Gamma Knife vs. cyberKnife by fdh56iuoui


									From research to Pr actice

Gamma Knife vs. cyberKnife
by S. Christopher Hoffelt, MD

               he	 procedure	 called	 “stereotactic	 radiosur-        How Gamma Knife Works
               gery”	 (SRS)	 is	 based	 on	 a	 simple	 concept.	 A	   Accuracy	 of	 the	 Gamma	 Knife	 is	 achieved	 by	 fixation	
               series	of	radiation	beams	converges	on	a	target	       of	 the	 patient’s	 skull	 to	 the	 treatment	 table	 by	 use	 of	 a	
               from	 various	 angles	 (Figure	 1).	 With	 proper	     fixed	head	frame	(see	Figure	2).	A	Gamma	Knife	treat-
               planning,	a	high	dose	of	radiation	is	given	to	a	      ment	begins	with	placement	of	this	head	frame,	usually	
target,	usually	a	tumor,	with	minimal	dose	to	the	surround-           by	a	neurosurgeon,	on	the	morning	of	treatment.	After	
ing	tissue.	Ideally	this	dosage	results	in	                                                 local	anesthetic	is	applied,	four	screws	

                                                                                                                                                             Photo courtesy of elekta, Inc.
destruction	of	the	tumor	while	sparing	                                                     are	used	to	tightly	secure	a	rigid	metal	
function	 of	 crucial	 organs	 or	 tissues	                                                 frame	 to	 the	 patient’s	 skull.	 Once	 the	
adjacent	to	the	treatment	area,	such	as	                                                    frame	 is	 properly	 secured,	 the	 patient	
the	optic	nerve	or	brainstem.                                                               is	brought	to	the	CT	scanner	and	fixed	
     For	 nearly	 100	 years,	 this	 use	 of	                                               to	the	scanning	table	in	a	supine	posi-
“cross-firing”	 beam	 techniques	 has	                                                      tion.	A	CT	scan	is	acquired	for	treat-
evolved	 to	 allow	 treatment	 for	 both	                                                   ment	planning,	and	the	patient	is	then	
intracranial	 and	 extracranial	 sites.	                                                    released	 from	 the	 table	 and	 awaits	
The	 most	 widely	 accepted	 use	 for	                                                      treatment.	
SRS	 is	 still	 for	 intracranial	 disease.	                                                      The	 treatment	 planning	 process	
SRS	 is	 now	 a	 standard	 option	 for	                                                     begins	as	a	cooperative	effort	between	
many	malignant	and	benign	lesions	of	                                                       neurosurgeon,	 radiation	 oncologist,	
the	 brain,	 as	 well	 as	 some	 functional	 	     Figure 1: Schematic of                   and	 physicist.	 The	 crucial	 first	 step	
conditions	(Table	1).                              stereotactic radiosurgery                is	 the	 delineation	 of	 the	 target	 and	
     Arguably	the	best	known	stereo-               with the Gamma Knife®                    nearby	 critical	 tissues.	 These	 struc-
tactic	radiosurgery	unit	is	the	Gamma	                                                      tures must	be	outlined	by	hand	on	the	
Knife®,	manufactured	by	Elekta.	The	                                                        planning	CT	scan.	In	many	cases,	the	

