419

THE USES OF A COBALT UNIT FORRADIOTHERAPY

By PAUL STRICKLAND, M.R.C.P., F.F.R.Consultant Radiotherapist, Mount Vernon Hospital, Northwood, Middlesex

X-rays generated at a kilovoltage of 250 havebeen used in the treatment of malignant diseasefor about 30 years. It was found that when highdoses were given, damage to the skin and sub-cutaneous tissues was frequent. After desquama-tion of the skin had subsided, pigmentation fol-lowed and during the succeeding i8 months per-manent and unsightly telangiectasis developed.This was often associated with a variable amountof fibrosis of the soft tissues. With conventionalX-ray therapy the maximum dose fell on the skin,which therefore suffered most severely.The absorption of X-rays in tissue increases

with increase in the atomic weights of the elementscomposing the tissues. Thus, absorption of X-raysin bone is two and a half times higher than in softtissues, mainly due to the high calcium content ofthe skeleton.- In clinical practice this had two disadvantages:firstly, in the irradiation of lesions deep to a bonystructure, such as tumours of the mouth or max-illary antrum, the dose actually received by thetumour was materially less than the calculateddose, because the overlying bone acted as anabsorbing shield; secondly, the very high dosereceived by the bone predisposed towards fracturein later years, and in the presence of infection,osteomyelitis and its complications.The beam produced by an average 25o-kV.

X-ray therapy machine falls to 30 per cent. ofits intensity by the time it has passed halfwaythrough the body. It is thus very difficult to give ahigh dose of irradiation to deep-seated tumours,particularly where differential sensitivity betweennormal and malignant tissues is low.These difficulties have been largely overcome by

the use of X-ray and gamma-ray beams generatedat a voltage of one million or more, so-calledsupervoltage irradiation therapy.With supervoltage rays the maximum dose falls

not on the skin, but at a level 0.5 cm. deep to itfor a i,ooo-curie 60Co source. The skin is thuscomparatively spared.The absorption of supervoltage irradiation in

bone is no greater than its absorption in soft tissues.It follows that the risk of fracture and radio-necrosis is greatly diminished. Moreover, thescreening effect of bone is not important, whichmeans that the dose received by tumours deep tobone is adequate.Many supervoltage machines have a high output.

We can take advantage of this by increasing thedistance from radiation source to skin surface toas much as ioo cm., compared with an average of50 cm., with a 25o-kV. machine. If the inversesquare law is recalled, it will be clear that the ratiobetween the dose received by the skin and the dosereceived at a depth within the body is much greaterwith supervoltage therapy. This ratio is known asthe percentage depth dose, and it follows that deep-seated tumours can be treated with much greaterfacility because of the improved percentage depthdose.There are many types of supervoltage machine.

In some powerful X-ray beams are produced bythe impact of high-energy electrons on metallictargets; in others use is made of a penetratingbeam of gamma rays produced by certain radio-active isotopes manufactured in the atomic pile.One of this latter class of machine is the

'Theratron,' which combines the advantages ofsupervoltage irradiation with rotation of the treat-ment head.

The ' Theratron' Cobalt 6o UnitThis machine, which was constructed by Atomic

Energy of Canada Ltd., has been in clinical use atMount Vernon Hospital since March I954. It isintended primarily for rotation therapy.The radioactive source is cobalt 6o, which has

a half-life of 5.3 years. This isotope produces apowerful gamma ray beam equivalent to an X-raybeam of 2 to 3 million volts. The source is a shortcylinder of 2 cm. diameter, housed in a massivehead of lead-tungsten alloy. The head is carriedon a C-shaped arm, at the other end of which is acounterweight. This weight also serves as adirect radiation shield (see illustration).

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420 POSTGRADUATE MEDICAL JOURNAL August 1958

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The 'Theratron' x,ooo-curie cobalt unit.

