International Journal of Applied Radiation and Isotopes, 1975, Vol. 26, pp. 694-695. Pergamon Press. Printed in Northern Ireland
Rad/at/on
from
Needles
l~dim
(Received 25 April
extent a n d the sensitivity o f the counter for g a m m a rays changes with the thickness o f the a l u m i n i u m absorber foils. We, therefore, also m e a s u r e d absorption curves with the needle s u r r o u n d e d b y art extra p l a t i n u m sheath or covered by a p l a t i n u m foil o f a b o u t 0.2 c m thickness. Similar measurements were m a d e with a n extra a l u m i n i u m cylinder or foil o f a b o u t 1 m m thickness. T h e difference b e t w e e n the
1975)
DURINO a panel discussion in Vienna on radium standards it was pointed out that about 20 ~o of the beta-rays of RaC(S14Bi) have a m a x i m u m energy of 3.18 M c V which means that their m a x i m u m range exceeds the normal wall thickness of a radium needle, i.e.0.5 m m platinum. It seemed a question of some importance whether the beta-particles which penetrate the platinum wall provide a noticeable contribution to the absorbed dose in tissue. For this reason w c have used a Geigcr Mtiller counter, which has a high sensitivity for electrons and a low sensitivity for photons, to measure absorption curves of the radiation emitted by low-activity radium needles with a 0"5ram platinum wall. As absorbers aluminium or lead foils wcrc placed between the nccdlc and the counter and different geometries wcrc adopted. Interpretation of such an absorption curve is not easy. Scattered radiation isalways present to a certain .
a
I
~
I
i
I
i
I
'
I
'
shapes of the absorption curves in a constant geometry should bc nearly independent of scattering effects and changes in counter efficiency. F r o m Fig. I it is sccrt that for absorber thicknesses > 300 m g Al/cm s g a m m a radiation only is observed. If this part of the curve is extrapolated to lower absorber thicknesses, g a m m a count-rates arc obtained which lic well b d o w the observed values. Evidently softer components arc also present. T h e similarity of the absorption curves obtained with and without an extra platinum sheath indicates that the soft component consists essentially of secondary radiation, presumably mainly electrons arising from the conversion of photons in the platinum wall. Penetrating/S-rays provide only a very minor contribution. Comparing the absorption curves of the radiation from the bare nccdlc and from the necdic surrounded I
i
I
i
I
i
I
i
I
'
I
'
l
3
.--
.. "°.%
CR
\
~
"'.... ""o..,. "~, ,,,,.
""'-.,,,.. ,,,,,. ~
,
I
,
I
l
°'.-
I
,
I
i
I
....
, 500
.. .....
I
,
I
,
I
,
I
i
I 1000
I
l
i
I
m g Allcm z
Fzo. 1. Count-rates t h r o u g h a l u m i n i u m absorber foils measured with a t h i n - w i n d o w O. M. counter. Observations on a bare 0"5 m m p l a t i n u m - w a l l e d r a d i u m needle are shown as - - , on a needle surr o u n d e d by 0.3 m m P t as - - - , a n d those on a needle s u r r o u n d e d 0.8 m m A1 as . . . . . . (The needle s u r r o u n d e d b y a n extra cylinder was lying o n the same support as the bare needle. T h u s the distance b e t w e e n the r a d i u m a n d the counter was not the same in all cases. This accounts for the fact t h a t t h r o u g h thick a l u m i n i u m cylinders the count-rate is a little higher t h a n w i t h o u t a n a l u m i n l u m sheath). 694
Tedmical notes by aluminlum shows the presence of a very short range component issuing from the platinum surface. This component makes a noticeable contribution to the count-rate up to an absorber thickness of 80 m g Al/crns. It is observed with approximately the same relative intensity if the needle is surrounded by platinum, but it is absent if the needle is surrounded by aluminium. It is reasonable to assume that this radiation is produced by the photoelectric effect in the platinum. Conversion electrons due to Compton absorption must also be present, but these would not be recognized as a difference in absorption of radiation outside a platinum and outside an aluminium surface, the Compton mass attenuation coefficient being even a little higher in aluminium than in platinum. Compton electrons