Improving molecular radiotherapy dosimetry using anthropomorphic calibration

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.

This 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.

Patient-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.

For 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.