Two types of movement of the treatment headare possible: partial or complete rotation in thevertical plane and oscillation at right angles to itover a limited arc. Thus, whatever the position ofthe source, the central axis of the beam-as definedby diaphragms-passes through the same fixedpoint, which is the centre of rotation. Speed ofrotation can be fixed at anything up to a maximumof two and a half revolutions per minute. Thetreatment couch has a full range of horizontalmovements and vertical adjustments are hydraulic-ally operated.By means of these devices the centre of rotation

can be made to coincide with any point within thebody.The output of the machine is about 45 r6ntgens

per minute in air at 75 cm. from the source andtreatment times vary between five and .15 minutes,depending on the depth of the lesion below thebody surface. The radioactive cobalt source isapproximately I,000 curies, which is equivalent tonearly 2,000 g. of radium element.

Advantages ofRotating Supervoltage TherapyIn addition to those -advantages of supervoltage

treatment already mentioned, the use of partial orcomplete rotation of the treatment head gives themaximum possible ratio between the dose de-

livered to the tumour and the dose received by thesurrounding normal tissues. During rotation thetumour is under continuous irradiation, whereasthe normal tissues through which the beam ispassing are constantly changing. Thus, normaltissues between skin surface and tumour receive aminimum dose and the risk of late radiation damageis therefore diminished.The penetration of a supervoltage beam is so

great that up to 25 per cent. of the beam mayemerge on the other side of the patient. This highexit dose is a disadvantage; it is obviated by theuse of a rotating head which ' distributes ' the totaldose over a larger volume of tissue and con-sequently over a larger exit portal.

Radiation DoseThe biological effect of irradiation depends ulti-

mately on physical and chemical changes takingplace at the molecular level within the cell, or,looked at another way, on the number of ionizationsoccurring per unit volume of tissue.

Because the density of ionization per unit lengthof track increases as the energy of the beam in-creases, there were strong reasons for assumingthat the dose of supervoltage irradiation in r6ntgenswould have to be increased if biological effects

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STRICKLAND: The Uses of a Cobalt Unit for Radiotherapy

comparable to those caused by 25o-kV. X-rayswere to be achieved. This has proved to be so.

It had been known for some time that a dose of5,ooo r. in five weeks at 25o kV. correspondedapproximately to 6,5oo r. from a Io-g. teleradiumunit given in the same overall time. This tele-radium gamma ray beam was roughly equivalentto an X-ray beam of i.6 million volts. It was there-fore thought that for beam energies in the 3 to 4million volt range-such as the ' Theratron ' beam-a dose of 7,500 r. to a small tissue volume couldreadily be given in about six weeks.Most cases have been treated with doses of this

order.

Tissue ReactionsWhen a high-energy or gamma ray quantum

strikes matter, most of the resulting electrons areprojected forwards. This is why the maximumdose from a supervoltage beam falls below the skinsurface. The appearance of the skin during treat-ment is thus no guide to tissue tolerance.

After two or three weeks' treatment slighterythema of the skin may occur. Towards the endof treatment this may deepen and perhaps beaccompanied by a little dry desquamation. In veryfew cases does moist desquamation ever develop,and then only if higher doses are given. It is neversevere.The reaction of mucous membranes is corre-

spondingly slight. Towards the end of irradiationa fibrinous film may be seen, but it is usually notsevere and only very rarely a reason for stoppingtreatment earlier than intended.

Systemic reactions are generally negligible.Nausea is an occasional complaint, but vomiting israre. The patient's energy and well-being areusually maintained normally throughout treat-ment. A very striking feature is the infrequencyof diarrhoea, even when large volumes of abdominaland pelvic viscera are treated.

It is too early to be dogmatic about late tissuedamage, since the first group of patients wastreated only three and a half years ago. However,if severe damage were to have been common, itwould have been evident by now, whereas, in fact,radio-necrosis has not been seen.During the months following the end of treat-

ment progressive thickening of the subcutaneoustissues develops. This is usually not severe. In afew, dense fibrosis of the soft tissues occurs with' leathering' of the muscles. Pigmentation of theoverlying skin is slight and telangiectasis does notoccur.

Selection of Cases for TreatmentIn the absence of past experience selection

criteria had to be evolved empirically. It was

decided that only those groups of cases whichcould not be effectively treated with conventionaldeep X-ray therapy would be considered for super-voltage irradiation, provided that the tumours wereof types known to be radiosensitive in some degree.

