OCTOBER 1977
Correspondence (The Editors do not hold themselves responsible for opinions expressed by correspondents) Mr. Higginbottom takes us to task for saying that the lower segment of the uterus was taken as that area within 5.0 cm of the internal os. This was the criterion adopted by In an excellent recent article (Dunster et al., 1976) com- Morrison et al. (1969), whereas Kohorn et al. (1969) placed parison was made between the ultrasonic and isotopic the upper limit 5.0 cm above the midpoint of the symphysis methods of localizing the placenta. Although not dis- pubis. Reed (1973) gave the upper limit of the anterior agreeing for one moment with the authors' eventual con- margin of the lower segment as halfway up the posterior clusion that the ultrasonic method was preferable, perhaps wall of the full bladder and the posterior margin an equal two other factors should be considered. Firstly, the lower distance from the internal os up the sacrum. We shared the segment cannot be regarded as "an area within 5 cm of the dilemma of Kukard and Freeman (1973) when they wrote internal cervical os". In a 20-week pregnancy, the lower "a further problem was that whilst the lower edge of the uterine segment hardly exists at all and yet by the time placenta was usually easily visualized, it was impossible to labour commences at term it may include the whole area of define the extent of the lower segment in relation to the the uterus between the internal os and the pelvic brim, internal os despite the use of a full bladder". We took an rather more than 5 cm. The lower segment develops mainly empirical distance in the full knowledge that there was no at the end of pregnancy and thus many cases with a placenta unanimity on the limits of the lower segment, nor the timing reported low or even praevia in the middle of pregnancy of its formation in relation to the stage of the pregnancy. may have a normally situated placenta by term as the lower Mr. Higginbottom did not offer an alternative description segment develops below the lower edge of the placenta and of the ultrasonic limits of the lower segment, nor an account the placenta appears to "move" out of the pelvis as time of the time relationship of its development to the stage of the goes by. One ultrasound examination is rarely sufficient and pregnancy. We will look forward to his clarification in due it should be repeated until delivery is imminent at the end of course. pregnancy. Yours, etc., F. G. M. Ross, An important advantage of the ultrasound method is the G. D. DUNSTER, safety factor enabling serial readings to be taken which can E. RHYS DAVIES, hardly be done by any other method of placental site A. H. JOHN. localization. Serial ultrasound examination will also be useful in assessing growth of the fetus which may also be an Bristol Royal Infirmary Bristol BS2 8HW. important factor in these cases. I would entirely agree with the original conclusion but REFERENCES would suggest that serial examinations every two to four DUNSTER, G. D., DAVIES, E. R., ROSS, F. G. M., and JOHN, weeks would be of even more value. A. H., 1976. Placental localization: a comparison of Yours, etc., isotopic and ultrasonic placentography. British Journal of J. HlGGINBOTTOM. Radiology, 49, 940-943. Research Fellow. KING, D. L., 1973. Placental migration demonstrated by Department of Obstetrics and Gynaecology, ultrasound. A hypothesis of dynamic placentation. University Hospital of South Manchester, West Didsbury, Radiology, 109,) 67-170. Manchester M20 8LR. KOHORN, E. I., SECKER WALKER, R. H., MORRISON, I., and CAMPBELL, S., 1969. Placental localisation: a comparison between ultrasonic Compound B-Scanning and radioREFERENCE isotope scanning. American Journal of Obstetrics and DUNSTER, G. D., RHYS DAVIES, E., ROSS, F. G. M., and Gynaecology, 103, 868-877. JOHN, A. H., 1976. Placental localization: a comparison of isotopic and ultrasonic placentography. British KUKARD, R. P. F., and FREEMAN, M. E., 1973. The clinical application of ultrasonic placentography. Journal of Journal of Radiology, 49, 940-943. THE EDITOR—SIR, COMPARISON OF ISOTOPIC AND ULTRASONIC PLACENTOGRAPHY
Obstetrics and Gynaecology of the British
Commonwealth,
80, 433-437. (in reply)
MORRISON, J., KOHORN, E. I., ASHFORD, C , TREDGOLD, C , SECKER WALKER, R. H., and BLACKWELL, R. J., 1969.
THE EDITOR—SIR, PLACENTAL LOCALIZATION: A COMPARISON OF ISOTOPIC AND ULTRASONIC PLACENTOGRAPHY.
