Dose rate constants for the quantity H_p(3) for frequently used radionuclides in nuclear medicine

Abstract

According to recent studies, the human eye lens is more sensitive to ionising radiation than previously assumed. Therefore, the dose limit for personnel occupationally exposed to ionising radiation will be lowered from currently 150 mSv to 20 mSv per year. Currently, no data base for a reliable estimation of the dose to the lens of the eye is available for nuclear medicine. Furthermore, the dose is usually not monitored. The aim of this work was to determine dose rate constants for the quantity H_p(3), which is supposed to estimate the dose to the lens of the eye. For this, H_p(3)-dosemeters were fixed to an Alderson Phantom at different positions. The dosemeters were exposed to radiation from nuclides typically used in nuclear medicine in their geometries analog to their application in nuclear medicine, e.g. syringe or vial. The results show that the handling of high-energy beta (i.e. electron or positron) emitters may lead to a relevant dose to the lens of the eye. For low-energy beta emitters and gamma emitters, an exceeding of the lowered dose limit seems to be unlikely.

Zusammenfassung

Die Augenlinse ist nach aktuellem Stand der Wissenschaft strahlensensibler als bislang angenommen. Der Grenzwert für beruflich strahlenexponierte Personen wird künftig von 150 mSv/a auf 20 mSv/a abgesenkt. In der Nuklearmedizin ist weder eine hinreichende Datenlage verfügbar, aus welcher sich belastbare Dosiswerte abschätzen lassen, noch wird die Organdosis in der Regel messtechnisch erfasst. Das Ziel dieser Arbeit war die messtechnische Bestimmung von Dosisleistungskonstanten für die Messgröße H_p(3), mit welchen eine Abschätzung der Augenlinsendosis ermöglicht werden soll. Hierzu wurden H_p(3)-Dosimeter an einem Aldersonphantom an verschiedenen Trageorten befestigt. Als Strahlenquellen wurden Spritzen oder Vials verwendet, wie sie in der Nuklearmedizin verwendet werden. Die Messergebnisse zeigen, dass speziell beim Umgang mit hochenergetischer Betastrahlung (d. h. Elektronen oder Positronen) relevante Dosisbeiträge entstehen können, und dass für niederenergetische Betastrahler und Gammastrahler kaum mit einer Überschreitung des zukünftig herabgesetzten Grenzwertes zu rechnen ist.

Introduction

The aim of radiation protection for the eye lens is to avoid cataract formation caused by ionising radiation. Currently, a dose limit of 150 mSv/a applies for occupationally exposed persons [1], [2]. According to studies within the last decade (see references in [3]), the human eye lens is more sensitive to ionising radiation than previously assumed. A re-evaluation of epidemiological studies supports the assumption that possibly a threshold dose of about 0.5 Gy cumulative dose is required for the induction of a cataract; however, it is unclear, whether such a threshold exists at all [3].

Hence, the International Commission on Radiological Protection (ICRP) has recommended a reduction of the dose limit [4] to 100 mSv cumulative dose in five years. In addition, a dose of 50 mSv/a shall not be exceeded in a single year. Following this recommendation, the Council of the European Union has laid down these new limits in the Directive 2013/59 / EURATOM as of 05.12.2013 [5] which has to be implemented into national law within a period of four years in member states of the EURATOM treaty.

The impending reduction of the dose limit for the eye lens requires a reassessment of individual exposure conditions for occupationally exposed personnel. As the eye lens is located behind the cornea and anterior chamber in a total depth of about 3 mm, the quantities H_p(10) (usually used as a measure of the effective dose) and H_p(0.07) (usually used as a measure of the local skin dose) do not seem to be appropriate for estimating the dose to the lens of the eye. Instead, H_p(3) should be used, at least in beta radiation fields. H_p(3) is the dose at 3 mm depth in the human body below the point where an eye dosemeter is worn (preferably as close as possible to the eye). Thus, only radiation being able to penetrate 3 mm of tissue (with an average density of 1 g/cm³ assumed) contributes to H_p(3). Accordingly, H_p(0.07) and H_p(10) are the dose in 0.07 and 10 mm tissue depth, respectively. However, at present only few dosemeters for H_p(3) are available and, at least in Germany, none is approved by regulation bodies. Finally, in Germany the need for a special dosemeter is still under discussion.

In nuclear medicine, a variety of different nuclides is in use for the treatment of patients and for research purposes. The lens of the eye of the medical staff is exposed to mixed fields of photons, electrons and positrons over a broad energy range. In most nuclear medicine departments and institutions, the individual exposure of the eye lens is neither measured nor calculated [6]. The estimation of doses to the lens of the eye has been facilitated with the recent publication of tabulated dose rate per activity values [7] (these are currently implemented in a revision of reference [8]). This is, however, only applicable to β⁻ emitting radionuclides but not to β⁺-emitters.

In this work the term “beta radiation” is used for both electrons and positrons. Depending on the context, the term refers to the whole spectrum emitted from a radionuclide, the fraction of that spectrum reaching the detector or only a part of that spectrum. In case only positrons are meant this is explicitly outlined.

The aim of this study is to determine dose rate constants for nuclides often used in nuclear medicine utilising dosemeters which allow the estimation of the dose to the lens of the eye per activity and time interval at different working distances.