -old b o y su f f er i ng f r om a tumor i n his e y e, w as t h e fi rs t t o u n de r g o X - r a y t r e a t m e n t f r om a m e d i c al l inear ac c e l e r a t or th a t w as d e v e l oped by He nr y K aplan, M D w i th c ampus p h y s i ci s ts. • • The t r e a t m e n t s a v ed h is v i s i on i nt ac t . the ch i ld's s i g h t and he l i v ed the r e s t of his l i f e w i th He nr y K a p l an ( l e f t ) , a n d h ea d o f r a d io l o g ic f irs t p h y s i ci s t The fir s t p a t i e n t t o r ecei v e r a d i a t ion t he r a p y f r om t he med i c al l i ne a r acce l e r a t or a t S t a n f o r d w as a 2 - y e ar -old b o y . p h y s i cs Mi t c h e l l W e i s s b l u t h , t he K a p l an h i re d, a t t he w o r king e n d of t h e S t a n f o r d acce l e r a t o r .

5. Gene r a tion ’ s of Linac • • • • The fi r s t one w as in s t al l ed in Ham m e r s m ith in 1 9 5 2. In 1956 , the fi r s t p a ti e n t w as t r e at ed a t S t a n f o r d Uni v e r s i ty in U S A . The LIN A C had an 8-MV X ra y beam w i th l i m i t ed g a n t r y mot i on. These LIN A Cs w e r e la r g e and b u lk y .

6. Se c ond g ene r a tion • The se c o n d g e ne r a tion w as i s oce n tr i c and c o uld r o tat e 36 o a r o und t h e g a n t r y a x i s . Th e y w e r e b u i l t b e t w een 1962 -198 2 . Th e y inc r eas e d the ac c u r acy and p r eci s ion of Dose del i v e r y . • •

7. Thi r d g ene r a tion • B e tt er ac c ele r at or w a v eg u i d es and b en d i n g ma g n e t beam mod i f ying ac c es s or i es. s ys t e m s and mo r e • • Wider r ange of ene r gies , dose r at es, fi e ld s i z es and ope r a ting mode s . Higher r e l iabi l ity and c ompu t er dri v en.

8. Linear ac c el e r a t or c o nsi s ts of : 1) 2) 3) 4) 5 ) 6 ) 7 ) 8 ) 9 ) El e ct r on i n jection s y s t em Mic r o w a v e s y s t em P o w er su p p l y s y s t em B e am t r ansport be a m m o n i t oring Auxi l ia r y s y s t em Sa f e t y i nt erlo c k s y st em s y s t em C o m pu t er c o n t r olled f e e d b ack s y s t em B e am c ollim a t or / ap p li c a t or s y s t em C o o li n g s y s t em 10 ) C o n t r ol c ons o le s y s t em

9. TYPE OF S Y S T E M C O M PONEN T S Elect r on inj e cti o n s y st em E l e c t r on G un, t o p r ov i d e e l e c t r o ns f o r ac ce le r a t i o n . Mic r o w a v e s y s t e m Mag n e t r o n mic r o w a v e s . or Kl y s t r on to p r o v i d e P o w e r s upp ly s y s t e m Mod u l a t or t o p r o v ide H igh v olt a g e & s h ort du r a tion pu lses in s yn ch r on i z a ti o n. Beam t r an sp o rt be am mo n i t oring s y s t e m Acc el e r a t i n g w a v e g u i d e , p r o v id e s ac ce l e r a t io n t o e l e ct r on s . Auxi l ia r y s y st em V a c u um p u m p, ci r c ul a ti ng coo l i ng w a t er ,RF f r eq uen cy t un e , P r e s s u r i z e d d i e l e ctr i c g as f or R F t r an s mi s s i on , R F is o l a t or & T h y r a t r o n . S a f e ty in t e rl oc k s y s t e m Both Ha r d w a r e and s o f t w a r e in t e r l o c k i n g s y s t e m. Com pu t e r c ont r ol l e d f eedb ack s y s t e m Moni t or c h a m b e r , H a r d w a r e p o s it io n en c od e r s, limiti n g mic r o s wi t c h e s . Beam c ol l im a to r / ap p l i c a t or s y s t e m J a w c o l l i m a t o r s , m u lt i - l e a f co l l im a t o r s(M L C), mic r o m u lti l e a f c ol l im ato r s ( mM L C ) .