                                                                                                                                              Photo courtesy of elekta, Inc.
first	 Gamma	 Knife	 unit	 in	 North	                                                       CT	images	alone	suffice.	If	necessary,	
America	was	installed	at	the	Univer-                                                        clearer	 images	 from	 an	 MRI,	 done	
sity	 of	 Pittsburgh	 Medical	 Center	 in	                                                  within	 1-2	 weeks	 prior	 to	 treatment,	
1987	(Figure	2).	As	SRS	has	become	an	                                                      can	be	fused	to	the	CT	to	allow	more	
accepted	 standard	 for	 malignant	 and	                                                    accurate	contouring.	When	complete,	
benign	conditions,	the	Gamma	Knife	                                                         radiation	dose	delivery	is	planned	by	
and	 other	 SRS	 systems	 have	 become	                                                     placing	 one	 or	 more	 isocenters.	 Each	
more	affordable.	Hence,	the	technol-                                                        isocenter	 represents	 a	 point	 at	 which	
ogy	 has	 expanded	 beyond	 primarily	 Figure 2: The Gamma Knife®                           all	 of	 the	 beams	 will	 converge	 for	 a	
university	hospitals	into	private	hos-            uses a light-weight head frame,           certain	 amount	 of	 time	 (see	 Figure	
pitals	and	freestanding	centers.                  which is affixed to the patient,          3).	The	result	is	a	high	radiation	dose	
     The	CyberKnife®	is	one	notewor-              for immobilization.                       around	the	isocenter	with	a	sharp	fall-
thy	example	of	these	“newer”	systems,	                                                      off	in	the	surrounding	tissue.	Spheri-
and	 resembles	 the	 Gamma	 Knife	 in	                                                      cal	 lesions	 require	 a	 single	 isocenter,	
more	 than	 just	 half	 its	 name.	 [And	                                                   while	 irregular	 shapes	 require	 multi-
despite	 the	 name—stereotactic	 radio-                                                     ple	isocenters	adjacent	to	one	another	
surgery	 is	 “knifeless”	 surgery.]	 Both	                                                  to	conform	to	the	desired	shape.	Beam	
systems	 can	 effectively	 and	 accurately	                                                 size	 can	 also	 be	 adjusted	 to	 achieve	
perform	 SRS	 for	 intracranial	 lesions.	                                                  necessary	 shape—the	 Gamma	 Knife	
The	 CyberKnife	 differs	 from	 the	                                                        has	 circular	 collimators	 of	 4,	 8,	 14,	
Gamma	Knife	by	employing	real-time	                                                         and	 18	 millimeters	 that	 can	 be	 used	
X-ray	images	to	guide	treatment;	and	as	                                                    interchangeably.
a	result	has	expanded	SRS	to	sites	out-           Figure 3: A typical Gamma                       When	 planning	 is	 complete,	 the	
side	the	brain.	A	basic	understanding	of	 Knife® treatment plan. Yellow                     patient	is	placed	supine	on	the	Gamma	
both	the	Gamma	Knife	and	CyberKnife	 demarcates the prescribed                              Knife	table	and	the	head	frame	is	fixed	
is	crucial	for	any	institution	to	evaluate	 dose; four isocenters are used                  to	 the	 unit.	 The	 position	 of	 the	 head	
the	potential	for	acquiring	and	support-          in this plan.                             frame	is	adjusted	with	a	series	of	dials	
ing	this	technology.                                                                        such	that	the	target	is	at	the	center	of	

18	                                                                                         Oncology Issues		September/October 2006
                                  the	beam,	as	defined	by	the	plan.	The	                                                   with	 multiple	 beams	 simultaneously,	
                                  most	recent	Gamma	Knife	unit	makes	                                                      the	 CyberKnife	 uses	 a	 single	 high-
                                  these	 adjustments	 automatically.	 The	                                                 energy	photon	beam	fixed	to	a	robot	
                                  patient	 is	 then	 monitored	 from	 out-                                                 arm.	The	arm	moves	the	beam	to	dif-
                                  side	the	room	while	treatment	is	given,	                                                 ferent	 positions	 during	 the	 course	 of	
                                  usually	 30	 to	 90	 minutes.	 Usually	 a	                                               treatment,	all	converging	in	the	treat-
                                  single	 operator	 is	 necessary	 to	 assure	                                             ment	 area	 (see	 Figure	 4).	 Unlike	 the	
                                  safe	 treatment	 delivery	 and	 patient	                                                 Gamma	 Knife,	 a	 head	 frame	 is	 not	
                                  monitoring,	 most	 often	 a	 physicist	 or	 Figure 4: The CyberKnife®                    required	for	sub-millimeter	accuracy.	
                                  radiation	therapist.                              uses a plastic mask for patient        Instead,	the	patient	is	fixed	to	the	treat-
Photo courtesy of accuray, Inc.