will certainly contribute to the countrates measured above the gamma contributionthrough absorbers between 80 and 250 mg/Al/cms. However, this part of the absorption curve is very difficult to interpret. As the thickness of the aluminium absorber foil is raised the efficiency of the counter for gammarays is also increased. This in part compensates for the reduction of the count-rate from the softer radiation emitted by the needle. Evidently, if one would take a platinum needle surrounded by a sufficiently thick aluminium cylinder and if the aluminium absorber foils would then be fitted snugly around this cylinder, one would observe only a reduction of the count-rate due to gamma-absorption in the various layers of aluminium. We have actually observed a strong dependence of the shape of the absorption curve measured with aluminium covered needles on the geometry. Thus we may conclude that there is a contribution from a low penetration component to the absorbed dose in the first one or two millimeters of tissue around a radium needle, but that this component cannot be noticeably reduced by increasing the thickness of the platinum wall. In as far as thissoftcomponent consistsof Compton dcctrons it only serves to establish the normal radiation cquilibrlum between photons and Compton electrons as it normally cxists in tissue. (A slight diffcrcncc will rcmain due to the transitionfrom a high Z to low Z matcrlal). The photo-electrons, on thc other hand, provide an extra contribution to the radiation field,which will penetrate for scvcral dccimctcrs the air around an uncovercd needle. In therapy this short range radiation will hardly be noticed becausse of the 1/r-dependence of the doserate. Under certain other circumstances, however, one might possibly wish to provide a shielding against this part of the radiation emitted by radium needles. This can easily be done by using 1 m m thickness of plastic or some similar light material.
695
Acknowledgement--This work has been performed in the Instituut voor Kemphysisch Onderzoek as part of the programme of the Foundation for Fundamental Research on Matter (FOM) with financial support of the Organization for Pure Research.
A. H. W. ATEN*
Subfaculty of Chemistry University of Amsterdam Holland * Correspondence: Instituut voor Kcrnphyslsch Onderzoek, Oosterringdijk 18, Amsterdam, Holland.
International ~ournal of Applled Radiation and Isotopes, 1975, Vol. 26, 695-697. Pergamon Press. Printed in Northern Ireland
pp.
N e w M e t h o d s l'or I n t r o d u c i n g 75Se i n t o R a d l o p h a r m a c e u t i c a l s
(R~¢dved 6 February 1975) Introduction T o SYZ~rHESIZ~a gamma-ray emitting radiopharmaceutlcal it is often necessary to introduce a radionuclide of an element not present in the original pharmaceutical. This process is sometimes referred to as foreign labeling.iX) The most c o m m o n foreign label used in radiopharmaccuticals has been radioiodlnc bccausc of the many, wcll-establishcd,iodlnatlon mcthods currently available. Although scle. nium-75 has desirable physical characteristics for in vivo radiopharrnaccutical imaging and can be a bioisostcricreplacement for sulfur in m a n y products, selenium labeled radiopharmaccutlcals are not extensivelyavailabledue to a lack of a simple method for introducing the label into the pharmaceutical. A further consideration in the synthesis of labeled pharmaceuticals is the practice of introducing the label into the product at the latestpossiblesynthetic step to prevent undue product handling thus decrcaslng personnel exposure and minimizing product decay. Iodinatcd radiopharmaceuticals, synthesizod by isotope exchange, illustratethe principle of last step labeling. Until now, labclcd sclcnium has not bccn availablein a form snitahlcfor laststcp labcling. Selenium-75 has scvcral advantages in radiopharmaceutical design over iodlne-131: (a) The betaparticle absorbed dose is about 7 % of that of iodine131; (b) Between 100 and 400keV there are 1.74 g a m m a rays per disintegration as compared to 0.91 for iodlne-131; (c) The combined contribution of (a) and (b) means that smaller administered doses of VaSe rather than IT M radlopharmaccutlcals would be neodcd; (d) The longer physical half-lifeof VaSe
