3D image-based dosimetry for Yttrium-90 SIR-therapy of HCC: comparison of three different techniques

Objectives: Most of the dosimetric approaches for Yttrium-90 SIRT rely on 1D averaged tumor and healthy liver tissue doses, while 3D image-based dosimetry for treatment verification would be preferred. Depending on the institute, either ⁹⁰Y bremsstrahlung (BRS) SPECT or ⁹⁰Y PET is used for post-therapeutic imaging. Although ⁹⁰Y BRS SPECT imaging is limited by its low quantification capability, it has been used for years in post-SIRT imaging. Whilst ⁹⁰Y PET imaging is gaining more and more importance, it is not yet implemented in the post-therapeutic SIRT workflow of all institutions. Aim of this study was to investigate differences in dose values and distributions for ⁹⁰Y PET and ⁹⁰Y BRS SPECT in comparison to a simplified Technetium-99m-MAA SPECT based 3D dosimetry for institutions without access to ⁹⁰Y PET imaging.

Methods: Five patients suffering from unresectable hepatocellular carcinoma (HCC) received a ^99mTc-MAA SPECT/CT before ⁹⁰Y SIRT and accordingly post-therapeutic ⁹⁰Y BRS SPECT/CT and ⁹⁰Y PET/CT. The therapy volume was segmented from low resolution ⁹⁰Y BRS SPECT and the ^99mTc-MAA SPECT was scaled with the ⁹⁰Y therapy activity within this volume. This scaled ^99mTc-MAA SPECT based activity image as well as the ⁹⁰Y BRS SPECT and ⁹⁰Y PET activity images were convolved with a pre-simulated ⁹⁰Y dose kernel for soft tissue to obtain 3D dose images. To compare tumor does a spherical VOI with 2 cm diameter was drawn and manually placed in the tumor. Additionally, cumulative dose volume histograms (DVH) were created and the 75 % dose percentile (D75P) was analyzed.

Results: The tumor doses based on the scaled ^99mTc-MAA SPECT averaged over all patients were 100.0 (± 16.2) Gy/GBq, based on ⁹⁰Y BRS SPECT were 62.0 (± 4.9) Gy/GBq and therefore they were deviating 23 % and respectively 47 % from the ⁹⁰Y PET based tumor dose values with on average 129.5 (± 10.1) Gy/GBq. The dose images based on ⁹⁰Y PET and on the scaled ^99mTc-MAA SPECT were showing comparable DVH curves. Whilst the ⁹⁰Y BRS SPECT based DVH curve indicates a systematic underestimation of dose values. The scaled ^99mTc -MAA SPECT and ⁹⁰Y BRS SPECT based D75P are showing a mean deviation of 28 % and 39 % compared to ⁹⁰Y PET based D75P.

Conclusions: With our study on 5 HCC SIRT patients, we observed a significant tumor dose underestimation for ⁹⁰Y BRS SPECT based 3D dosimetry. Dosimetry based on a scaled ^99mTc-MAA SPECT showed comparable tumor dose values within the given standard deviation and the deviation compared to ⁹⁰Y PET tumor doses was overall smaller than those of ⁹⁰Y BRS SPECT based tumor doses. We therefore conclude, that our investigations are indicating that post-therapeutic 3D dosimetry based on scaled ^99mTc-MAA SPECT is feasible while showing comparable tumor dose values to ⁹⁰Y PET. Further investigations for larger patient cohorts are required.