In practice this meant that certain deep-seatedtumours would be treated where an otherwise in-sufficient tumour dose would have been given withconventional X-rays. Carcinomas of the oeso-phagus, bronchus and bladder fell within thisgroup. In addition, certain accessible tumourswere treated where the violence of the mucosalreaction might have precluded the giving of a suf-ficient dose with 200-kV. X-rays. Carcinomas ofthe posterior part of the tongue, pharynx, post-nasal space and extrinsic carcinomas of the larynxand post-cricoid region fell into this category.The low absorption of supervoltage rays in bone

commended their use in bone sarcomas and insoft tissue sarcomas adjacent, or fixed, to bone.One group of cases chordomas was treated

to test the accepted notions of tumour sensitivity.

Tumours of the Upper Air PassagesThe site of origin of these tumours has been

between the posterior one-third of the tongue andthe upper trachea. The primary tumours haveoften been locally advanced and sometimesassociated with fixed metastatic glandular masseswhen first seen.

It has been possible to treat most of these caseswith one field, using a partial rotation technique.Tumour resolution is often noted within three

weeks of starting treatment and, in favourablecases, is well marked at the end of six weeks'irradiation.

In the minority who develop a brisk mucosalreaction it has been necessary to reduce the dailydose a little or sometimes to suspend treatment for afew days. The reaction usually subsides rapidly.

Deep-seated TumoursCarcinomas of the bronchus have to be carefully

selected. Radical irradiation is only worthwhile inhistologically proved squamous carcinomas, andthis automatically excludes the majority of referredcases. Those treated have previously been deemedinoperable, either because of technical difficulty inremoval of the tumour-for example, infiltrationof the trachea or because of the patient's poorvital capacity.The tuiuour is accurately localized and irradi-

ated to a tumour dose of about 6,Soo r. in sixweeks. Towards the end of treatment retrosternaldiscomfort and burning are almost invariable andare due to radiation-induced oesophagitis. Thissettles rapidly. Some degree of progressive lungfibrosis is inevitable; as it is not yet known how

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POSTGRADUATE MEDICAL JOURNAL

disabling this will finally prove, the dose of 6,5oo r.should not be much exceeded.Most patients referred with carcinoma of the

oesophagus usually have extensive lesions andwhen first seen are only able to swallow fluids withdifficulty. The tumour is localized very accuratelyby barium swallow and oesophagoscopy andtreated by a complete rotation technique to thesame order of dose as a carcinoma of bronchus.

Within a fortnight dysphagia improves and, inmany cases, the patient may be eating solid foodby the time of his discharge from hospital. Thetumour can often be eradicated locally and afibrotic stricture usually follows. These stricturesneed cautious dilatation later.Most cases of carcinoma of the bladder have

been fairly advanced when first seen. The volumeof pelvic tissue to be treated is determined byexamination under anaesthesia and cystoscopywith the insertion of markers. In nearly all casesit is necessary to treat the whole bladder with afull rotation technique. Symptomatic improve-ment is usual.

ChordomasThese tumours arise from the remnants of the

primitive notochord. They have a characteristichistological picture with many small dark nuclei inlarge vacuolated cells. They may be found any-where over the site of the notochord, but are mostfrequent in the sacral region.

Until recently chordomas were universally re-garded as radio-resistant tumours and this belief,together with their surgical inaccessibility, madethe outlook particularly poor.

Over 20 chordomas have been treated withdramatic results. The tumours are clearly radio-sensitive and respond well to only moderate dosesof supervoltage irradiation. Large masses fillingthe hollow of the sacrum have melted away andthe disease has remained quiescent for observationperiods of up to three years. Calcification of chor-domas after irradiation is uncommon.

Sarcomas of BoneA large group has been treated. Most sarcomas

have limited radio-sensitivity and treatment policyhas consisted in radical irradiation, followed byamputation in selected cases.A tumour dose of 7,000 to 8,ooo r. has been

delivered in about* seven weeks, often to largevolumes of tissue without particular difficulty. Theusual immediate result has been improvement inthe patient's condition, disappearance of pain,marked regression in tumour size and recalcifica-tion of the area radiologically.

