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Advanc,es in Pfysics
AEVANCE NOTICE ARTICLE
Friday 21 Jt$tr S@ Afiernoon I rlour s nfnries
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Medicaf uses of gamma rays
Radlotherapy and modlcal lmaglng
It has been known frcr a long time that gamma rays are electrwtagmetic photons of high energy, usually more than about 100 keV. During that time, gamma rays tsve given scientists inbrmation at scales rarqging from the very srnalbst - tre energy bre of fle nucleus - to the very largest, with mysterious astronomical phenomena called gsmrna rqy bursts sending us eignals from the eadbst days of the Univerce.
Modicine is one important area where gamma rays have ficund uscs. Affir*W is one of the earliest uses of gamma radiation, but uses in medlcaf tfitqg$Ng have developed considerably over th€ past few years as imaging techniques ard: qwv:*qlr.6Ee ol gamma radiation have become available. Because of the hazards aseociated;Witfi gpmma mdia$on, the protectlon of pattcnte and medlcal staff is an important point to ooni*ider in any medical apllcatisn of radioactivity.
Rdiothcrapy
Gsncprous tissues are rnore susceptiile b radiation damage than heaW tbgues, so normal tissue will recover from a dosa that vrould destroy a tumour. Intense beams of X.rap or gamma rrys can prwide the radbtion, althougrh partic-le beams produced by linear mebrators are nolv Oemming more common. Substantiai Oces, a brr siererts (Sv) in sirc,8t€ n€Gded, altfnugh thoee are usually administered over several sessions in order to albrr fre healthy tissue to recuprate. Such high dooes are many orders of magnitude greater tfian lfesg eprienced in ensVOaV life, as shown in the taUe following.
sourue of radatlon dos6In pSv
transaflantic fiight 3
typicaldaily dose due to bad<grcund
radiation in the UK 5
one chest X-ray 50
The gamma source usually used in radiotherapy is cobalt-60, which has a half-life of 5.3 years and emits very penetrating gamma rays with two different energies, 1.17 and 1.33 MeV. To ensure the maximum dose reaches the tumour without giving surrounding healthy tissues a 25 large dose, the radiation source can be rotated during treatment, as shown in Fig. 1.
moveable radiation source
patient (tumour shown in black)
F i g . l
Medical imaging
Gamma scans
This is a valuable method of investigating whether organs are functioning properly. A chemical which is normally metabolised by a particular organ is 'labelled'with a radioactive isotope of an element, This radiochemical is then injected into the body, and the blood stream quickly carries it to the organ in question, where it is absorbed by the healthy tissue. A gamma-detector near the body can then detect which parts of the organ concerned are working properly, for they will be emitting gamma photons. Non-functioning parts will not absorb the radiochemical, and so will not emit gamma photons.
An efficient detector of gamma photons needs a large mass of material containing nuclei of large proton number. This is done with a crystal of sodium iodide doped with thallium. This crystal absorbs gamma photons and emits visible photons. The visible photons are detected by photomultiplier tubes mounted on the crystal.
30
35
Using an array of photomultiplier tubes allows the radiographer to obtain an image of the region 40 of int-erest, but it is important that each photomultiplier detects only the gamma photons heading directly towards it. This is done by collimating the gamma radiation with a lead block containing holes perpendicular to the crystal. The basic design for a gamma camera is shown in Fig. 2.
photomultiplier tube
lead sodium iodide crystal
doped with thallium s h i e l d i n g
face of gamma camera collimating channel
Fig.2
The visible photons are detected by photomultiplier tubes, as shown in Fig. 3 below. Light shields ensure that only a photon emitted by the sodium iodide crystal immediately adjacent to 45 each photomultiplier tube can strike the sensitive photocathode. This light-sensitive photocathode
emits a single electron when it absorbs a visible photon. This electron is accelerated onto a photomultiplying electrode, knocking off extra electrons as it collides. These electrons pass between a series of these photomultiplying electrodes, exponentially increasing the number of electrons present. Eventually, a cascade of electrons reaches the anode.
