The IAEA-WHO postal dose comparison service for cobalt-60 teletherapy dosimetry is an ongoing programme aimed at improving dosimetric accuracy in IAEA-WHO Member States. LiF dosemeters were irradiated under specified conditions in a water phantom at the participating radiotherapy centres and returned to the IAEA's Dosimetry Labotatory for evaluation. The participants filled in a data sheet giving numerical values of physical factors (such as per cent depth dose, tissue-air ratio, etc.) that they employed. Nearly 65% of the comparisons showed deviations within +/- 5%, a figure which is believed to represent the overall uncertainty of the method itself. Almost 10% of the measurements deviated more than +/- 15%. It appears that participants applying 'in phantom' output measurements have better control over the dose they administer. Many of the errors observed are due to the use of faulty values for the various physical factors employed, and to the fact that often a check of the output just before the irradiation of the LiF dosemeters was not carried out.

E infiihrung.In Fortffihrung einer Arbeit yon H. H6NL 1 fiber den Atomfaktor in der N/tahe der K-Absorptionskante soll die WALLERsche Dispersionsformel ffir die koh/trente Streuung von R6ntgenstrahlen an idealamorphen Stoffen unter besonderer Berficksichtigung der anomalen Dispersion an der L-Kante diskutiert werden. Anlal3 zu dieser Untersuchung gaben t944 durchgeffihrte Messungen von R. GLOCKER fiber R6ntgen-streuintensit/tten an amorphem Tellur 2, wobei zlm/tChst Abweichungen yon den fiblichen Streukurven festgestellt wurden, die um so gr6Ber waren, je n/ther die Wellenl/tnge der verwendeten R6ntgenstrahlung an den L-Absorptionskanten lag. Es lag die Vermutung nahe, diese Tatsache mit der anomalen Dispersion der R6ntgenstrahlung in der N/the der L-Kanten in Verbindung zu bringen. Dies zu kl/tren, wurde die Dispersionstheorie erneut herangezogen; diesmal unter besonderer Berficksichtigung der L-Elektronen.Die vorliegende Arbeit behandelt die Auswertung des die anomale Dispersion der L-Schale beschreibenden Gliedes der WALLERschen Streuformel, unabh/tngig von dem zun/~chst gegebenen AnlaB. In Teil I wird der ffir die anomale Dispersion der L-Schale verantwortliche Bestandteil des Streumomentes eines wasserstoffithnlichen, schweren Atoms berechnet. Wie bei den entsprechenden Rechnungen von H. H/3NL for die K-Schale gelingt es auch hier, das Streumoment mit Hilfe eines AdditionStheorems fiir zugeordnete Kugelfunkti0nen nach Multipolen zu entwickeln, wobei als Entwicklungsparameter das Verh/tltnis von L-Schalenradius zu Prim/trwellenl/tnge auftritt. Jeder Multipol besitzt seine eigene, charakteristische Frequenzabh/tngigkeit.

During 1975-1982 the International Atomic Energy Agency (IAEA) in co-operation with the World Health Organization (WHO) carried out 603 absorbed dose determinations for 60Co gamma-ray beams for 278 radiotherapy institutes in 60 countries. The participants were asked to irradiate capsules containing LiF powder to 2 Gy in 5 cm of water. The percentage of the determinations in which the deviation between the institutes quoted dose and the IAEA measured dose was within +/- 5% was 62%. In 17% of the determinations, the deviation was more than +/- 10%. For around 70% of the institutes the deviation decreased when further measurements were carried out.

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