The superior predictive value of 166Ho-scout compared with 99mTc-macroaggregated albumin prior to 166Ho-microspheres radioembolization in patients with liver metastases.

PURPOSE: As an alternative to technetium-99m-macroaggregated albumin (99mTc-MAA), a scout dose of holmium-166 (166Ho) microspheres can be used prior to 166Ho-radioembolization. The use of identical particles for pre-treatment and treatment procedures may improve the predictive value of pre-treatment analysis of distribution. The aim of this study was to analyze the agreement between 166Ho-scout and 166Ho-therapeutic dose in comparison with the agreement between 99mTc-MAA and 166Ho-therapeutic dose. METHODS: Two separate scout dose procedures were performed (99mTc-MAA and 166Ho-scout) before treatment in 53 patients. First, qualitative assessment was performed by two blinded nuclear medicine physicians who visually rated the agreement between the 99mTc-MAA, 166Ho-scout, and 166Ho-therapeutic dose SPECT-scans (i.e., all performed in the same patient) on a 5-point scale. Second, agreement was measured quantitatively by delineating lesions and normal liver on FDG-PET/CT. These volumes of interest (VOIs) were co-registered to the SPECT/CT images. The predicted absorbed doses (based on 99mTc-MAA and 166Ho-scout) were compared with the actual absorbed dose on post-treatment SPECT. RESULTS: A total of 23 procedures (71 lesions, 22 patients) were included for analysis. In the qualitative analysis, 166Ho-scout was superior with a median score of 4 vs. 2.5 for 99mTc-MAA (p < 0.001). The quantitative analysis showed significantly narrower 95%-limits of agreement for 166Ho-scout in comparison with 99mTc-MAA when evaluating lesion absorbed dose (- 90.3 and 105.3 Gy vs. - 164.1 and 197.0 Gy, respectively). Evaluation of normal liver absorbed dose did not show difference in agreement between both scout doses and 166Ho-therapeutic dose (- 2.9 and 5.5 Gy vs - 3.6 and 4.1 Gy for 99mTc-MAA and 166Ho-scout, respectively). CONCLUSIONS: In this study, 166Ho-scout was shown to have a superior predictive value for intrahepatic distribution in comparison with 99mTc-MAA.

Comparing adaptation strategies in MRI-guided online adaptive radiotherapy for prostate cancer: Implications for treatment margins.

PURPOSE: To quantify the difference in accuracy of adapt-to-position (ATP), adapt-to-rotation (ATR) and adapt-to-shape (ATS) workflows used in MRI-guided online adaptive radiotherapy for prostate carcinoma (PCa) by evaluating the margins required to accommodate intra-fraction motion of the clinical target volumes for prostate (CTVpros), prostate including seminal vesicles (CTVpros + sv) and gross tumor volume (GTV). MATERIALS AND METHODS: Clinical delineations of the CTVpros, CTVpros + sv and GTV of 24 patients with intermediate- and high-risk PCa, treated using ATS on a 1.5 T MR-Linac, were used for analysis. Delineations were available pre- and during beam-on. To simulate ATP and ATR workflows, we automatically generated the structures associated with these workflows using rigid transformations from the planning-MRI to the daily online MRIs. Clinical GTVs were analyzed as ATR GTVs and only ATP GTVs were simulated. Planning target volumes (PTVs) were generated with isotropic margins ranging 0.0-5.0 mm. The volumetric overlap was calculated between these PTVs and their corresponding clinical delineation on the MRI acquired during beam-on and averaged over all treatment fractions. RESULTS: The PTV margin required to cover > 95% of the CTVpros was equal (2.5 mm) for all workflows. For the CTVpros + sv, this margin increased to 5.0, 4.0 and 3.5 mm in the ATP, ATR and ATS workflow, respectively. GTV coverage improved from ATP to ATR for margins up to 4.0 mm. CONCLUSION: ATP, ATR and ATS workflows ensure equal coverage of the CTVpros for the current clinical margins. For the CTVpros + sv, ATS showed optimal performance. GTV coverage improves by additional adaptations to prostate rotations.