OPERATIONS EXPERIENCE AT THE BEVALAC RADIOTHERAPY FACILITY

OPERATIONS EXPERIENCE AT THE BEVALAC RADIOTHERAPY FACILITY* J.K. A l o n s o , T.L. C r i s w e l l , J . Howard, W.T. Chu, R.P. Singh, D. G e l l e r , and M. Nynan Lawrence Berkeley Laboratory U n i v e r s i t y o f C a l i f o r n i a , Berkeley, C a l i f o r n i a , 94720 Introduction Tlie use o f the Bragg peak o f charged p a r t i c l e s for medical treatments was f i r s t proposed i n 1946 by Wilson*. The g r e a t l y enhanced d o s e d e p o s i t e d a t the end o f the beam range compared t o the e n t r a n c e d o s e , and the sharp f a l l o f f o f dose beyond the beam range and a t the edge o f the c o l l i m a t e d f i e l d due t o small h e a v y - i o n s t r a g g l i n g and m u l t i p l e s c a t t e r i n g lead t o very p r e c i s e l o c a l i z a t i o n o t the d o s e d e l i v e r e d t o the t r e a t e n t volume, thus maximizing the sparing o f normal tissue. In a d d i t i o n , the enhanced b i o l o g i c a l e f f e c ­ t i v e n e s s due t o the high LET (dE/dx) o f heavy ions has a l s o pointed t o t h e s e beans a s p o t e n t i a l l y being a very s u p e r i o r modality for the treatment of c a n c e r . T e s t i n g o f these i d e a s became p o s s i b l e when heavy i o n beams of adequate energy and i n t e n s i t y became a v a i l a b l e with the c r e a t i o n o f the Bevalac i n 1974.2 During the f i r s t y e a r s o f Bevalac o p e r a t i o n the biomedical e f f o r t c o n c e n t r a t e d on r a d i o b i o l o g y work, l a y i n g the foundation for p a t i e n t radiotherapy.-* »^ A d e d i c a t e d radiotherapy area was created i n 1978,-^ and i n 1979 f u l l - s c a l e p a t i e n t treatment was begun. As o f now over 500 treatments with carbon, neon and argon beams have been d e l i v e r e d t o about 50 p a t i e n t s , some as b o o s t s from o t h e r m o d a l i t i e s and some as complete heavy ion t r e a t m e n t s . Up t o 12 p a t i e n t s per day have been t r e a t e d i n t h i s f a c i l i t y . Continuing e f f o r t s in r e f i n i n g t e c h n i q u e s and o p e r a t i n g p r o c e ­ dures a r e i n c r e a s i n g e f f i c i e n c y and accuracy of t r e a t m e n t s , and a r e c o n t r i b u t i n g t o the a l l e v i a t i o n of s c h e d u l i n g d i f f i c u l t i e s caused by the unique requirements of radiotherapy with human p a t i e n t s . Facilities The treatment a r e a , beam d e l i v e r y and dosimetry instruments were reported in an e a r l i e r paper so w i l l only be b r i e f l y d e s c r i b e d . Figure 1 shows the treatment room with support a r e a s for t h e r a p i s t s , p h y s i c i s t s , o p e r a t o r s and t e c h n i c i a n s . Beam e n t e r s the treatment room from the lower r i g h t c o r n e r , p a s s e s a l o n g o p t i c a l t a i l s c o n t a i n i n g beam shaping, range modulation and dosimetry apparatus and f i n a l l y i n t o the p a t i e n t . The p a t i e n t i s supported on a P h i l i p s Mark I r a m - s t y l e c o u c h with attachments t o h o l d t h e p a t i e n t i n a wide v a r i e t y o f p o s i t i o n * t o b e s t u t i l i z e our s t a t i c h o r i z o n t a l bean. Alignment o f t h e p a t i e n t i s performed with x-ray and l a s e r systems c a r e f u l l y p o s i t i o n e d i n r e l a t i o n t o t h e beam a x i s . Bean preparation u t i l i z e s the s c a t t e r i n g f o i l - occluding ring system^'** shown i n Figure 2 . The t i g h t l y focused a c c e l e r a t o r beam i s passed through the f i r s t s c a t t e r i n g f o i l , 9 mm o f l e a d placed 10 Meters upstream o f the p a t i e n t . At a p o i n t 6 meters from the p a t i e n t o c c l u d i n g r i n g s block out p o r t i o n s o f the Gaussian beam, and a second s c a t t e r e r d i f f u s e s the beam passing through open s p a c e s i n the r i n g s y s t e m . As the beam moves towards the p a t i e n t the h o l e s are f i l l e d i n by s c a t t e r e d beam, and i f e v e r y t h i n g i s done c o r r e c t l y , r i g h t beam w i d t h , r i n g diameters and d r i f t d i s t a n c e s , the beam i n t e n s i t y d i s t r i b u t i o n w i l l be f l a t t o w i t h i n + 2% over the d e s i r e d 20 t o 25 cm diameter f i e l d used for t r e a t m e n t . 1st scatterer Figure 2. Occluding Rin^ beam d e l i v e r y system. JH Figure 1. The Bevalac Radiotherapy F a c i l i t y . The s e l e c t i o n of parameters i s e n t i r e l y e m p i r i c a l , so that a s u b s t a n t i a l amount of beam-development i s necessary t o o b t a i n the d e s i r e d f i e l d s i z e and u n i f o r m i t y . To aid in t h i s development and in f i n e - tuning parameters for d i f f e r e n t ions and e n e r g i e s heavy use has been made o f MEDUSA, a m u l t i - p l a n e m u l t i - w i r e p r o p o r t i o n a l chamber capable o f recon­ s t r u c t i o n in a few seconds beam i n t e n s i t y p r o f i l e s over a f u l l 25 cm diameter c i r c l e ^ « 1 0 . This chamber, with a s p a t i a l r e s o l u t i o n of a few m i s m capable o f d e t e c t i n g i n t e n s i t y v a r i a t i o n s as small a s 1 or 2%. Because of i t s rapid turn-around, one can perform a s u b s t a n t i a l amount o f beam-development in a short period of time. Beam range modulation, t o p l a c e the Bragg peak a t the treatment depth and t o d i s t r i b u t e the stopping p a r t i c l e s over the f u l l th^ *ness of the tumor i s performed with b r a s s s p i r a l ridge f i l t e r s and an a d j u s t a b l e water c o l u m n . Dose monitoring i s done with t h r e e m u l t i - segmented i o n i z a t i o n chambers** and a secondary e l e c t r o n monitor (StM) l o c a t e d a l o n g the bean path. Each i o n chamber has two c o l l e c t i n g f o i l s , the f i r s t i s d i v i d e d i n t o quadrants providing e a s t / w e s t and top/bottom balance i n f o m a t i o n , t h e second i s d i v i d e d i n t o c o n c e n t r i c r i n g s g i v i n g data on the r a d i a l d i s ­ t r i b u t i o n o f the bean i n t e n s i t y . A t o t a l o f e l e v e n independent s i g n a l s are generated by each ion chamber. The i o n chambers a r e l o c a t e d a s f o l l o w s ; one j u s t upstream o f the second s c a t t e r e r (at the beam entrance t o the treatment room), the second half-way t o the p a t i e n t , and the t h i r d j u s t upstream o f t h e T h i s work was suaaortna by t h e A s s i s t a n t S e c r e t a r y f o r Health and Environnrntal Research D i v i s i o n of the U . S . Departs*** o f Kncrgy wnoer Contract W-7405-ENC-4S, ana by the National I n s t i t u t e s o f Health Grant CA151S4, ***tl