Results from an EANM survey on time estimates and personnel responsible for main tasks in molecular radiotherapy dosimetry

Molecular radiotherapy (MRT) is the selective delivery of radionuclides to target and destroy malignant cells, mainly by exposure to the emitted beta or alpha particles [ 1 , 2 ]. In most cases, these radionuclides are labelled to carrier molecules, also called vectors, for which tumour cells show avidity [ 3 , 4 , 5 ]. There is a wide variety of radionuclides and vectors used to treat a diversity of diseases [ 6 , 7 , 8 , 9 ], and their number is increasing, favoured by intense research in the field of theranostics in nuclear medicine [ 10 , 11 , 12 ]. In other therapeutic techniques with ionising radiation, such as external beam radiotherapy (EBRT) or brachytherapy, the International Commission on Radiation Units and Measurements (ICRU) has standardised the prescription, recording and reporting of treatments [ 13 , 14 , 15 , 16 , 17 , 18 ]. Moreover, the International Atomic Energy Agency (IAEA) has released documents on the determination of absorbed dose in EBRT, calibration of sources in brachytherapy and commissioning and quality assurance of treatment-planning systems [ 19 , 20 , 21 ]. Thus, EBRT and brachytherapy treatments are carried out on the basis of accurate dosimetric characterisation of all equipment involved in planning and delivering the treatment. Currently, this is not the case in MRT, as shown in a survey performed by the former Internal Dosimetry Task Force (IDTF) [ 22 ] of the European Association of Nuclear Medicine (EANM). However, the European Directive 2013/59 Euratom [ 23 ] establishes the obligatory nature of treatment optimisation and verification in MRT. In order to address the implementation of the Directive, the EANM recently released a position paper [ 24 ] in which three levels of dosimetry are proposed. These levels include an activity-based prescription with patient-averaged dosimetry, an activity-based prescription with patient-specific dosimetry and, a dosimetry-based prescription and post-therapy dosimetry verification. A report by the IDTF [ 3 ] addressed the potential and prospect of treatment planning for the main treatments of MRT. However, whilst several dosimetric approaches were included in the report, the resource implications were not thoroughly examined. Current practices of dosimetry for MRT were investigated in a topical report of the Institute of Physics and Engineering in Medicine (IPEM) [ 25 ], including the potential barriers in setting up a clinical dosimetry service. It was concluded that in the UK, most medical physics groups are well equipped to provide a simple form of dosimetry service, but in most cases refrain to perform dosimetry routinely by ‘lack of clinical evidence and practice’ and that more complex dosimetry will require additional staffing. Previous documents [ 26 , 27 , 28 ] have addressed the subject of resourcing in nuclear medicine, including estimates of medical physics time, pertinent to dosimetry and radiation safety across different therapies, such as thyrotoxicosis, thyroid carcinoma and neuroendocrine tumours [ 27 , 28 ]. However, those estimates were not specific for the particular dosimetry workflow that is specific to each therapeutic procedure. For instance, in the treatment of neuroblastoma with [ I]I-mIBG, dosimetry may be performed for the whole-body dosimetry utilising portable radiation detectors or at the lesion level using image-based techniques. [ 29 ]. Moreover, as with EBRT and brachytherapy dosimetry [ 29 , 30 , 31 , 32 ], time has to be dedicated to initial protocol development and configuration of equipment (Fig. 1 ). Additionally, several disciplines may be involved in the different tasks associated with the dosimetry workflow. Dosimetry workflow To better understand the potential resources being dedicated to the main tasks within a dosimetry workflow in MRT (Fig. 1 ) and the personnel groups undertaking these tasks, a survey was conducted among different experts working in MRT dosimetry. The present document reports on the results of that survey. The survey was prepared by the Dosimetry Committee of the EANM in the form of an electronic questionnaire and was distributed amongst experts working in MRT. Respondents to the survey were mostly members of the former IDTF or Dosimetry Committee of the EANM. Table 1 summarizes the structure of the survey, and all included questions are shown in table A.I of Appendix I . An introductory page which contained instructions and explanation of the rationale for the survey was given to participants. The survey was divided into three main sections corresponding to the principal stages of MRT dosimetry (see Fig. 1 ). Each section contained further introductory explanations. Where necessary, each section of the questionnaire was split into further