TY - JOUR T1 - Impact of Modality (2D Planar, 2D/3D Hybrid, 3D SPECT) on Kidneys Absorbed Dose in <sup>177</sup>Lu-based PRRT JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 391 LP - 391 VL - 59 IS - supplement 1 AU - Wencke Lehnert AU - Karl Schmidt AU - Sharok Kimiaei AU - Tobias Meyer AU - Marcus Bronzel AU - Andreas Kluge Y1 - 2018/05/01 UR - http://jnm.snmjournals.org/content/59/supplement_1/391.abstract N2 - 391Aim: 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 the therapeutic treatment regimen and therefore also treatment outcome. Dosimetry can be performed as (1) planar dosimetry using multiple time points of conjugate view imaging, (2) hybrid dosimetry using multiple time points of conjugate view imaging and one quantitative SPECT/CT imaging time point for improved quantification or (3) 3D dosimetry using multiple time points of quantitative SPECT/CT. The choice of modality is expected to impact the dosimetry 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 had received 177Lu-Dotatate PRRT with injected activities between 6780 and 8505 MBq and imaging time points at 0.5h, 4h, 24h and 96h post-injection were analyzed. SPECT data had been reconstructed with 3D OSEM including attenuation and scatter correction. The SPECT camera sensitivity factor had been estimated using a small point-like source [1]. Planar corrections included attenuation (including source self-attenuation) 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 according to MIRD pamphlet 16 [2]. Phantom-based dose calculations were performed using IDAC-Dose 2.1 [3]. Results Comparing modalities, using 3D SPECT as reference, the absorbed dose to the kidneys was overestimated by 95% when calculated with standard 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 would be preferable. If planar imaging is used, overlapping activity, e.g. from the intestine, can cause significant overestimation of activity and hence absorbed dose for the kidneys. As absorbed dose limits (23 Gy - 29 Gy) are commonly applied to the kidneys in PRRT or other 177Lu-based radionuclide therapy, dosimetry results based on planar imaging could potentially impede treatment efficacy by artificially restricting the administered therapeutic activity. For the set of patient data analyzed here, hybrid dosimetry provided a sufficiently accurate compromise between the higher acquisition time demands of 3D dosimetry and limited accuracy of planar dosimetry. ER -