P086] Impact of modality (2D planar, 2D/3D hybrid, 3D SPECT) on kidneys absorbed dose in lu-177-based PRRT

Abstract

Purpose In peptide receptor radionuclide therapy (PRRT) the kidneys are considered an organ at risk for radiation-induced damage. Thus the absorbed dose to the kidneys is one important safety parameter with the potential to influence treatment regimen and therefore treatment outcome. Dosimetry can be performed using planar, hybrid (planar/SPECT) or SPECT imaging. The choice of modality is expected to impact the results. In this work we aimed to compare (1) the different modalities and (2) different planar correction/calibration methods. Methods Retrospective planar, hybrid and 3D dosimetry analysis was performed using the internal dosimetry software suite QDOSE. Five patients with neuroendocrine tumours, who received 177Lu-Dotatate PRRT and were imaged at time points at 0.5 h, 4 h, 24 h and 96 h post-injection, were analyzed. SPECT data were reconstructed with 3D OSEM including attenuation and scatter correction. Planar corrections included attenuation correction using a broad beam μ and background correction. The activity calibration of the planar data was performed either using the activity in the whole body (WB) or a reference vial. Phantom-based dose calculations were performed using IDAC-Dose 2.1. Results Comparing modalities, using 3D SPECT as reference, the absorbed dose to the kidneys was overestimated by 95% when calculated with planar methodology and by 13% when hybrid dosimetry was applied. Regarding planar corrections, there was only a moderate difference between vial and WB calibration of −14% when all corrections were applied. However, vial calibration without attenuation correction resulted in a large deviation of −58% compared to full correction. When WB calibration was applied, no correction performed slightly better (−8% deviation) than only attenuation correction (+9%) or only background correction (−14%) compared to full correction. Conclusions For quantitative accuracy 3D SPECT dosimetry is preferable. If planar imaging is used, overlapping activity can cause significant overestimation of activity and hence absorbed dose for the kidneys. As absorbed dose limits (23Gy–29Gy) are commonly applied to the kidneys, dosimetry results based on planar imaging could potentially impede treatment efficacy by artificially restricting the administered therapeutic activity. Hybrid dosimetry provided a sufficiently accurate compromise between the higher acquisition time demands of 3D dosimetry and limited accuracy of planar dosimetry.