I
)
sodium iodide
crystal photocathode
F i g . 3
SO Early applications of gamma scanning used nuclides such as iodine-121, which is absorbed by the thyroid gland in the neck. Unfortunately, iodine-121 emits beta particles as well as gamma photons during the decay.
'u'lt -t?Ix"+ _!e+v
photomultiplying electrodes
II
section
55
60
65
70
75
The beta particles are scattered too readily to contribute any useful data to the scan at all, and many of them are absorbed in the body, increasing the dose to the patient.
The commonest gamma emitter currently in use is technetium-99m, where the 'm' stands for metastable - the nucleons are arranged in such a way that the nucleus is at a higher energy level than the ground state. lt therefore decays to the ground state, emitting a 140 keV gamma photon as it does so. Technetium-99m has the advantage over nuclides such as iodine-121 that it does not emit a beta particle with the gamma photon, so reducing the dose that the patient receives.
PET scans
A modern development of gamma scanning uses nuclides that emit positrons, the anti-particles of electrons. The familiar beta decay (more correctly termed beta-minus decay) occurs when a neutron in the nucleus is converted into a proton, an electron and an anti-neutrino. This happens in nuclides where the nucleus contains too many neutrons for stability, as in the iodine- 121 example above. Positron emission occurs in beta-plusdecay when a proton in the nucleus is converted into a neutron, a positron and a neutrino. This occurs in nuclides where the nucleus contains too many protons for stability. One common example used is oxygen-15:
' 3 o + t ? t l * * ! e + v
A positron, once emitted, does not travel far before annihilating with an electron to give a pair of gamma photons of characteristic energy. These photons travel in opposite directions, and are detected by a pair of gamma cameras on opposite sides of the patient.
The fact that two identical gamma photons are produced allows the source of radiation to be p i n - p o i n t e d w i t h g r e a t p r e c i s i o n . C o i n c i d e n c e c i r c u i t r y i n t h e d e t e c t o r e n s u r e s t h a t o n l y simultaneous gamma photons in opposite directions, arising from positron-electron annihilation, are counted. This ability to discount noise, together with the rotation of the detectors around the patient to observe photons emerging in different directions, allows the source of positrons to be identified with an accuracy ol + 2mm. With a resolution of 2mm in each direction, the emissions from a brain of volume 600cm3 would be coded in a set of 75000 pixels. These pixels can be accessed by the computer in different ways to give two-dimensional "slices" through the brain, as shown in Fig.4, where a light pixel indicates high emission and a dark pixel low emission.
F i g . 4 80
Grnerathrg anduclng short-llved po{trcn ry
85 Fositron emitters are rather scarce in nafi.lre, and must be made artfrci@. Thb is done by ircroasing the proton content of nuclei by tking' exha prdons, a$ha Fr&os or &.rtercns {hydrogen-Z nuclei) into stable nuclei. Particle accolorators egch ae linaar amelcrators or qdotrcrns, whlcfi can give particle erreq;ies in the rang€ 3-18 M€V are rsed.
A widely-used radiochemical that emits positrons is FDG (frroro&oxygilucose), a form of W gilucose fabslled with the riadioactiw nuclide ffuotine-l8. Fluorins-l8 is produced either by bombarding oxygsftl8 with protons or by bombarding neon-2O with deuterons. lt is then wnbined hto fie Ediochemical FDG in the accebrator laboratory anil banrysrbd to the PET scanner ficr immediate use.
Fositron smittsrs used in mdicine harn very sftort haff-lines, wfrft*t is bo#r arl dtsr@p and a 95 probbm. The adm*ap b that the activity of the radioclpn*cal lnlected Hb'ee pe$€nt furys
rery rapidly, reducing th€ doss rec€id by the petient. The di@g k t!€ difficulty in prcducing the rf,.lclides ard cornbiniqg tham into a suita$e radoc*rsnbd mfrpotlnd in,h€ short tirne availade. The half-lits of ffuorine-l8 is onfy 110 minutes, ard the cfiernbat pr,o*$ion of FDG trakes about haff frat, so there ie'ftfla time to delay. For this t€son, PET scryurer suites lff are h'rilt alongside the acelerator laboratorbs servtr€ srem.
