Planar Images-Based Internal Dosimetry using Patient-Specific Organ S-Value after 177Lu-DOTATATE Therapy

Introduction: Post-therapeutic dosimetry after ¹⁷⁷Lu-DOTATATE is required for the optimization of further therapy. The voxel-based dosimetry using sequential SPECT/CT images is potentially more accurate than organ-based dosimetry but requires additional time and cost. Hybrid dosimetry using a single SPECT/CT and sequential planar images was suggested in order to reduce this burden. On the other hand, planar dosimetry with patient-specific organ S-values (PSS) could be used to further simplify the dosimetry procedure as it requires only one SPECT/CT image for dosimetry of the entire cycle treatments. Here, we investigated whether the planar dosimetry with PSS could be an alternative approach to the hybrid dosimetry. The planar dosimetry with PSS was compared with full 3D dosimetry using Monte Carlo simulation and hybrid dosimetry.

Methods: The SPECT/CT and planar image sets from 6 patients who underwent ¹⁷⁷Lu-DOTATATE therapy at Seoul National University Hospital were used for dosimetry. All patients received at least 2 cycles of therapy. The mean administered activity was 7.07 <m:omath><m:scr m:val="roman"><m:sty m:val="p"></m:sty></m:scr></m:rpr>±</m:omath> 1.11 GBq. SPECT/CT and planar images were acquired at 4, 24, 48, and 120 hours after injection. Full 3D dosimetry using sequential SPECT/CT images was performed through Monte Carlo simulation with GATE v.8.2. For the simulations, the time-integrated activity maps derived from sequential SPECT images and the corresponding CT images were used as voxelized source maps and phantoms. Hybrid dosimetry was performed using dose rate maps acquired from Monte Carlo simulation and time-activity curves derived from planar images. Planar dosimetry was performed using PSSs (PSS dosimetry) which were calculated using the multiple VSV method with a CT image of the first cycle treatment (JNM 2018:1133-9). The radioactivity over time of each organ was estimated using sequential planar images for the planar dosimetry. The absorbed doses of kidneys, bone marrow in the L-spine, liver, and spleen were assessed using each method. The relative difference of absorbed doses obtained using full 3D dosimetry and the other two simplified methods was analyzed. Wilcoxon signed-rank test was performed to compare the hybrid dosimetry and the planar dosimetry using PSS.

Results: The average absorbed doses were 3.42 ( 1.05), 1.63 ( 1.49), 6.83 ( 5.89), and 3.01 ( 2.06) Gy for kidneys, bone marrow, liver, and spleen, respectively when full 3D dosimetry was performed. In hybrid dosimetry, the average absorbed doses were 4.01 ( 1.30), 1.71 ( 1.64), 6.17 ( 5.50), and 3.12 ( 2.03) Gy in the same order. In PSS dosimetry, the average absorbed doses were 4.13 ( 1.22), 1.64 ( 1.57), 6.28 (<m:omath><m:scr m:val="roman"><m:sty m:val="p"></m:sty></m:scr></m:rpr></m:omath> 5.05), and 3.02 ( 1.99) Gy in the same order. The hybrid dosimetry and PSS dosimetry yielded similar absorbed doses regardless of the treatment cycle (averaged difference: 7.65, 7.03, 7.89, and 15.72 % for kidneys, bone marrow, liver, and spleen, respectively). The dose estimations acquired using hybrid dosimetry and PSS dosimetry showed a strong correlation when Pearson’s correlation analysis was performed (P < 0.05 for all organs). Wilcoxon signed-rank test showed that there is no significant difference between the results obtained using hybrid and PSS dosimetry methods (P > 0.1 for all organs). The cycle number (i.e. first or second) does not affect the tendency of the results.

Conclusions: In this study, we confirmed that the planar dosimetry with PSS yielded almost similar dose estimation compared to hybrid dosimetry. As the PSS derived from CT images used actual patient anatomical information, the planar dosimetry with PSS provided reliable results. In addition, the results of the PSS dosimetry showed similar results to the hybrid dosimetry for later cycle treatment even though PSSs were derived using a CT image of the first cycle. As a result, PSS dosimetry could simplify the dosimetry procedure compared to the hybrid dosimetry and full 3D dosimetry.

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