Of course, all that Mr. Higginbottom says about our paper (Dunster et al., 1976) is absolutely true and it is well known to us and others. The paper reported a retrospective study on the results of isotope and ultrasonic examinations that had already been completed. It did not attempt to describe the method of performing placentography nor to analyse the circumstances in which serial ultrasonic examinations would be indicated nor their frequency. It is common practice to repeat the examination as a check if the first one shows that the placenta could be low-lying or praevia. The interval between the examinations relates to the maturity of the pregnancy, being widely spaced in the second trimester and more frequent in the third trimester. These serial examinations will frequently show the placenta to "move" away from the region of the internal os (King, 1973) with the passage of time.
Ultrasonic scanning in obstetrics 2. The diagnosis of placenta praevia. Australian and New Zealand Journal of Obstetrics and Gynaecology, 9, 206-208. REED, M. F., 1973. Ultrasonic placentography. British Journal of Radiology, 46, 255-258.
THE EDITOR—SIR, 134 Cs—A POTENTIAL RADIONUCLIDE FOR RADIATION THERAPY
Caesium 134, with a half life of 2.06 years and emitting 2.23 photons of 0.7 MeV average energy per disintegration, has not been considered so far in radiotherapy. It decays (Lederer et ah, 1967) by j8~ emission to stable 134Ba as shown in Fig. 1. The maximum energy of the electrons is 0.66 MeV. The expected photons and the respective yields are given in 2Table 1I. The calculated specific y-ray constant is 8.8 R cm mCi" h"1. The half value layer, in a narrow beam
761
VOL.
50, No. 598 Correspondence factors, we Since the final activity depends on several calculated the ratio of specific activity of 134Cs to that of 60 Co as being 0.9, assuming that the irradiation time, thermal neutron flux and self shielding for 133Cs target59 are 59 same as for Co target. The activation cross-section for Co is taken as 37 barn (Mughabghab and Garber, 1973). Because nuclear reactors contain epithermal neutrons in the irradiation positions and because some targets show strong resonances in the epithermal region of the neutron flux, the effective activation cross-section can be considerably larger than the thermal neutron capture cross-section. Mughabghab and Garber (1973) report a value of 415 ± 15 barn for the resonance integral of 133Cs compared to 75.5 ± 59 1.5 barn in the case of Co. Depending on the epithermal fraction of the neutron flux at the irradiation position,134 the effective cross-section can be as high as 60 barn for Cs production. However, due to long irradiation times and the large thermal neutron activation cross-section (140 barn) for 134 Cs, small amounts of 135Cs are expected to be produced. 135 Fortunately Cs, with a half life of more than million years, is a pure /J~ emitter (Eo=O.21 MeV) and therefore no y-ray emitting contaminants are expected in the reactor production. In fact, no impurities are observed134 in a Nal (Tl) y-ray spectrum of a commercially obtained Cs source. Because of these desirable physical properties, its usefulness in radiation therapy should be evaluated. Yours, etc., D. V. RAO,
2.09 h '
2.06 Y 1.9696 MeV
'Cs
1.6430 MeV
71%
1.4004 MeV 1.36 MeV 1.1677 MeV
0.6046 MeV
F. ELLIS, J. T. MALLAMS.
STABLE
l34
Ba
FIG. 1. Decay scheme of 134 Cs. The half life of 134 Cs m should be read as 2.9h.
Photon energy (MeV)
REFERENCES LEDERER, C. M., HOLLANDER, J. M., and PERLMAN, I., 1967.
Table of Isotopes, 6th edn (John Wiley & Sons, New York). MUGHABGHAB, S. F. and GARBER, D. I., 1973. NeutronCross-
TABLE I PHOTON YIELDS IN THE DECAY OF
Department of Radiology, CMDNJ, New Jersey Medical School, Newark, New Jersey 07103, USA
134
Sections, Vol. 1, Resonance parameters, Brookhaven National Laboratory report, BNL-325.