Onc€ in the body, FDG is absorbd by aH tisues metabolising &@s6. As gfirw is requir€d br normaf respriratioh, aS haalthy, actiw fiscn es wHl abm6 FFG, #tt*!S fis Wr&,F b b€
ured to investiggte brein ac{ivity and a rarqe of cortditions frorn featt'd*sa$s b egrcen Rrofiectlon ol pathntr and nedbd steff
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105 Atrrougrh patibnts underyoirq racfotherapy nsedlo rmiw high docs,lfuss'tr*J$t ba ttsffied b the regilons uruder trea8nsnt. To limit tfe mdiation &se to lualtty Sssrre, fuise obstrucsng sfiields are used. Radiographers, the medical pereonnel invohpd in trreatirq patients, must Sviously avoid €xposure to radiation therlselves. The high intenslty ef radlation in a radiotherapy suite is an obviqus source of hazard, so the entire ars* i$ scrwd sff and 11O sfrfiafded by tfiick, &nse corlcr{rte walls. kr phces $frers it is rnwary b erb $$ rafttion
. Flors efdocfirrefy, such as fie si(bs of fte gilnma-sfrrrets mntairer, a dswr, maEr,id srdt as bad is ussd, becauee it has a much smaHer haff-tridmees. The tptr-{tHsuss ef a rnate#l is
$rat *lickness sfibh trd,*T the intensig ol a beam of radiation to one-lmffi of iF eflH rnlue.
A simder, ard cheaper, way of limiting radia$on &se is to make rse of the hnlerue-eqrare law:
| |5 if pl dqJh the distance betu€en the es.iree and arprrc like$ to b oxrcd to ta$dion, then the radiation dose is reducod four-fold. For this Feason, ths distafrcs bdwwn radioactive ffin@s and patients and straff b alwap msde as great as p@&b S,*ft fs Wrnatry af the radiotherapy suite. ln a similar way, wtnn strong rnedicat sources haw e be transported by plane, they are canbd in special capetdes in the wing-tlps cf ste Hams, pnfulg $em at a tff substanfral distance lrorr cruw and pasmrgprs in the aircraft and rcd.rdrq *ta,anutnt of te*vy
shblding needed.
The dose to the patient is admir*stered autonratir:afly onoe the ra@s are deer of the Fatrnent arsa. In this way tie radkrgrrytiers witl not rccsir€ a &e e$ni$en$y abov€ that ufticfi ftry wwld got from background rsdiafron. The wrtinrnl uso of lctt"lenn*s)tsroEro, such as tg5 fF rdi remicals uEd in gnmma scars, has rnort potsn$*l dar,qer !o & tffi ucing them.
Bscanee Sre*r activity is rda$re$ low, it wgxid be eary to treat lhem as mtrqdetdy sab. But, as ' fuy ars uead-by the radiograplnrs over a long period thror.rgilu$,Seh yyor*irq [tes, lor-bnd
Bourc€s could produce an unaccosaHe dos€ or"r time.
To monitor the radiation dose received by staff, film badges, as shown in Fig. 5, can be used.
pin to attach to clothing plastic foils
of different
densities open window
light-proof
envelope photographic
film
metalfoil f i l m holder
film holder metal foils of different densities
plastic
foil open
window
130
front view section through AB
F i g . 5
Film badges use different absorbers to produce regions absorbing different types and energies of radiation. The blackening in these regions shows the total dose the wearer of the badge has received. The film in the badges is developed and replaced regularly, so that the dose received over the time since the badge was last changed can be assessed. lf there has been a sudden increase in dose, however, it could be some time before this is realised. lonisation dosimeters, shown in Fig. 6, are preferable for staff working in hazardous environments, as they can be examined at any time to see whether the wearer has exceeded the permitted dose.
135