10. Ene r gies of L IN A C Elect r on Beam e ne r gy 6 MeV 9 MeV 12 MeV 15 MeV 18 MeV P h o t ons beam ene r gy 6 MV 1 8 MV

11. The maj o r c omp o ne n ts of medi c al Linear Accele r at or • • • • • P o w er Su p ply Mo d u l a t or Mag n e t r on Or Kl y s t r on Elect r on G u n W a v e G u i de s y s t e m • • • • Acc e l e r a t or T u b e Be n d i n g M agnet T r e a t m e n t H e ad T r e a t m e n t t abl e (Co u ch)

12. Block diag r am of LIN A C

13. Modul a t or and po w er supply • This i mpor t a n t c omp o n e n t of the l i n e ar ac c ele r a t or i s u s ual l y l o c a t ed i n the t r e a tme n t r oom In so m e Un i ts. The Modul at or c abin e t c o n t ai n s th r ee ma j or c ompone n ts • i. F an c o n t r o l ( c o o li ng the p o w e r -di s t ri but i o n s y s t em). i i . A u xi lia r y p o w e r d i s t r ibu t i o n s y s t e m ( c o n t a i ns the e me r g ency o f f bu t t on th a t s h uts o f f the p o w er t o the t r e a tme n t un i t ) . Prima r y p o we r- d i s tr i b u t i on s y s t em . i i i. • A p o w e r s upp ly p r o vides di r e c t c ur r e n t ( D C ) p ow er t o t he m o du l a t o r , w h i c h i n cl udes the pu l s e - f orm i ng n e t w ork and a s w i t c h tube kn o wn as h y d r o g e n t h y r a t r o n . • Hi gh v o l t a g e pu l s e s f r om t h e mod u l a t or s e c t i on a r e fl a t - t opp e d D C pu l s e s of a f e w m ic r ose c onds i n du r a t i on. • T h e se pu l s e s a r e d e li v e r ed t o the ma g n e t r on or k l y s t r on a nd simul t an e o u sly t o the elec t r on g un .

14. Magn e t r on The magn e t r on i s a d e v i ce th a t p r oduc e s mic r o w av e s . It i s a hi g h - p o w er osci l l a t o r , g en e r a t es m i c r o w a v e p u lses of f r equ e ncy o f about ~ 3 , 000 MH z . Ma g n e t r o n i s c ylin d r i c al c on s truction c onsi s ts o f e v a c u a t ed ce n t r al ca tho d e and a n ou t er anode w i t h r esona n t c a v it i es machined out o f a sol i d piece o f c oppe r . The ca thode i s hea t ed b y a n inner f i lame n t and the elec t r ons a r e g en e r a t ed b y thermi o nic emissio n . Both the elec t r on g u n and the Magnet r on a r e f ed with Hi g h • • • • • v o l t a g e p o wer supply & sho r t du r a ti o n pu l ses in with the Modu la t ed p o w er supply s y s t e m . s ynch r o n y • T y p i c al hi g h v o l t a g e pulse o f about 50k V p i s a f e w m i c r o ti m es p e r s e c onds l o ng and i s r ep e a t e d a f e w h un d r ed s e c ond. • Pulse r ep e tition f r eq u e ncy ( P RF) OR Pulse per second d i f f e r s a c c o r di n g to manu f a c tu r e r but pulse width r em a ins c on st a n t.(P u lses a r e o f about 4 µs du r a tion & a r e del i v e r ed at a P R F o f 250 Hz .) PRF o r PPS d e t ermines the dose r a t e f r om a LIN A C . •

15. W OR K ING: • A s ta t ic magn e tic f i e l d i s ap p l i ed perpen d icular t o the plane of the c r o ss sec t ion of the c a v i t i es and a p u l s ed D C e l ectric field i s app l i e d b e t w een the c a t h o d e and the ano de . The e l ect r ons e m i t t ed f r om the c a th o de a r e ac c e l e r a t ed t o w a r d the a n o de b y the action of the puls e d DC e l ect r ic fi e l d . Under the s im ul t ane o us i n flue n ce of the magn e tic fi e ld. The e l ec t r ons m o v e i n c omp l e x spi r a l s t o w a r d the r esona n t c a v i t i e s, r a d i a t ing ene r g y i n the f o r m of m i c r o w a v e s . The g ene r at ed m ic r o w a v e pu l s es a r e l e d t o the ac c e l e r at or s truc t u r e vi a the w a v eg u ide. • •

16. Kl y s t r on The Kl y s t r o n is a mic r o w a v e amplifie r . It is dri v en b y a low-power mi c r o w a v e o scill a t o r . • • The e l ec t r o n s p r o du c ed by t h e c a thode a r e acc e le r a t ed b y a ne g a ti v e pulse of v o l t a g e i n t o bu n cher c a vity which is ene r gi z ed b y lo w -po w er mi c r o w a v e s . • The mi c r o w av es set up a n al t ern a ti n g elec t ric field ac r o ss the c a vit y . • The v elocity o f the e l ec t r o ns b y the a c tion o f this el e c t ric v a r y i ng d e g r ee by a p r o cess v elocity mod u l a ti o n . is al t e r ed field t o a kn o wn as

