Wednesday Case of the Day Physics Authors: Matt Vanderhoek and Beth Harkness Henry Ford Health System, Detroit, Michigan History: A 58 year-old, female patient with liver metastases was treated via 90Y microsphere radioembolization. As per protocol, SPECT/CT imaging of the bremsstrahlung was performed 1 hour post-therapy to assess 90Y distribution. However, SPECT image quality was insufficient for accurate evaluation of 90Y distribution. Repeat SPECT/CT bremsstrahlung images were acquired 11 days post-therapy and 90Y distribution was successfully assessed. Bremsstrahlung Images PMMA Lead SPECT SPECT/CT Bremsstrahlung spectra for 90Y beta particles interacting with PMMA (similar to tissue) and lead. PMMA spectrum ranges from about 70 – 300 keV. Jodal 2009 1 hour post-tx 11 days post-tx Question The 1 hour post-therapy SPECT images exhibited inferior image quality because: 99mT (and not 90Y) was the radionuclide selected during SPECT acquisition Low energy collimators were used during SPECT acquisition rather than medium energy collimators Attenuation correction was not performed on the SPECT data SPECT imaging was acquired too early, as the 90Y distribution had not yet stabilized in the patient

Bremsstrahlung Images Good Attenuation Poor Attenuation SPECT SPECT/CT 1 hour post-tx 11 days post-tx Attenuation characteristics of low energy collimator (in color) overlaid on Bremsstrahlung spectra for 90Y beta particles. Jodal 2009 Diagnosis: Answer B is correct. Low energy collimators are unable to substantially attenuate the higher energy photons in the 90Y bremsstrahlung spectrum. This results in a loss of spatial resolution. Spatial resolution was degraded in the 1 hour post-therapy images, which were acquired with low energy collimators. Consequently, there was very little structure to the 90Y distribution. Image quality was markedly improved on the 11 day post- therapy images, which were acquired with medium energy collimators. The structure of the 90Y distribution was now evident.

99mTc Energy Window Typical 99mTc energy window (purple) overlaid on Bremsstrahlung spectra for 90Y beta particles. Jodal 2009 Discussion: Answer A is incorrect. Selecting 99mTc would typically set an energy window of 140 keV +/- 10 keV (see graph above). Photon detection would be significantly reduced since most of the spectrum would be excluded by this narrow energy window. This would result in much noisier images, which was not seen in the 1 hour post-therapy images. In addition, there is no 90Y radionuclide selection on a SPECT/CT scanner, since 90Y is a beta emitter. Rather, the user must enter an energy window appropriate for imaging the 90Y bremsstrahlung spectrum. Answer C is incorrect. Without attenuation correction, shallow liver (closer to the body surface) would have overall higher counts and deep liver would have overall lower counts on the SPECT images. This was not seen on the 1 hour post-therapy images. Answer D is incorrect. The size of the 90Y microspheres causes them to become embolized in the tumor vasculature almost immediately after injection. There is no subsequent redistribution.

References/Bibliography: Jodal, L. Beta emitters and radiation protection. Acta Oncologica 2009; 48: 308–313.