CS
No. of Group " photons No. of photons weighted per 100 disin- average energy per 100 distegrations integrations (MeV)
THE EDITOR—SIR, 134 C S FOR THE TREATMENT OF CERVICAL CANCER
One projected use of 134Cs is for the treatment of cancer of the uterine cervix. The isodose curves for cancer of the 0.4753 0.58 cervix commonly have the shape shown in Fig. 1 A and B. As 0.5631 8.18 can be seen, the tumour volume is not the same shape as the 0.5692 0.597 121.68 14.84 high dose treatment volume. With moderate energy radio0.6046 98.08 nuclides it should be possible to use filtration to try tofitthe 0.7958 85.84 0.796 95.36 isodose shape to the tumour volume using a single central 0.8018 9.52 line source with a suitable variation of linear activity along 1.0384 0.42 its length. This implies changing the isodose shape consider1.1677 1.92 1.279 5.98 ably in the coronal plane (see Fig. 1 c) but, because of the 1.365 3.64 proximity of rectum and bladder, producing little change in the sagittal section. 223.02 0.700 This may be relatively simple with low dose-rate sources in which treatment times are measured in hours. For high geometry, for platinum, lead and water is calculated to be dose-rate (e.g. cathetron) treatments, however, suitable sources capable of filtration (e.g. intracervical) so as to pro0.32 cm, 0.56 cm and 8.2 cm respectively. High specific activity combined with low cost is very duce the desired isodose shape have proved difficult to find. important for any radionuclide to be useful in therapy. Such sources (see Table I) should (a) have high specific Caesium 134 can be easily and economically produced in activity and exposure rate, (b) produce y rays of sufficiently nuclear reactors. The target material is 100% naturally low energy so that filtration can influence the shape of the abundant 133 Cs. The134thermal neutron activation cross- isodose surfaces, (c) have long half life. Cobalt 60 fulfils the 133 section for Cs(n, y) Cs is reported (Mughabghab and first but not the second of these criteria. Cesium 137 fulfils Garber, 1973) to be 29 ±1.5 barn. The cross-section for the the second but not the first. Both fulfil the third. Iridium 192 formation ofm134 Cs m is 2.5 barn. As shown in Fig. 1, the half- satisfies the first two, but not the third. A half life of two 134 long enough to be practicable. It life of Cs is 134 only 2.9 hours and the decay is completely years might be considered is possible with 134Cs, because of its very high specific to the long lived Cs.
762
Correspondence (The Editors do not hold themselves responsible for opinions expressed by correspondents) Mr. Higginbottom takes us to task for saying that the lower segment of the uterus was taken as that area within 5.0 cm of the internal os. This was the criterion adopted by In an excellent recent article (Dunster et al., 1976) com- Morrison et al. (1969), whereas Kohorn et al. (1969) placed parison was made between the ultrasonic and isotopic the upper limit 5.0 cm above the midpoint of the symphysis methods of localizing the placenta. Although not dis- pubis. Reed (1973) gave the upper limit of the anterior agreeing for one moment with the authors' eventual con- margin of the lower segment as halfway up the posterior clusion that the ultrasonic method was preferable, perhaps wall of the full bladder and the posterior margin an equal two other factors should be considered. Firstly, the lower distance from the internal os up the sacrum. We shared the segment cannot be regarded as "an area within 5 cm of the dilemma of Kukard and Freeman (1973) when they wrote internal cervical os". In a 20-week pregnancy, the lower "a further problem was that whilst the lower edge of the uterine segment hardly exists at all and yet by the time placenta was usually easily visualized, it was impossible to labour commences at term it may include the whole area of define the extent of the lower segment in relation to the the uterus between the internal os and the pelvic brim, internal os despite the use of a full bladder". We took an rather more than 5 cm. The lower segment develops mainly empirical distance in the full knowledge that there was no at the end of pregnancy and thus many cases with a placenta unanimity on the limits of the lower segment, nor the timing reported low or even