17. K l y s t r on • E l e ct r ons f orm bunches du e t o v ar i a tion in v e l ocity r esu l t i ng i n b unchi n g of e l ect r ons as the v e l ocit y -modul at ed beam pas s es th r o u gh a fi e ld-f r ee space i n the drift tu be . A s the ele c t r o n b u n c hes arr i v e a t the c at c h er c a v i t y , th e y in du ce c ha r g es on the ends of the c a v ity and the r e b y g ene r at e a r e t a r ding e l ectr i c fi e l d . The e l ec t r ons su f f er de c el e r a tion , a n d b y t he principle of c o nse r v a tion of ene r g y , the kin e tic ene r g y of e l ect r ons i s c o n v er t ed i nt o h i g h - p o w er m i c r o w a v e s . • •

18. Elect r on Gun • It i s r e spons i ble f or p r o du cing e l ect r ons and injecti n g them i nt o t he ac c e l e r at or s truc t u r e . T u n g st en Me s h / c o i l p r o d u c es a s t r a y of e l ec t r ons d u e t o ther m i o nic e m i s s i on when v ol t a g e i s ap p l i ed i n t er m s of “ F i l am e n t c u r r e n t ” . The e l ec t r on gun a n d the sou r ce a r e p u l sed s o th a t t he hi g h v e l ocity e l ec t r ons a r e i nje c t ed i nt o the ac c e l e r a t ing w a v eg u ide a t t he sa m e t i m e as i t i s ene r gi z ed b y the m i c r o w a v e s . The n umber o f e l ect r ons ejec t ed dep e nds u p on the t e m pe r a tu r e of t h e fi l am e n t . The e l ect r on g un a n d w a v eg u ide s y s t em a r e e v ac u at ed t o a l o w p r es s u r e • • • • t o ma k e the m ean f r ee p a th of e l ect r ons b e t w een at o mi c c ol l isions c ompa r ed t o p a th i n t h e s y s t e m . lo n g

19. W a v e Guide Accele r a t or Guide : s y s t em Also c al l ed as the ac c e l e r a t or s t r uc t u r e mo u n t ed i n the g a n tr y : , i ) H o ri z o n t a l ly ( H i g h -e n e r gy machine s ) ii ) V e rti c ally ( l o w -e n e r gy machines ) . • Mic r o w a v e po w er (p r o duc e d i n the kl y s t r on) i s t r an s por t ed t o the a c c e le r a t or s tructu r e, i n which c or r u g a ti o ns(wrinkle) a r e us e d t o sl o w the w av e s . • Accele r a ti n g e l e c t r ons t e n ds to d i v e r g e , partly by the mutual c ou l o mb r epuls i on and ma i nly b y the r adial c o m pone n t o f electric f i eld i n w a v e g u ide s tructu r e . Elect r o ns a r e f ocus e d ba c k t o their p a th by the use o f c o- a x ial m agne t ic f oc u sing fi e ld gene r a t ed b y th e c o a x i al c o i ls which a r e c o a xi a l wi t h a c c e le r a ti n g w av eguide.( A lso c alled a s s t e e r i ng c o i ls) •

20. T ypes of w a v egui d e s y st em 1) T r a v e l l i ng w a v eg u ide s y s t em 2 ) S t an d ing w a v e g u ide s y s t em T r a v elli n g w a v e g u i de s y s t em • T r a v e l l i ng w a v e g u ide s truc t u r e r equi r e r e l a ti v e l y lon g er ac c e l e r a ting w a v eg u ide. Fu n ctiona l l y , t r a v e l ing w a v e s tructu r es r equi r e a t e r m in a tin g , or " d u m m y ," load t o ab s orb the r es i d u al p o w er a t the end of the s truc t u r e, th u s p r e v e n ting a b ack w a r d r e fl e c t ed w a v e. •

21. S t anding w a v e guide s y st em • S t an d ing w a v e g u ide s truc t u r e hel p s i n r e d u c i n g t he ac c e l e r a ting l e n g t h du e t o o p tion of s i de c ou p l i ng c a v i ti es . The s t an d i n g w a v e s truc t u r e s p r o v i de m a xim u m r e fl e ction of the w a v es a t both e n ds of the s truc t u r e s o th a t the c o m bin a tion of f o r w a r d an d r e v e r s e t ra v e l ing w a v es wil l gi v e r i s e t o s ta tiona r y w a v e s as the m i c r o w a v e p o w er i s c ou p l e d i nt o the s truc t u r e vi a s i de c ou p l i ng c a v i ti es . Suc h a des i gn t ends t o b e m o r e e f fici e n t than the t ra v eling w a v e des i g n s s i n c e a x i a l, b eam t r ansport c a v i ties, a n d t he s i de c a v i t ies c an be indepe n de n tly o p ti m i z e d . • •

22. T r e a tme n t head T r e a t me n t he a d c ompr i s e s of c om p one n ts a n d mon i t or the t r e a tm e n t bea m . • th a t a r e de s i g n e d to shape          B e nd ing magn e t : S h ieldi n g m a t eri a l: X - r a y t a r g e t : Prima r y c ollim at or B eam fl att eni n g fil t e r : S c a tt ering f oi l : B eam mo n i t oring d e vice s : Se c o n d ary c ollim at o r s : Field li g h t :

