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Comparison of 90Y SIRT predicted and delivered absorbed doses using a PSF conversion method
Allison J Craig1, Iain Murray1, Ana M Denis-Bacelar2
1Joint Department of Physics, Royal Marsden NHSFT, Sutton, United Kingdom; The Institute of Cancer Research, London, United Kingdom.
Summary
This study developed a resolution conversion method (RCM) to accurately compare Technetium-99m (99mTc) SPECT and Yttrium-90 (90Y) SPECT imaging for selective internal radiation therapy. The RCM improved absorbed dose predictions, especially for normal liver tissue.
Area of Science:
- Nuclear Medicine
- Medical Imaging
- Radiotherapy Physics
Background:
- Selective internal radiation therapy (SIRT) involves administering radioactive isotopes like Yttrium-90 (90Y) to treat liver tumors.
- Accurate dosimetry is crucial for effective treatment and minimizing toxicity.
- Partial volume effects in SPECT imaging can lead to discrepancies between pre-therapy (99mTc-MAA) and post-therapy (90Y) imaging, impacting dose calculations.
Purpose of the Study:
- To develop and validate a resolution conversion method (RCM) to correct for partial volume effects in 99mTc-MAA and 90Y SPECT imaging.
- To improve the quantitative comparison of predicted and delivered 90Y absorbed doses in SIRT.
- To assess the predictive accuracy of 99mTc SPECT for 90Y absorbed dose.
Main Methods:
- Developed an RCM by converting the spatial resolution of 99mTc SPECT data to match 90Y SPECT data using point spread functions.
- Validated the RCM using phantom studies with both 99mTc and 90Y SPECT data.
- Applied the RCM to clinical SIRT data to evaluate its effectiveness in predicting absorbed doses.
Main Results:
- The RCM significantly reduced absorbed dose differences in phantom spheres from 178% to 27%.
- In clinical data, mean absorbed dose differences in the normal liver were reduced from 38% to 20% after RCM application.
- While predicting doses to tumors >100 cm³ improved to within 52%, smaller tumors (<100 cm³) showed poor predictability (up to 210% difference) even with RCM.
Conclusions:
- The resolution conversion method (RCM) effectively corrects for partial volume effects in 99mTc and 90Y SPECT imaging.
- 99mTc pre-therapy imaging, when corrected by RCM, can predict 90Y absorbed dose to the normal liver within 20%.
- The RCM demonstrates poor predictability for smaller tumors (<100 cm³), highlighting limitations in current imaging resolution for precise tumor dosimetry.
