Quantifying respiratory motion effects on dosimetry in hepatic radioembolization using experimental phantom measurements

Objective: In radioembolization, SPECT/CT planning scans are often acquired during free breathing, which can introduce motion-related blurring and misregistration between SPECT and CT, leading to dosimetric inaccuracies. This study quantifies the impact of respiratory motion on absorbed dose metrics—tumor-to-normal tissue (T/N) ratio, dose volume histograms, and mean dose—using several voxel-based dosimetry methods. This study supports standardization efforts through experimental measurements using a motion-enabled phantom. Approach: Motion effects in pre-therapy imaging were evaluated using a Jaszczak phantom filled with technetium-99m, simulating activity in lesion and background volumes. SPECT/CT scans were acquired with varying cranial-caudal motion amplitudes from the central position ±0, ±5, ±6.5, ±10, ±12.5, and ±15 mm. The impact of motion-related misregistration during scanning on dosimetry was also examined. Five dosimetry methods, including Monte Carlo simulation with uniform reference activity (MC REF), Monte Carlo simulation based on SPECT images (MC SPECT), Simplicity™ (Boston Scientific), local deposition method, and voxel-S-value convolution. Absorbed dose metrics of mean dose, dose volume histogram dosimetric indices (D50, D70, D90), and T/N ratio were obtained to quantify motion effects and evaluate clinical suitability. Main results: Mean absorbed dose values for the lesion and background were consistent across methods within uncertainties, though discrepancies were noted in non-lesion low-density regions. Respiratory motion reduced lesion dose by 16-25% and increased background dose by 13-32%, although the latter represented only a 1-2 Gy change. These shifts led to a 28-43% decrease in the T/N ratio at ±12.5 mm motion amplitude. Misregistration due to motion also significantly impacted dosimetric accuracy. Significance: The study demonstrated agreement between five dosimetry methods and revealed that respiratory motion can lead to substantial underestimation of the lesion dose and T/N ratio. Since T/N ratio is critical for patient selection and activity prescription, accounting for respiratory motion is essential for accurate radioembolization dosimetry.