23. Bending magn e t • The e l ec t r ons e x i t the w a v eg u ide and e n t er the ‘fl a y t u b e’ whe r e e l ect r o n beam i s r edi r ec t ed t o w a r ds the t a r g e t, the e l ec t r o n s t ra v el along a ‘ S l a l om’ p a th tu b e . w i thin the f l a y • Th r ee pai r s of magn e ts on t h e e i th e r s ide of the Fl a y tu b e, c a u se the e l e c t r on beam t o bend t h r o u gh the turns of t h e Slalo m . This p r ocess not on l y posit i ons the be a m t o s tr i k e t h e t a r g e t, bu t a l so f ocuses the beam t o a dia m e t er of 1 mm . The des i gn of the ma g n e ts enables them t o f o c u s t h e e l ec t r o n s of s l ig h tly di f f e r e n t ene r gies o n t o the sa m e p oi n t on the t a r g e t ( A c h r om a tic beh a v i or) • •

24. Slal o m 9 Be n d ing (A ch r om a t i c ) Ch r om a tic 270 Be n d i n g • In t he h i g h e r -ene r g y l i na c ’ s, h o we v e r , an d , the r e f o r e, i s placed h o r i z o n t a l ly hori z o n t al. the ac c e l e r at or or a t an a n gle s t r u c tu r e i s t oo lo n g w i th r espect t o the • The e l ec t r ons a r e then be n t th r o u gh a sui t a b le an g le (u s ually abo u t 9 or 27 de g r ee s ) b e t w een the ac c e l e r at or s truc t u r e and the t a r g e t . 9 deg r ee m agn e ts ( A c h r om a tic) h a v e the p r ope r ty th a t a n y ene r g y sp r ead r esu l ts i n sp a tial dispe r s i on of the bea m . 27 deg r ee magn e ts ( c h r om a tic) des i g n ed t o e l i m in at e sp a tial dispe r s i on. • •

25. • S h ield i n g m a t eri a l : The t r e a t m e n t head co n s i s t s of a thick she l l of hig h - d ensity s hie l ding m a t e r ial such as l e ad, tu n g st en, or l e ad-tu n g st en al l o y . S h i e lding m a t e r i a l i s used t o a v oid the unne c e s sary • i r r a di a tion t o t he sur r oundi n gs, p a ti e n t t h e r adi a tion w or k e r s. as w e l l as • X - r a y t a r g e t : T he pen c il e l ect r on beam t h e x - r a y t a r g e t t o p r od u ce p h o t ons. s tr i k es on • X - r a y t a r g e t u sed i s t r ansm i s s i o n type t a r g e t .It u sed i s m a inly made of T ung s t en du e t o its high at o m ic 337 C. num b e r (Z = 74 ) & H i gh m e l ti n g p o i n t

26. • Primary c o l l i m a t or : T he t r e a t m e n t b eam is fi rs t c ol l im a t e d b y a fi x ed p r i m ar y c ol l im a t or lo c at ed i m m e di at e l y b e y ond the x - r a y t a r g e t . I n the c ase of x - r a y s, the c ol l im a t ed beam then pass e s th r ough the fl att ening fi l t e r . In the e l ec t r on m o de, the fi l t er i s m o v ed o ut of the w a y . • Fl a t t en i ng f i l t e r : M od if i es t he nar r o w , n on- u n i f orm p ho t o n beam a t the i s oce n t er i n t o a clin i c a l ly us e f u l beam th r ough a c omb i n a t i on of att enu a ti o n of the ce n t er of the beam and s c att er i nt o the per i p h e r y of the bea m . It i s made u p of l e ad, s t eel or c op p e r . •

27. • S c a tt ering f oi l : In t h e e l ect r on mode of l i nac o p e r a tion, nar r o w pen c il e l e ct r on beam, a b o ut 3 m m i n dia m e t e r ., in s t ead of s t r ik i ng the t a r g e t, is made sp r ead to s t r i k e an e l ect r on as w ell s c att e r ing f o il to the b eam as g e t a u ni f orm e l ect r on fluence ac r oss the t r e a tm e n t fi e ld. • The s c att e r ing f oil c o nsi s t s of a thin hig h -Z m e t al l ic f oil ( e . g ., l e ad, t a n t alu m ) . • The thick n ess of t he f oil i s such th a t mo s t of the e l ec t r o n s a r e s c att e r ed in s t ead of su f f er i ng b r em s s t r a h lu n g. C a r r o u sel i s a d e v i ce i n t r e a t m e n t head which hel p s i n the m o v e m e n t of ’Fl at t en i ng fi l t e r s of d i f f e r e n t ene r gies as w e l l as Sc a tt er i ng f oi l s ’ . •

