Abstract
The optimised delivery of Molecular Radiotherapy requires individualised calculation of absorbed dose to both targeted lesions and neighbouring healthy tissue. To achieve this, accurate quantification of the activity distribution in the patient by external detection is vital. MethodsThis work extends specific anatomy-related calibration to true organ shapes. A set of patient-specific 3D printed organ inserts based on a diagnostic CT scan was produced, comprising the liver, spleen and both kidneys. The inserts were used to calculate patient-specific calibration factors for 177Lu. These calibration factors were compared with previously reported calibration factors for corresponding organ models based on the Cristy and Eckerman phantom series and for a comparably sized sphere. Monte Carlo calculations of the patient-specific radiation dose were performed for comparison with current clinical dosimetry methods for these data. ResultsPatient-specific calibration factors are shown to be dependent on the volume, shape and position of the organ containing activity with a corresponding impact on the calculation of the dose to the patient. The impact of organ morphology on calculated dose is reduced when the dominant contributor to dose is beta particles. This is due to the small range of beta particles in tissue. Overestimations of recovered activity and hence dose of up to 135% are observed. ConclusionFor accurate quantification to be performed calibration factors accounting for organ size, shape and position must be used. Such quantification is vital if accurate, patient-specific dosimetry is to be achieved.
Highlights
Molecular Radiotherapy (MRT) is growing in popularity as a combined therapy and imaging procedure
Accurate activity quantification is vital for accurate, patient-specific dosimetry to be performed
Cristy and Eckerman (C&E) calibration factors are suitable for patients presenting with organs similar in size, shape and position to C& E defined organs
Summary
Molecular Radiotherapy (MRT) is growing in popularity as a combined therapy and imaging procedure. In current practice a standard activity is often administered. This results in a wide range of absorbed doses to organs of interest in different patients, as much as 14–32 Gy to the kidneys in 177Lu-Dotatate therapies [1]. The optimisation of MRT therapies requires individualised calculation of absorbed dose to both targeted lesions and organs at risk. Such patient-specific dosimetry is regarded as an essential component of MRT therapies [2]. In a recent review of the use of dosimetry in the clinical practice of MRT it was concluded that ‘evidence strongly implies a correlation between the absorbed doses delivered and the response and toxicity’ and that ‘dosimetry-based personalised treatments will improve outcomes and increase survival’ [3]
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