Subsequent management has depended onclinical and X-ray findings. If there is any clinical

evidence of activity three to six months afterirradiation and a chest X-ray is normal, amputationis advised; if there is no clinical evidence ofactivity, it is reasonable to postpone amputation.The rationale for this policy is as follows:

because of early blood-borne metastases amputa-tion in bone sarcomas has given such generally badresults that it is being increasingly abandoned.By delaying operation-and using the interval fora radical radiotherapeutic attack on the tumour-natural selection is allowed to act and those patientswho develop pulmonary metastases are spared anunnecessary amputation.

Similar considerations apply to many soft tissuesarcomas.

ResultsThe follow-up period is too short for any pre-

sentation of survival rates and we are still at thestage of dealing in clinical impressions.Tumours of the upper air passages are nearly all

squamous carcinomas; most are radio-sensitiveand many disappear with a moderate dose of super-voltage irradiation. Many are very advanced whenfirst seen and are suitable for palliative treatmentonly. In the less advanced group a number oftumours have regressed completely and have notrecurred so far.Though large numbers of carcinomas of the

bronchus are referred for treatment, rigid selectionhas meant that only a comparatively small serieshas been treated. Immediate results are en-couraging and many tumours can certainly beeradicated locally. This is equally true of even theadvanced groups of carcinomas of the oesophagus,which have been treated radically. The problemhere is not the destruction of the primary, whichcan usually be achieved, but that most patients dieof their metastases.

Results in the irradiation of carcinoma of thebladder are difficult to assess. The radiotherapistis asked to treat a very advanced group of caseswhere often little more than palliation can beoffered. No great developments can be expecteduntil the effect of supervoltage irradiation can betried on a less advanced group.Time alone will decide the worthwhileness of

the bone sarcoma experiment. It is certain thatmany useless amputations have been avoided andit is equally sure that the majority of cases can behelped in an important symptomatic way. Amputa-tion specimens, obtained after radical irradiation,are now becoming available and in a number ofcases no viable tumour has been found. This workis continuing.

It has been established that radiation is thetreatment of choice of chordomas. This tumour isundoubtedly commoner than had been thought and

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August I958 STRICKLAND: The Uses of a Cobalt Unit for Radiotherapy 423

many cases have hidden their identity under falsehistological labels. The importance- of- making -thediagnosis accurately is enhanced now that aneffective treatment exists.

Availability of ApparatusA number of different models of rotating cobalt

supervoltage apparatus are now available. NewtonVictor Ltd.,I32 Long Acre, London, W.C.z, makethe ' Orbitron' with a price range of fi1,000 to£I4,000 and a delivery time of iz months.Nuclear Engineering Ltd., Woolwich Road,London, S.E.7, also make rotating cobalt apparatus,the cost of which is in the region of £2I,000 and

a delivery date of io to I2 months. The Britishagents for the' Theratron' are Watson & Sons(Electro - Medical) Ltd., North Wembley,Middlesex. All prices exclude the cost of theradioactive cobalt source, -which would be ap-prQximately £5,000 for a i,ooo-curie source.

AcknowledgmentsThe 'Theratron' was the gift of a Canadian

philanthropist, Mr. Jack McConnel, to the BritishEmpire Cancer Campaign. The unit was installedat Mount Vernon Hospital under the direction ofSir Stanford Cade and Professor B. W. Windeyer.

A*E*C Ltd., COBALT 6oTELETHERAPY UNITS

THERATIRON JUNIORAn economical unit for fixed beam and rotationaltechniques. Source-to-skin distances of 35 to 65 cm.permit the use of a Cobalt 60 source of small diameter toproduce high dose rates.

THERATRON (Model B)A larger source than the Theratron Junior (up to 2,000curies) providing dose rates of 80 r per minute at source-to-skin distances of 70 cm.

ELDORADO (Model G)A high output unit for fixed beam or multiportal therapy.Provides dose rates of up to 100 r per minute at source-to-skin distances from 50 to 150 cm.

ATOMIC ENERGY OF CANADA LTDSole representatives in Great Britain

WATSON & SONS (Electro-Medical) LTDEAST LANE, NORTH WEMBLEY, MIDDLESEX

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