28. • Beam moni t or i ng d e vic e s : The fl att e ned x - r a y beam or the e l ec t r on beam i s incide n t on t h e d ose mon i t or i ng c h ambe r s. • The mon i t or i ng s y s t em a r e t r ansm i s s i on ty p e i o n c h ambe r s or a s i n g le c hamber w i th mu l tiple pl at e s. c y l i n d r i c al thimb l e c h ambe r s h a v e a l so b een used in so m e l i n ac ’ s. • The f u n c tion o f the i o n c hamber i s t o mon i t or d ose r at e, i n t eg r at ed dose, a n d fi e ld s ym m e t r y . A s the c hambe r s a r e i n a hig h -i nt en s ity r adi a tion fi e ld a n d t he b eam i s p u l s ed, the ion c ol l ec t ion e f fici e n c y of the c h amb e r s should r e m ain u nchan g e d w i th cha n g es i n the dose r at e. Bias v ol t a g es i n the r a n g e of 30 t o 1 , 00 V a r e a p pli e d ac r oss the c h amber e l ect r odes, depe n ding on the c h amber des i g n . The mon i t or c hambe r s i n the t r e a t m e n t head a r e usual l y sealed s o t h a t their r esponse i s not i n fluenced b y t e m pe r a tu r e and p r es s u r e of the outside ai r . • • •

29. • Se c onda r y co l l im a t o r s : the b eam i s further c o l l i m a t ed by a c o n tin u o u s l y m o v able x - r a y c ol l i m at o r s. This c ol l im a t o r s c o nsi s ts of t w o pai r s of l e ad of t un g st en bloc k s ( j a w s} which p r o v i de a r ec t an g ular openi n g (f r o m X t o 4 X 4 cm 2 ) p r ojec t ed a t a s t an d a r d di s t an c e such as 10 c m f r om the x - r a y sou r ce. The c ol l im a t or bloc k s a r e c o n s t r a ined t o m ov e s o th a t the block e d g e i s al wa y s along a r adial l ine p assi n g th r o u gh the x - r a y sou r ce posit i o n . • • • Fie l d lig h t : T h e fi e ld s i z e d e finit i on i s p r o v i ded b y a l i g h t lo c al i zing s y s t em i n the t r e a tm e n t hea d . A c ombi n a tion of mir r or a n d a lig h t s o u r ce lo c at ed i n the space b e t w een t he c hambe r s and the j a w s p r ojects a l i g h t beam as i f e m i t ting f r om the x - r a y f o c al spot. Thus the l i g h t fi e ld i s c ong r u e n t w i th the r adi a t ion fi e ld. • • al l ow s ac c u r at e posit i oning of the r adi a tion fi e ld in r e l a tionship t o sk i m ma r k s or other r e f e r en c e poi n ts.

30. f d Field Lig h t and lase r s • • Field Lig h t : It i s a F i e l d lo c al i zing d e v i ce, Use d t o displ a y the p os i tion of the r adi a tion fi e ld on the p a ti e n t sk i n . A n h i gh accu r acy b u lb i s placed a t 45 an g le w i th the • Me r c u ry m i r r or placed in the p a th of the beam ( T r a n s m i s s i on ty p e m i r r or) . • The l i g h t field s i z e fi e ld s i z e. F i e ld s i z e ‘Lig h t fi e ld s i z e) i s i n c o n g ruen c e w i th the r adi a tion c an b e v ar i ed w i th t h e help of this • • Lase r s : The ac c u r a c y of t h e la s er g u ides in d e t ermining Isoce n t er posit i on. I s oce n t er i s a v i r tual poi n t w he r e the ce n t r al a x is o Ga n t r y , Col l i m at or and c ou c h m e e ts. 2 Side la s e r s, sa g gi t al and C e i l ing la s e r s a r e mou nt e on w al l s of LIN A C u n it. T ole r a n ce of la s er posit i on i s 2 mm • • •

31. T r e a tme n t t able(Couch) • T r e a tme n t t a ble (Cou c h) : T r e a tm e n t t able is a m e c h ani c al l y m o v able mo t or dri v en c ou c h . P a t i e n t i s positioned o v er t h e t r e a t m e n t t able ac c o r di n g t o the des i r ed c o-o r din a t es of plan n in g . P a ti e n t i s i m mob i l i z ed using the I m mob i l i z a tion d e v i ces. T r e a t m e n t t ab l e c an b e m o v e d H or i z o n t al, V ert i c a l as w e l l as R o ta tional di r ections. • • • • H a nd P en d e n t : It c o n t ains all the c o n t r ol s w i t c h es which c an b e used t o ac c ess t h e m o v e m e n t of Ga n t r y , C o u c h , Col l i m at or j a w s(Fie l d s i z e), S SD e t c .,