subsections relating to different procedures or dosimetry approaches. The section dedicated to protocol development did not include any subsections as it was assumed a similar time was required for developing a dosimetry protocol irrespective of the therapeutic procedure. The section relating to initial set-up and preparation of equipment was divided into five subsections. Four of these sections addressed the main equipment used in MRT dosimetry, namely portable radiation detectors, gamma well counters and liquid scintillation counters, thyroid uptake probes and SPECT/CT or PET/CT scanners. The final subsection concerned data analysis of the equipment configuration. The section of the survey related to patient measurement and dosimetry calculations was also subdivided into five subsections. The first four subsections addressed the resources dedicated to activity measurement using the aforementioned equipment. The final subsection covered the resources for absorbed dose calculations using the activity measurements. For the online survey, questions relating to time resources appeared as drop-down lists covering a wide range of available options (see Appendix I ). Questions relating to responsible personnel were multiple-choice and included option for medical physicist, medical doctor, technologist, nurse, engineer and other. Participant responses were exported for analysis to a spreadsheet. The survey was completed by 19 medical physicists and two nuclear medicine physicians working at 18 different centres across 13 countries (Canada, Czech Republic, Finland, France, Germany, Greece, Italy, Netherlands, Norway, Spain, Sweden, Switzerland and the UK). Not all participants responded to each question as in some cases, a particular method of dosimetry may not have been undertaken at that centre. A detailed analysis of the responses to each question is presented in Appendix II . Results for time estimates were summarised as box whisker plots in which the box extends from the first to third inter quartile range about the median value. The whiskers correspond to the maximum and minimum values of all responses. The percentage of responses in which each personnel group was selected was summarized in bar diagrams, for which the following abbreviations are used: Phys. = medical physicist, M.D. = medical doctor, Tech. = technologist, Eng. = engineer. As questions relating to personnel allowed for more than one choice, the total percentage exceeded 100% in some cases, indicating more than one personnel group was responsible for that task. Using these data, some specific examples for different MRT dosimetry tasks are provided giving estimates of the potential time dedicated to dosimetry and which personnel group or groups could be primarily responsible. Time estimates are given as median (1 quartile, 3 quartile) and are summed for each step in the dosimetry process to give an indication of the total resource required to prepare and undertake a dosimetry study. In treatments of neuroblastoma with [ I]I-mIBG, patients often spend several days in the treatment room for radiation-protection purposes [ 29 ]. Whole-body measurements can be used to track the activity clearance from the body so as to determine an appropriate time for discharge. In addition, these results can be used for dosimetry as a surrogate for bone marrow dosimetry and predicting haemotoxicity. Several measurements per day are performed, from which the whole-body activity at each time point is determined. A function is fitted to the time activity data and integrated, to obtain the time-integrated activity. The whole-body absorbed dose is calculated from this using an S-value scaled according to the patient body mass [ 29 ]. For 20 whole-body measurements, a total required time of 2.5 h is estimated to obtain these data. The responsibility of the measurements is shared mainly by medical physicists and technologists. A further 1.4 h is required for analysis and interpretation of data (activity and absorbed dose determination), which is generally carried out by the medical physicist. Initial set-up of portable radiation detectors would need 2 h as results from the survey indicate, but it is not strictly necessary if a conversion factor from dose rate to activity is obtained from the first patient measurement [ 29 ]. Table 2 summarises the separate tasks, together with the time estimates and personnel responsible. In treatments of metastatic differentiated thyroid cancer, treatment planning can be performed after administration of a tracer activity to determine the activity to be administered to reach a maximum tolerable red marrow absorbed dose of 2 Gy [ 33 ]. For this example, five blood extractions and five whole-body dose-rate measurements are assumed [ 34 ]. Table 3 summarises the tasks, together with the time estimates and personnel responsible, as