Internal Dosimetry Task Force Report

October 2017

Cancer and benign diseases have been treated with radiopharmaceuticals since the 1940s. Although internal dosimetry was initially investigated for benign and malignant thyroid disease with radioiodine, this was subsequently omitted. For over 60 years, radiotherapeutic administrations have been primarily governed by fixed levels of activity, sometimes modified by patient weight or body surface area.

The aim of this report is to examine the potential for personalised, dosimetry-based treatment planning and verification of the absorbed dose delivered. The main sections evaluate whether dosimetry is feasible for the therapeutic procedures currently used, examine the evidence for absorbed dose-effect correlations, and speculate on how personalised treatment planning may be further developed. The results of a Europe-wide survey on current practices in MRT are also presented, demonstrating the range of practices currently offered and the need to promote European standardisation and optimisation. Finally, consideration is given to the resources needed to deliver a comprehensive therapy service.

The European directive 2013/59/Euratom (1) is concerned with basic safety standards for protection against the dangers arising from exposure to ionising radiation. Of particular relevance to medical procedures is the need for justification of medical exposures and the recording and reporting of absorbed doses from medical procedures. The general principle of optimisation is applied to radiotherapeutic procedures in terms of patient dosimetry:

Article 56 Optimisation

‘For all medical exposure of patients for radiotherapeutic purposes, exposures of target volumes shall be individually planned and their delivery appropriately verified, taking into account that doses to non-target volumes and tissues shall be as low as reasonably achievable and consistent with the intended radiotherapeutic purpose of the exposure.’

The term ‘radiotherapeutic’ is specifically defined as ‘including nuclear medicine for therapeutic purposes’ (Definition 81).

This form of treatment has been known by many names. In general, the most widely used term has been radionuclide therapy. In recent years the term ‘molecular radiotherapy’ (MRT) has gained acceptance to describe the use of radiotherapeutics informed by patient-specific absorbed dose calculations, as this acknowledges that for any given procedure, for any given patient, treatment outcome is dependent on the absorbed doses delivered to tumours, target volumes and healthy organs. However, it should be noted that not all therapy procedures employ a molecular process, a notable exception being the use of 90Y microspheres for hepatocellular carcinoma and liver metastases. In this respect, this generic term emulates that of ‘molecular imaging’, which is also widely applied to functional imaging procedures. In this report, ‘radionuclide therapy’ is used as a general term to refer to treatment with radiopharmaceuticals, and the term  ‘molecular radiotherapy’ is used where dosimetry is a key element.

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