32. Cooling s y s t em • He a t dissi p a tion i n l i near ac c e l e r at or i s an i m por t a n t s t ep i n m a i nt enan c e i n la r g e s e tup and he a v y p a ti e n t load i n hospi t al s . The x - r a y s p r o d u ced a r e almo s t the 1 pe r ce n t of the e l ect r on e ne r g y wh i ch i s s tr i king on the t a r g e t . Hence 99 % of the ene r gy i s c o n v er t ed t o he a t. This he a t i s needed t o b e c ool e d a nd th a t i s ac h i e v ed b y the ‘Co o li n g s y s t em ’ . Cooling s y s t e m c onsi s ts of ‘ wat e r c h i l l e r ’ f or c o ol i ng t he w at e r and w a t er inl e ts and outl e ts t o v ar i ous parts of LIN A C inclu d ing X - r a y t a r g e t . • • • •

33. Radi a tion S a f e ty/i n t erlock s yst em • Sim i lar t o t r e a tme n t f r om r a d i a tion t h e S a f e ty f r om r a d i a tion al s o p l a y s an i m p or t a n t r ole in R a d iothe r ap y . V ar i o u s I n t er l o c k s a r e p r es e n t in LINAC t o a v oid t h e m i s -ha p pens or w r ong t r e a tme n t t o th e p a ti e n t. I n t er l o c k s in d i c at es t h e p r o b l e m in p art i c u lar d e v i ce in t h e LINAC as s embly a n d i n t er l o c king s y s t em h e l ps in so l v i n g the p art i c u la r ly a n d eas i l y . S a f e ty I n t erloc k s in c lu d e: • • • 1) 2) 3) La s t Man Out S w i t ch ( LMO) D oor i nt er l ock Beam O N/ O F F K e y e t c., • Eme r g en c y s wi t c h es a r e p r o v i ded a t all t h e s y s t e m s of an L IN A C u n it t o c omp l e t e l y turn O f f the e n ti r e Unit w i th only s i n g le s w i t ch d uring e m e r g en c y s i tu a tions.

34. W orking: Pho t on Beam The r a p y Ph o t o n s of e n e r gies 6 MV & 18 MV c an b e p r o d u c ed f o r t r e a t m e n t in Me d i c al l i near ac c e l e r at o r . M o dul a t or p r o d u c es the ne c e s sary po t e n tial f or the whole ac c e l e r at or ma i nly the Magn e t r on/Kl y s t r on and the e l ect r on gu n. E l e ct r o n s a r e p r oduced b y t he e l ect r on gu n a n d p r ocee d ed t o the w a v e g u ide s y s t e m . A n 3 m m e l ect r on ‘ p en c il beam’ i s p r o d uced du e t o acce l e r a t ion a nd the f o cu s i n g c oi l ’ s fi x ed all a r ou n d the w a v eg u ide. F o cu sed beam i s be n t b y t he be n di n g m ag n e ts and al s o the beam is further f ocused du e t o r e fl e cti v e f o r ce of the bending m agn e ts. The e l ect r on beam i s made t o s tr i k e on t h e X - r a y p r od u cing t a r g e t . • • • • • • • X - r a y s/Pho t o n ene r g y p r o d u ced is n o t u ni f orm in fl a t n e s s & is a s ymm e tr i c al i n n a tu r e, which i s made u n i f orm b y the fl att ening fi l t e r . • The u n i f orm f l att ened beam i s then pas s e s b y the m o ni t or i ng c h amb e r s and the dose r at e as w e l l as the s ym m e t r y i s v e r if i ed. The beam i s then r eady t o t r e a t the tumor/ P a ti e n t . •

35. W ork i ng: Elect r on Beam T h e r a p y E l e ct r on of e n e r gies 6 Me V , 9 Me V , 1 2 Me V , 15 Me V , 1 8 MeV & 20 MeV ca n • be p r o d u c e d f or t r e a tm e n t in Medi c al l i n e ar ac c e l e r at o r . M o dul a t or p r o d u c es the ne c e s sary po t e n tial f or the whole ac c e l e r at or ma i n ly the Ma g n e t r on / Kl y s t r on and the e l ect r on gun . E l e ct r o n s a r e p r oduced b y t he e l ect r on gu n a n d p r ocee d ed t o the w a v e g u ide s y s t e m . A n 3 m m e l ect r on ‘ p en c il beam’ i s p r o d uced du e t o acce l e r a t ion a nd the f o cu s i n g c oi l ’ s fi x ed all a r ou n d the w a v eg u ide. F oc u s e d beam i s be n t b y t he be n di n g m agn e ts and a l so the beam is f u rther f oc u sed du e t o r e fl e cti v e f o r ce of the ben d ing magn e ts. The e l ect r on beam i s made t o s tr i k e on t he Sc a tt er i ng f oil t o ma k e t he e l ect r o n s t o di s tr i b u t e i n f o r w a r d di r ectio n . E l e ct r on beam p r oduced i s un i f orm i n fl a tness & i s a s ym m e t r i c al i n n a tu r e, which c an b e di r ectly used f or t r e a t m e n t. The u n i f orm f l att ened beam i s then pas s e s b y the m o ni t or i ng c h amb e r s a n d the dose r at e as w e l l as the s ym m e t r y i s v er i fi e d . The beam i s then r eady t o t r e a t the tumor/ P a ti e n t . • • • • • • •