Rad/at/on
from
Needles
l~dim
(Received 25 April
extent a n d the sensitivity o f the counter for g a m m a rays changes with the thickness o f the a l u m i n i u m absorber foils. We, therefore, also m e a s u r e d absorption curves with the needle s u r r o u n d e d b y art extra p l a t i n u m sheath or covered by a p l a t i n u m foil o f a b o u t 0.2 c m thickness. Similar measurements were m a d e with a n extra a l u m i n i u m cylinder or foil o f a b o u t 1 m m thickness. T h e difference b e t w e e n the
1975)
DURINO a panel discussion in Vienna on radium standards it was pointed out that about 20 ~o of the beta-rays of RaC(S14Bi) have a m a x i m u m energy of 3.18 M c V which means that their m a x i m u m range exceeds the normal wall thickness of a radium needle, i.e.0.5 m m platinum. It seemed a question of some importance whether the beta-particles which penetrate the platinum wall provide a noticeable contribution to the absorbed dose in tissue. For this reason w c have used a Geigcr Mtiller counter, which has a high sensitivity for electrons and a low sensitivity for photons, to measure absorption curves of the radiation emitted by low-activity radium needles with a 0"5ram platinum wall. As absorbers aluminium or lead foils wcrc placed between the nccdlc and the counter and different geometries wcrc adopted. Interpretation of such an absorption curve is not easy. Scattered radiation isalways present to a certain .
a
I
~
I
i
I
i
I
'
I
'
shapes of the absorption curves in a constant geometry should bc nearly independent of scattering effects and changes in counter efficiency. F r o m Fig. I it is sccrt that for absorber thicknesses > 300 m g Al/cm s g a m m a radiation only is observed. If this part of the curve is extrapolated to lower absorber thicknesses, g a m m a count-rates arc obtained which lic well b d o w the observed values. Evidently softer components arc also present. T h e similarity of the absorption curves obtained with and without an extra platinum sheath indicates that the soft component consists essentially of secondary radiation, presumably mainly electrons arising from the conversion of photons in the platinum wall. Penetrating/S-rays provide only a very minor contribution. Comparing the absorption curves of the radiation from the bare nccdlc and from the necdic surrounded I
i
I
i
I
i
I
i
I
'
I
'
l
3
.--
.. "°.%
CR
\
~
"'.... ""o..,. "~, ,,,,.
""'-.,,,.. ,,,,,. ~
,
I
,
I
l
°'.-
I
,
I
i
I
....
, 500
.. .....
I
,
I
,
I
,
I
i
I 1000
I
l
i
I
m g Allcm z
Fzo. 1. Count-rates t h r o u g h a l u m i n i u m absorber foils measured with a t h i n - w i n d o w O. M. counter. Observations on a bare 0"5 m m p l a t i n u m - w a l l e d r a d i u m needle are shown as - - , on a needle surr o u n d e d by 0.3 m m P t as - - - , a n d those on a needle s u r r o u n d e d 0.8 m m A1 as . . . . . . (The needle s u r r o u n d e d b y a n extra cylinder was lying o n the same support as the bare needle. T h u s the distance b e t w e e n the r a d i u m a n d the counter was not the same in all cases. This accounts for the fact t h a t t h r o u g h thick a l u m i n i u m cylinders the count-rate is a little higher t h a n w i t h o u t a n a l u m i n l u m sheath). 694
Tedmical notes by aluminlum shows the presence of a very short range component issuing from the platinum surface. This component makes a noticeable contribution to the count-rate up to an absorber thickness of 80 m g Al/crns. It is observed with approximately the same relative intensity if the needle is surrounded by platinum, but it is absent if the needle is surrounded by aluminium. It is reasonable to assume that this radiation is produced by the photoelectric effect in the platinum. Conversion electrons due to Compton absorption must also be present, but these would not be recognized as a difference in absorption of radiation outside a platinum and outside an aluminium surface, the Compton mass attenuation coefficient being even a little higher in aluminium than in platinum. Compton electrons will certainly contribute to the countrates measured above the