Entropy of tumor FDG uptake measured in-vivo from PET images reflects the tumor entropy seen in autoradiography

Objectives There is an increasing interest for Texture Indices (TI) computed from PET images to predict treatment response or survival in some types of cancer. Yet, whether TI reflect the microscopic heterogeneity of the tracer distribution has not been investigated. We studied whether PET-derived TI correlated with the corresponding TI measured on autoradiographic (AR) images.

Methods 3 mice bearing orthotopically implanted mammary tumors derived from transgenic MMTV-PyMT mouse were scanned with the Inveon PET/CT, 60 min post-injection of 7.4 MBq FDG. The mice were immediately sacrificed and tumors were frozen, sliced (20 µm slice thickness) and used for AR. The AR volume was resampled to the PET voxel size (388x388x796 µm), smoothed to match the 1.6 mm PET image resolution, and registered to the PET volume. The registration parameters were also used to register the resampled AR volume (no smoothing) and the original AR volume (no resampling, no smoothing) to PET. 32 AR slices were selected (~10 per tumor with a 388 µm gap between selected slices) and the corresponding PET slices were identified. In each slice, the tumor region was automatically delineated and 6 TI were calculated both from the PET and AR slices. To characterize the consistency between in-vivo and ex-vivo TI, a contingency table was calculated for each TI and the relative risk (RR) was deduced.

Results All TI were significantly correlated between smoothed, resampled AR slices and PET slices, with RR varying from 2.2 (LRE) to 15 (HGZE). The correlation with PET TI remained significant without smoothing the resampled AR volume for 4 TI with RR from 2.2 (SRE, HGZE) to 7 (Entropy). Comparing the TI between the PET data and the original registered AR volume, the correlation was significant for Entropy only (p<0.01, RR=4.3).

Conclusions In spite of the modest spatial resolution in PET and change of scale between PET and AR, tumor texture as measured with Entropy on PET images accurately reflects the entropy of the uptake directly measured on AR images.