36. Ac c ess o ries used with LIN A C: • • • • • W ed g es D y na m ic w ed g e Bloc k s Mult i leaf Co ll i m a t o r (M L C) E le c t r o nic P o r t al Im a ging (EPID)

37. W ed g es • P h y s i c al w e d g es a r e g r a d u a t ed pieces of l ead th a t h a v e a thick end and a thin en d . The thin end c auses less at t e n u a tion than t he thick en d ; t his c aus e s a sh i ft i n the i s odose cu r v es w ith i n t h e t r e at ed v olu m e. The w e d g e i s deno t ed b y the an g le i t tilts t h e i s o d ose c u r v es e g . a 3 o w e d g e w ould c ause a 3 o ti l t i n the i s odose c u r v e s . P h y s i c al w e d g es a r e not i n c om m on use d ue t o the a b i l i ty of the independe n t j a w s t o per f orm dynam i c w e d ging . • • • •

38. Cu s t om Bloc k s Sig n if ic a n t ir r a d i a tion of the n ormal tis s u e o u ts i d e t h is v olume mu s t b e a v oided as m u ch as p o s s ible. These r e s tr i ctio n s c an gi v e r i se t o c ompl e x fi e ld s h a p es, which r eq u i r e i n tr i c at e b lockin g . C u s t om blocking s y s t em u se s a l o w mel t ing poi n t al l o y , L i p o w i t m e t a l ( Ce r r obe nd ) , which has a density of 9 . 4 g / c m 3 a t 20° C (83 % of the l e ad density) This m a t er i al c onsi s ts o f , 1 ) 50 . % bis m uth 2 ) 26 .7 % l e ad, 3 ) 13 .3 % tin, and 4 ) 10 % c adm i um • • • • • The ma i n ad v a n t a g e of Cer r obe n d o v er lead i s th a t it me l t s a t abo u t 7 ° C a nd ca n b e eas i ly c a s t i n t o a n y sha p e . A t r oom t e m pe r a tu r e, i t i s ha r der than l e a d . •

39. Multi Le a f Coll i m a t or • A mu l t i le a f c o l lim a t or (M L C) f o r pho t o n be a ms c onsi s ts o f a la r g e nu m ber o f c o l l i m a ti n g bl o c k s o r le av e s th a t c an be dri v e n au t om a ti c al l y , in d e p ende n t o f ea c h othe r , to g en e r a t e a f i eld o f a n y sh a p e . T y p i c ally the M L C s y s t ems c onsi s ts 60 t o 80 p ai r s, w hich a r e indepe n de n tly dri v e n . The in di v i d ual l eaf has a width o f 1 cm o r less a s p r ojec t ed a t the isoce n t e r . • • • The le a v es a r e made of tu n g st en a l l o y (p=17.0 to 18.5 g / cm3 ) and h a v e thickn e ss along the b e a m di r e c tion r an g i n g f r om 6cm t o 7.5cm, dep e n d i n g o n the t y pe of a c c e le r a t o r . The le a f thic k ne s s i s su f f i cie n t t o p r o v ide pri m a r y x - r a y t r an s missi o n th r ou g h the le av es o f less than 2%. The pr i ma r y b e am t r ansm i ssion m a y be fu r ther m i nimi z ed b y c o m bin i n g ja w s with the M L C i n shiel d i n g a r e a s outside the M L C f i eld openin g . • •

40. Elect r onic P or t al I m aging (EPID) • E l e ct r onic mak i ng p or t al i m aging o v e r c om e s t w o p r o ble m s by it poss i ble to v i e w the por t al imag e s in s t a n t aneously 1) r eal ti m e i m a g es c a n b e d i s pl a y e d o n co m p u t er sc r ee n b e f o r e i n iti a ti n g a t r e a tm e n t. 2) P or ta l ima g e s ca n a l so b e s t o r e d o n c o mp u t e r f or l a t e r viewi n g o r a r c h iving. E P I Ds use fl a t pa n el ar r a y s of so l id s t a t e d et ec t o r s b a sed on a m orph o us s i li c o n ( a-Si) t ec h nol o g y .Fl a t p anel ar r a y s a r e c ompact, & easier t o mo u n t o n a r e t r ac t able arm f o r posit i oning i n or out of the fi e ld. • • A sci n t i ll a t or c o n v erts the r adi a tion b e am i nt o v i s ible pho t ons. The l i g h t i s d e t e c t e d b y a n ar r a y of p ho t o diod e s i m pla nt ed on an a m orphous s i l ic on pane l .