gamma contributionthrough absorbers between 80 and 250 mg/Al/cms. However, this part of the absorption curve is very difficult to interpret. As the thickness of the aluminium absorber foil is raised the efficiency of the counter for gammarays is also increased. This in part compensates for the reduction of the count-rate from the softer radiation emitted by the needle. Evidently, if one would take a platinum needle surrounded by a sufficiently thick aluminium cylinder and if the aluminium absorber foils would then be fitted snugly around this cylinder, one would observe only a reduction of the count-rate due to gamma-absorption in the various layers of aluminium. We have actually observed a strong dependence of the shape of the absorption curve measured with aluminium covered needles on the geometry. Thus we may conclude that there is a contribution from a low penetration component to the absorbed dose in the first one or two millimeters of tissue around a radium needle, but that this component cannot be noticeably reduced by increasing the thickness of the platinum wall. In as far as thissoftcomponent consistsof Compton dcctrons it only serves to establish the normal radiation cquilibrlum between photons and Compton electrons as it normally cxists in tissue. (A slight diffcrcncc will rcmain due to the transitionfrom a high Z to low Z matcrlal). The photo-electrons, on thc other hand, provide an extra contribution to the radiation field,which will penetrate for scvcral dccimctcrs the air around an uncovercd needle. In therapy this short range radiation will hardly be noticed becausse of the 1/r-dependence of the doserate. Under certain other circumstances, however, one might possibly wish to provide a shielding against this part of the radiation emitted by radium needles. This can easily be done by using 1 m m thickness of plastic or some similar light material.
695
Acknowledgement--This work has been performed in the Instituut voor Kemphysisch Onderzoek as part of the programme of the Foundation for Fundamental Research on Matter (FOM) with financial support of the Organization for Pure Research.
A. H. W. ATEN*
Subfaculty of Chemistry University of Amsterdam Holland * Correspondence: Instituut voor Kcrnphyslsch Onderzoek, Oosterringdijk 18, Amsterdam, Holland.
International ~ournal of Applled Radiation and Isotopes, 1975, Vol. 26, 695-697. Pergamon Press. Printed in Northern Ireland
pp.
N e w M e t h o d s l'or I n t r o d u c i n g 75Se i n t o R a d l o p h a r m a c e u t i c a l s
(R~¢dved 6 February 1975) Introduction T o SYZ~rHESIZ~a gamma-ray emitting radiopharmaceutlcal it is often necessary to introduce a radionuclide of an element not present in the original pharmaceutical. This process is sometimes referred to as foreign labeling.iX) The most c o m m o n foreign label used in radiopharmaccuticals has been radioiodlnc bccausc of the many, wcll-establishcd,iodlnatlon mcthods currently available. Although scle. nium-75 has desirable physical characteristics for in vivo radiopharrnaccutical imaging and can be a bioisostcricreplacement for sulfur in m a n y products, selenium labeled radiopharmaccutlcals are not extensivelyavailabledue to a lack of a simple method for introducing the label into the pharmaceutical. A further consideration in the synthesis of labeled pharmaceuticals is the practice of introducing the label into the product at the latestpossiblesynthetic step to prevent undue product handling thus decrcaslng personnel exposure and minimizing product decay. Iodinatcd radiopharmaceuticals, synthesizod by isotope exchange, illustratethe principle of last step labeling. Until now, labclcd sclcnium has not bccn availablein a form snitahlcfor laststcp labcling. Selenium-75 has scvcral advantages in radiopharmaceutical design over iodlne-131: (a) The betaparticle absorbed dose is about 7 % of that of iodine131; (b) Between 100 and 400keV there are 1.74 g a m m a rays per disintegration as compared to 0.91 for iodlne-131; (c) The combined contribution of (a) and (b) means that smaller administered doses of VaSe rather than IT M radlopharmaccutlcals would be neodcd; (d) The longer physical half-lifeof VaSe