praevia in the middle of pregnancy of its formation in relation to the stage of the pregnancy. may have a normally situated placenta by term as the lower Mr. Higginbottom did not offer an alternative description segment develops below the lower edge of the placenta and of the ultrasonic limits of the lower segment, nor an account the placenta appears to "move" out of the pelvis as time of the time relationship of its development to the stage of the goes by. One ultrasound examination is rarely sufficient and pregnancy. We will look forward to his clarification in due it should be repeated until delivery is imminent at the end of course. pregnancy. Yours, etc., F. G. M. Ross, An important advantage of the ultrasound method is the G. D. DUNSTER, safety factor enabling serial readings to be taken which can E. RHYS DAVIES, hardly be done by any other method of placental site A. H. JOHN. localization. Serial ultrasound examination will also be useful in assessing growth of the fetus which may also be an Bristol Royal Infirmary Bristol BS2 8HW. important factor in these cases. I would entirely agree with the original conclusion but REFERENCES would suggest that serial examinations every two to four DUNSTER, G. D., DAVIES, E. R., ROSS, F. G. M., and JOHN, weeks would be of even more value. A. H., 1976. Placental localization: a comparison of Yours, etc., isotopic and ultrasonic placentography. British Journal of J. HlGGINBOTTOM. Radiology, 49, 940-943. Research Fellow. KING, D. L., 1973. Placental migration demonstrated by Department of Obstetrics and Gynaecology, ultrasound. A hypothesis of dynamic placentation. University Hospital of South Manchester, West Didsbury, Radiology, 109,) 67-170. Manchester M20 8LR. KOHORN, E. I., SECKER WALKER, R. H., MORRISON, I., and CAMPBELL, S., 1969. Placental localisation: a comparison between ultrasonic Compound B-Scanning and radioREFERENCE isotope scanning. American Journal of Obstetrics and DUNSTER, G. D., RHYS DAVIES, E., ROSS, F. G. M., and Gynaecology, 103, 868-877. JOHN, A. H., 1976. Placental localization: a comparison of isotopic and ultrasonic placentography. British KUKARD, R. P. F., and FREEMAN, M. E., 1973. The clinical application of ultrasonic placentography. Journal of Journal of Radiology, 49, 940-943. THE EDITOR—SIR, COMPARISON OF ISOTOPIC AND ULTRASONIC PLACENTOGRAPHY
Obstetrics and Gynaecology of the British
Commonwealth,
80, 433-437. (in reply)
MORRISON, J., KOHORN, E. I., ASHFORD, C , TREDGOLD, C , SECKER WALKER, R. H., and BLACKWELL, R. J., 1969.
THE EDITOR—SIR, PLACENTAL LOCALIZATION: A COMPARISON OF ISOTOPIC AND ULTRASONIC PLACENTOGRAPHY.
Of course, all that Mr. Higginbottom says about our paper (Dunster et al., 1976) is absolutely true and it is well known to us and others. The paper reported a retrospective study on the results of isotope and ultrasonic examinations that had already been completed. It did not attempt to describe the method of performing placentography nor to analyse the circumstances in which serial ultrasonic examinations would be indicated nor their frequency. It is common practice to repeat the examination as a check if the first one shows that the placenta could be low-lying or praevia. The interval between the examinations relates to the maturity of the pregnancy, being widely spaced in the second trimester and more frequent in the third trimester. These serial examinations will frequently show the placenta to "move" away from the region of the internal os (King, 1973) with the passage of time.
Ultrasonic scanning in obstetrics 2. The diagnosis of placenta praevia. Australian and New Zealand Journal of Obstetrics and Gynaecology, 9, 206-208. REED, M. F., 1973. Ultrasonic placentography. British Journal of Radiology, 46, 255-258.
THE EDITOR—SIR, 134 Cs—A POTENTIAL RADIONUCLIDE FOR RADIATION THERAPY
Caesium 134, with a half life of 2.06 years and emitting 2.23 photons of 0.7 MeV average energy per disintegration, has not been considered so far in radiotherapy. It decays (Lederer et ah, 1967) by j8~ emission to stable 134Ba as shown in Fig. 1. The maximum energy of the electrons is 0.66 MeV. The expected photons and the respective yields are given in 2Table 1I. The calculated specific y-ray constant is 8.8 R cm mCi" h"1. The half value layer, in a narrow beam
761
VOL.