                                       It	is	not	practical	to	leave	the	head	 immobilization.                              ment	 table	 with	 a	 firm	 plastic	 mask,	
                                  frame	in	place	for	more	than	one	or	two	                                                 and	the	robot	is	guided	by	a	series	of	
                                  days,	 so	 treatments	 with	 the	 Gamma	                                                 X-ray	images	of	the	skull	taken	during	
                                  Knife	 are	 generally	 limited	 to	 a	 single	 dose.	 The	 patient	 treatment.	The	position	of	the	skull	is	updated	real-time,	
                                  typically	remains	hospitalized	while	the	head	frame	is	in	          and	the	robot	adjusts	the	beam	to	account	for	any	skull	
                                  place. This	dosage	is	appropriate	for	most	standard	appli-          movement.	Thus,	SRS	for	cranial	lesions	is	non-invasive,	
                                  cations.	The	time	allowed	for	treatment	planning	is	lim-            and	 recently	 updated	 CyberKnife	 planning	 software	
                                  ited	but	usually	poses	no	difficulty	for	experienced	users.         allows	treatment	of	some	spine	lesions	using	image	guid-
                                                                                                      ance	alone,	without	fiducial	markers.	Some	CyberKnife	
                                  How CyberKnife Works                                                centers	are	treating	selected	patients	routinely	with	this	
                                  The	CyberKnife	is	another	capable	SRS	system	for	treat-             method,	though	experience	is	still	limited.
                                  ing	 brain	 lesions,	 but	 its	 design	 has	 expanded	 SRS	 for	         For	 most	 treatments	 outside	 the	 skull	 and	 spine,	 the	
                                  lesions	anywhere	in	the	body,	including	structures	that	            CyberKnife	requires	more	than	just	bone	anatomy	to	guide	
                                  move	 with	 respiration.	 While	 the	 Gamma	 Knife	 treats	         the	beam	position.	These	areas,	especially	lesions	that	move	
                                                                                                      with	 respiration,	 require	 the	 placement	 of	 gold	 fiducials	
                                                                                                      near	 or	 within	 the	 target.	 Fiducials	 are	 usually	 placed	 as	
                                                                                                      a	 simple	 outpatient	procedure,	similar	to	a	needle	biopsy.	      	
                                     taBLe 1: common conditions for Which                             X-ray	 images	 capture	 the	 position	 of	 these	 markers	 and	
                                     stereotactic radiosurgery is indicated                           guide	the	robot	during	treatment	to	correct	for	movement.	
                                                                                                      For	targets	that	move	with	respiration,	the	CyberKnife	can	
                                     Neoplasms                                                        correlate	the	beam	position	with	the	pattern	of	respiration	
                                     Brain	metastases	                                                and	 adjust	 accordingly.	 This	 feature	 is	 useful	 for	 treating	
                                     Acoustic	neuroma/Vestibular	schwannoma	                          tumors	in	the	lung	or	liver.	
                                     Meningioma	                                                           The	design	of	the	CyberKnife	results	in	a	different	
                                     Pituitary	adenoma	                                               treatment	 process	 for	 the	 patient.	 The	 fiducials,	 if	 nec-
                                     Glioma/astrocytoma	                                              essary,	 must	 be	 implanted	 several	 days	 before	 planning	
                                     Chordoma/Chondrosarcoma                                          images	can	be	acquired.	Images	are	then	acquired	as	an	
                                     Craniopharyngioma	                                               outpatient,	with	one	or	two	visits	for	a	CT	scan	and	MRI	
                                     Hemangioblastoma	                                                if	necessary.	Planning	is	performed	while	the	patient	is	at	
                                     Ocular	melanoma                                                  home,	without	the	time	pressure	associated	with	the	head	
                                     Nasopharynx	carcinoma                                            or	 body	 frame.	 The	 planning	 is	 typically	 a	 joint	 effort	
                                     Glomus	jugulare	tumors                                           between	 radiation	 oncologist,	 surgeon,	 and	 physicist,	
                                                                                                      usually	 within	 one	 day.	 Complex	 cases	 may	 take	 sev-
                                     Vascular Disorders of the Brain                                  eral	iterations,	occasionally	requiring	two	or	more	days.	
                                     Arteriovenous	malformations	(AVM)	                               Treatments	are	usually	delivered	by	a	radiation	therapist,	
                                     Arteriovenous	fistulas	(AVF)	                                    with	physicians	present	for	the	initiation	of	treatment.	
                                     Cavernous	malformations	
                                                                                                        CyberKnife vs. Gamma Knife
                                    Other Disorders                                                     A	 fair	 comparison	 regarding	 CyberKnife	 and	 Gamma	
                                    Trigeminal	neuralgia	(tic	douloureux)	(painful		                    Knife	must	be	limited	to	their	ability	to	treat	lesions	in	
                                    	 condition	of	the	face)	                                           the	head,	as	the	latter	cannot	treat	extracranial	sites.	The	
                                                                                                        need	for	precision	in	SRS	treatments	within	or	adjacent	

