Comparison of Y SIRT predicted and delivered absorbed doses using a PSF conversion method.

PURPOSE

The aims of this study were to develop and apply a method to correct for the differences in partial volume effects of pre-therapy Technetium-99 m (Tc)-MAA SPECT and post-therapy Yttrium-90 (Y) bremsstrahlung SPECT imaging in selective internal radiation therapy, and to use this method to improve quantitative comparison of predicted and delivered Y absorbed doses.

METHODS

The spatial resolution of Tc SPECT data was converted to that of Y SPECT data using a function calculated from Tc and Y point spread functions. This resolution conversion method (RCM) was first applied to Tc and Y SPECT phantom data to validate the method, and then to clinical data to assess the power of Tc SPECT imaging to predict the therapeutic absorbed dose.

RESULTS

The maximum difference between absorbed doses to phantom spheres was 178%. This was reduced to 27% after the RCM was applied. The clinical data demonstrated differences within 38% for mean absorbed doses delivered to the normal liver, which were reduced to 20% after application of the RCM. Analysis of clinical data showed that therapeutic absorbed doses delivered to tumours greater than 100 cm were predicted to within 52%, although there were differences of up to 210% for smaller tumours, even after the RCM was applied.

CONCLUSIONS

The RCM was successfully verified using phantom data. Analysis of the clinical data established that the Tc pre-therapy imaging was predictive of the Y absorbed dose to the normal liver to within 20%, but had poor predictability for tumours smaller than 100 cm.