50, No. 598 Correspondence factors, we Since the final activity depends on several calculated the ratio of specific activity of 134Cs to that of 60 Co as being 0.9, assuming that the irradiation time, thermal neutron flux and self shielding for 133Cs target59 are 59 same as for Co target. The activation cross-section for Co is taken as 37 barn (Mughabghab and Garber, 1973). Because nuclear reactors contain epithermal neutrons in the irradiation positions and because some targets show strong resonances in the epithermal region of the neutron flux, the effective activation cross-section can be considerably larger than the thermal neutron capture cross-section. Mughabghab and Garber (1973) report a value of 415 ± 15 barn for the resonance integral of 133Cs compared to 75.5 ± 59 1.5 barn in the case of Co. Depending on the epithermal fraction of the neutron flux at the irradiation position,134 the effective cross-section can be as high as 60 barn for Cs production. However, due to long irradiation times and the large thermal neutron activation cross-section (140 barn) for 134 Cs, small amounts of 135Cs are expected to be produced. 135 Fortunately Cs, with a half life of more than million years, is a pure /J~ emitter (Eo=O.21 MeV) and therefore no y-ray emitting contaminants are expected in the reactor production. In fact, no impurities are observed134 in a Nal (Tl) y-ray spectrum of a commercially obtained Cs source. Because of these desirable physical properties, its usefulness in radiation therapy should be evaluated. Yours, etc., D. V. RAO,
2.09 h '
2.06 Y 1.9696 MeV
'Cs
1.6430 MeV
71%
1.4004 MeV 1.36 MeV 1.1677 MeV
0.6046 MeV
F. ELLIS, J. T. MALLAMS.
STABLE
l34
Ba
FIG. 1. Decay scheme of 134 Cs. The half life of 134 Cs m should be read as 2.9h.
Photon energy (MeV)
REFERENCES LEDERER, C. M., HOLLANDER, J. M., and PERLMAN, I., 1967.
Table of Isotopes, 6th edn (John Wiley & Sons, New York). MUGHABGHAB, S. F. and GARBER, D. I., 1973. NeutronCross-
TABLE I PHOTON YIELDS IN THE DECAY OF
Department of Radiology, CMDNJ, New Jersey Medical School, Newark, New Jersey 07103, USA
134
Sections, Vol. 1, Resonance parameters, Brookhaven National Laboratory report, BNL-325.
CS
No. of Group " photons No. of photons weighted per 100 disin- average energy per 100 distegrations integrations (MeV)
THE EDITOR—SIR, 134 C S FOR THE TREATMENT OF CERVICAL CANCER
One projected use of 134Cs is for the treatment of cancer of the uterine cervix. The isodose curves for cancer of the 0.4753 0.58 cervix commonly have the shape shown in Fig. 1 A and B. As 0.5631 8.18 can be seen, the tumour volume is not the same shape as the 0.5692 0.597 121.68 14.84 high dose treatment volume. With moderate energy radio0.6046 98.08 nuclides it should be possible to use filtration to try tofitthe 0.7958 85.84 0.796 95.36 isodose shape to the tumour volume using a single central 0.8018 9.52 line source with a suitable variation of linear activity along 1.0384 0.42 its length. This implies changing the isodose shape consider1.1677 1.92 1.279 5.98 ably in the coronal plane (see Fig. 1 c) but, because of the 1.365 3.64 proximity of rectum and bladder, producing little change in the sagittal section. 223.02 0.700 This may be relatively simple with low dose-rate sources in which treatment times are measured in hours. For high geometry, for platinum, lead and water is calculated to be dose-rate (e.g. cathetron) treatments, however, suitable sources capable of filtration (e.g. intracervical) so as to pro0.32 cm, 0.56 cm and 8.2 cm respectively. High specific activity combined with low cost is very duce the desired isodose shape have proved difficult to find. important for any radionuclide to be useful in therapy. Such sources (see Table I) should (a) have high specific Caesium 134 can be easily and economically produced in activity and exposure rate, (b) produce y rays of sufficiently nuclear reactors. The target material is 100% naturally low energy so that filtration can influence the shape of the abundant 133 Cs. The134thermal neutron activation cross- isodose surfaces, (c) have long half life. Cobalt 60 fulfils the 133 section for Cs(n, y) Cs is reported (Mughabghab and first but not the second of these criteria. Cesium 137 fulfils Garber, 1973) to be 29 ±1.5 barn. The cross-section for the the second but not the first. Both fulfil the third. Iridium 192 formation ofm134 Cs m is 2.5 barn. As shown in Fig. 1, the half- satisfies the first two, but not the third. A half life of two 134 long enough to be practicable. It life of Cs is 134 only 2.9 hours and the decay is completely years might be considered is possible with 134Cs, because of its very high specific to the long lived Cs.
762