                                  Oncology Issues		September/October 2006                                                                                             19
The ability to treat any body site distinguishes
CyberKnife from the Gamma Knife.

to	 sensitive	 brain	 tissues	 is	 obvious,	 hence	 accuracy	 is	      obtained	from	each	company	can	vary	by	region	but	are	
a	reasonable	first	point	of	discussion.	Both	the	Gamma	                comparable,	 ranging	 from	 $3.4	 to	 $4	 million	 dollars.	 It	
Knife	 and	 CyberKnife	 employ	 regularly	 scheduled,	                 is	 also	 important	 to	 consider	 installation,	 physics,	 and	
strict	quality	assurance	measurements,	including	film,	to	             therapist	 training	 and	 support,	 and	 maintenance	 costs	
simulate	a	treatment.	For	both,	the	accuracy	of	treatment	             for	these	units.	Because	these	costs	also	vary	by	region,	it	
delivery	can	usually	be	confirmed	to	within	less	than	0.5	             is	important	to	discuss	these	factors	with	their	respective	
mm.	Other	sources	of	error	are	unaccounted	for	in	these	               companies.	
measurements.	The	CyberKnife	real-time	image	capture	                        The	long,	established	history	of	Gamma	Knife	cer-
may	generate	an	additional	0.5	mm	error.1	The	Gamma	                   tainly	contributed	to	the	development	of	the	CyberKnife	
Knife	head	frame	is	not	entirely	rigid,	and	may	account	               and	 other	 intracranial	 and	 extracranial	 SRS	 systems.	
for	less	than	0.5	mm	up	to	1.7	mm	inaccuracy. 2,3	Regard-              Direct,	formal	clinical	comparisons	will	likely	be	avail-
less	 of	 these	 potential	 sources	 of	 error,	 most	 users	 will	    able	in	the	future,	as	more	data	for	intracranial	and	extra-
agree	that	accuracy	of	both	units	is	more	than	adequate	               cranial	applications	emerge.	While	costly,	both	units	are	
for	treatment	of	appropriate	intracranial	lesions.                     practical	and	effective	for	appropriate	patients.	
      Clinical	efficacy	is	a	second	important	point	of	dis-
cussion.	 For	 the	 numerous	 applications	 listed	 in	 Table	         S. Christopher Hoffelt, MD, is medical director of
1,	use	of	the	Gamma	Knife	has	been	arguably	the	domi-                  Radiation Oncology at Southwest Washington Medical
nant	resource	for	published	literature	regarding	efficacy	             Center and assistant professor of Radiation Oncology at
of	SRS	for	the	past	30	years.	Literature	specific	to	the	              Oregon Health Sciences University in Portland, Ore.
newer	CyberKnife	is	far	less	abundant	but	continues	to	
emerge.	It	is	fair	to	consider	CyberKnife	efficacy	simi-               References
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