RT Journal Article SR Electronic T1 In Vivo Dosimetry Based on SPECT and MR Imaging of 166Ho-Microspheres for Treatment of Liver Malignancies JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 2093 OP 2100 DO 10.2967/jnumed.113.119768 VO 54 IS 12 A1 Maarten L.J. Smits A1 Mattijs Elschot A1 Maurice A.A.J. van den Bosch A1 Gerrit H. van de Maat A1 Alfred D. van het Schip A1 Bernard A. Zonnenberg A1 Peter R. Seevinck A1 Helena M. Verkooijen A1 Chris J. Bakker A1 Hugo W.A.M. de Jong A1 Marnix G.E.H. Lam A1 Johannes F.W. Nijsen YR 2013 UL http://jnm.snmjournals.org/content/54/12/2093.abstract AB 166Ho-poly(l-lactic acid) microspheres allow for quantitative imaging with MR imaging or SPECT for microsphere biodistribution assessment after radioembolization. The purpose of this study was to evaluate SPECT- and MR imaging–based dosimetry in the first patients treated with 166Ho radioembolization. Methods: Fifteen patients with unresectable, chemorefractory liver metastases of any origin were enrolled in this phase 1 study and were treated with 166Ho radioembolization according to a dose escalation protocol (20–80 Gy). The contours of all liver segments and all discernible tumors were manually delineated on T2-weighted posttreatment MR images and registered to the posttreatment SPECT images (n = 9) or SPECT/CT images (n = 6) and MR imaging–based R2* maps (n = 14). Dosimetry was based on SPECT (n = 15) and MR imaging (n = 9) for all volumes of interest, tumor-to-nontumor (T/N) activity concentration ratios were calculated, and correlation and agreement of MR imaging– and SPECT-based measurements were evaluated. Results: The median overall T/N ratio was 1.4 based on SPECT (range, 0.9–2.8) and 1.4 based on MR imaging (range, 1.1–3.1). In 6 of 15 patients (40%), all tumors had received an activity concentration equal to or higher than the normal liver (T/N ratio ≥ 1). Analysis of SPECT and MR imaging measurements for dose to liver segments yielded a high correlation (R2 = 0.91) and a moderate agreement (mean bias, 3.7 Gy; 95% limits of agreement, −11.2 to 18.7). Conclusion: With the use of 166Ho-microspheres, in vivo dosimetry is feasible on the basis of both SPECT and MR imaging, which enables personalized treatment by selective targeting of inadequately treated tumors.