indicated from the survey results. Blood extractions are generally carried out by a nurse or technologist, and samples prepared by a medical physicist or technologist. For tasks related to whole body, responsibilities are those of the previous example. Interpretation and processing of the results fell to the medical physicist. The whole process is expected to take about half a working day, but it is often split over many days as the blood samples are taken over a 4- or 5-day period, so it equates to less than 1 h per day of physics time. These values are similar to that expected for a glomerular filtration rate service. In treatments of benign thyroid disease with [ I]I-NaI, the activity to deliver the prescribed absorbed dose can be calculated by means of a pre-therapy dosimetry administering a tracer. Two measurements of the [ I]I-NaI uptake in the thyroid can be performed, and afterwards, the [ I]I-NaI uptake must be determined at each time point. With those values and the thyroid mass which is usually obtained from ultrasound imaging, the thyroid absorbed dose delivered by the tracer is calculated and then the activity to administer for the therapy [ 35 ]. A previous calibration of the thyroid uptake probe would take 0.7 h according to the survey. The thyroid uptake measurements would take 0.4 h and the data analysis to determine the activity and the absorbed dose to the thyroid 0.3 h. Responsibility for uptake measurements mainly fell to medical physicists and technologists and calculations of activity and absorbed dose to medical physicists. Table 4 summarises the separate tasks, together with the time estimates and personnel responsible. To perform the preparatory imaging tests on a SPECT/CT scanner prior to image-based dosimetry, a variety of phantoms can be prepared [ 36 , 37 , 38 ]. In this example, a cylindrical water-filled cylindrical phantom, used to determine the calibration factor, and a phantom with fillable inserts, used to determine the recovery curve (e.g. the NEMA IEC Body phantom set) are considered. Images of both phantoms are acquired, processed and analysed with image processing software. Lastly, the gathered data are analysed and the calibration factor and recovery coefficients determined. From the results of the survey, this task would generally be performed by the medical physicist and require a full working day to obtain and analyse the required data. Table 5 summarises the separate tasks undertaken, together with the estimated time required. It should be noted that this procedure is generally only completed once, prior to initiating a dosimetry service, or infrequently (e.g. annually) as part of a regular quality assurance programme. Two different approaches are considered for patient dosimetry measurement. In the first approach, image-based kidney dosimetry is performed using a single SPECT/CT acquisition [ 39 ] following treatment with [ Lu]Lu-DOTA-TATE. Acquired data are reconstructed and processed and the activity and volume (or the activity concentration) of the kidney determined. Time-integrated activities are calculated, and subsequently the absorbed doses. In this example, it is assumed that a spreadsheet is used for calculation of absorbed dose, rather than using a dedicated dosimetry software package. Table 6 summarises the tasks, together with the time and personnel responsible. Results indicate that a dosimetry study can be completed in just over 2 h, including time dedicated to imaging the patient and for the manual calculations of the absorbed dose. Responsibility was generally that of the technologist for scanning. For image processing, responsibility is shared between medical physicists and technologists, and for activity and volume determination, which implies organ outlining, between medical physicists and medical doctors. Lastly medical physicists were considered as responsible for absorbed dose calculations. The second approach considers a more complex scenario whereby the doses of two lesions and one kidney are of interest. Three SPECT/CT acquisitions are modelled in this scenario. The methodology is the same as described in the case of the kidney dosimetry summarised in Table 6 . Due to the additional scanning and image processing, technologist time increases accordingly. More physics resources are also required as the absorbed dose calculation is more complex and organ and lesion delineation is more time consuming, which also would increase the time of medical doctor. Whilst the results indicate that a full working day is necessary to perform these calculations, it should be noted that this dosimetry schedule is protracted over a full week so amounts to just over 1 h per day per patient. Table 7 summarises the tasks, together with the time and personnel responsible, as obtained from the survey.

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