41. On B o a r d I maging (OBI) • C T s c ans acqui r ed w i th d e t e c t o r s i mb e dded i n a fl a t pa n el i n s t ead of a ci r c ular r i ng i s kn o wn as Onboa r d Im a ging . C T s c an n i ng that u s es th i s type of g eome t r y i s kn o wn as c on e - beam c ompu t ed t omog r ap h y ( CB C T) . In c on e -b e am C T , planar p r oje c t i on i mages a r e o b t a i n e d f r om mult i ple d i r ections a s the so u r c e w i th the o p po s i ng d e t e c t or pa n el r o t a t es a r ound the p a t i e n t th r ou g h 180 d e g r e e s or mo r e . Th e se mult i di r e c t i onal i mages p r o vide s u f fi c i e n t i n f orm a t i on to r e c on s t r uct p a tie n t an a t o m y i n 3D , i ncluding c r os s - s e c t i ona l , s a gi tt al, and c o r onal plan e s . A fi l t e r ed b a c k-p r oject i on al g o ri thm i s u s ed t o r e c on s t r uct t he v o l ume t r i c i mage s . Th e y a r e mou n t ed on the a c c el e r a t or g a n try and c an b e u s ed to • • • • • acqui r e v o l u m e t ri c i mage d a t a unde r ac t ual t r e a tme n t c ondit i on s . Th e y e n able t h e • l o c al i z a t i on of plann e d t a r g e t v o l ume and c r i t i c al s t r uc t u r es b e f o r e each t r e a tme n t. The s y s t em c an b e i mplem e n t ed either b y u s i ng a ki lovo l t a g e x- r a y so u r c e or the m e g a v ol t a g e the r ap e ut i c so u r c e . •

42. • Kilo v ol t a g e CB C T : • Kil o v ol t a g e x - r a y s f or a ki l o v ol t a g e CB C T ( k V CB C T) s y s t em a r e g ene r at ed by a c o nv e n tional x - r a y tube th a t is m ou n t ed on a r e t r ac t able arm a t 9 t o t h e t h e r apy b eam di r ectio n . A fl a t p a n el of x - r a y d e t ec t o r s is mou n t ed o p posi t e the x - r a y tu b e. This imaging s y s t em is v e r s a ti l e a n d is capa b le of c o n e -beam C T as w e l l 2 -D r a d iog r a p h y a n d fluo r os c op y . A d v a nt a g es: P r od u ce v olu m e tr i c C T i m a g es w ith good c o n t r a s t an sub m i l l i m e t er sp a tial r eso l ution. ac q ui r e i m a g es i n the r a p y r oom c oo r din a t es, and U s e 2 -D r adiog r aphic and fluo r os c opic m odes t o v e r i f y p or t al ac c u r ac y , mana g e m e n t of p a ti e n t mot i on, and mak i ng p os i tional and dosi m e tr i c adj u s tm e n ts b e f o r e and d u r i ng t r e a tm e n t. • • • 1. 2. 3.

43. • Me g a v ol t a g e CB C T • MVCB C T uses the me g a v ol t a g e x - r a y beam of the l i near ac c e l e r at or and its E PID mou nt ed op p os i t e the sou r ce. E PIDs w i th the a -Si fl a t pa n el d e t ec t o r s a r e sens i ti v e enou g h t o al l o w r apid ac q uis i tion of m ultip l e, l o w -dose i m a g es as the g a n t r y is r o tat ed th r ou g h 180 or mo r e. M u lt i di r ectional 2 -D i m a g es, v olu m e tr i c C T i m a g es a r e r e c on s truc t ed f r om thes e . MV C B C T s y s t e m has g o o d i m a g e q u al i ty f or the b o n y a n a t o m y and, i n e v en f or soft ti s sue t a r g e ts. M V CB C T is a g r e a t t ool f or; On -l i ne or p r e t r e a tm e n t v er i fi c a tion of p a ti e n t posit i onin g , A n at om i c m a t c h ing of plan n ing C T , P r e t r e a tm e n t C T , a v oidan c e of crit i c al s truc t u r es such as spinal c o r d, and ide n tif i c a tion of i m pla n t ed m e t al ma r k e r s i f used f or p a ti e n t s e t up . • • • • a) b) c) d)

44. • A d v a nt a g es of M V CB C T o v er k V CB C T : a) 1 Le s s susce p t ibility t o art i f a cts du e t o hi g h-Z o b jects such as m e t a llic ma r k e r s i n the t a r g e t, m e t al l ic hip i m pla n ts, and de n t al fi l l i n g s b) 2 . No need f or e xt r a p ol a t ing att enu a tion c o e f fici e n t s f r om kV to m e ga v o l t a g e pho t on ene r gies f or dos im e tr i c c or r ecti o ns • F r om i ma g e s w e ca n s a y t h a t M V -C B C T ima g e o f 2.5c G y is s u f fic i en t an a t o m y v e ri f i c a ti o n du ring p a ti e n t p o s iti on ing. f or B o n y

45. E -ma i l:- augnasjohnson97 @gmai l . c om Thank Y ou for your patience . AUGNAS JOHNSON M.Sc R a diat i on P h ys i cs R adi a t i on Phys i cs Depa r t m ent KIMS THIRUVANANDAPURAM