Comparison of 68Ga-PSMA SUVmax and tumour heterogeneity in DW-MRI

Introduction: 68Ga-PSMA is a labelled ligand to a membrane protein overexpressed in many prostate cancer cells and has raised its interest to be highly specific in PET imaging for detection and staging of prostate cancers. DW-MRI is a standard technique that provides quantitative information about tumor cellularity and tissue structure is a useful tool for the detection and staging of prostate cancer in clinical practice. This study investigates the correlation between SUVmax and tumour heterogeneity computed from DWI sequences. Materials and Methods: 17 patients (age range, 49-75 years; mean, 64.3 years) of a prospective study aiming at multi-cohort investigation for clinical impact of mp-3TMRI and 68Ga-PSMA PET/CT in diagnosis of clinically-significant prostate cancer and presurgical staging. After co-registering MRI and PET sequences, Regions of Interest (ROIs) were manually delineated around prostate in MRI/T2 and the cancer lesion in the dominant sequence (high values of DWI or T2). SUVmax was computed on PET images and 60%-threshold of SUVmax was used for lesion contouring. A compound texture feature related to tumour heterogeneity (Hc) was computed within prostate ROIs in DWI and the outcome represented with colorimetric maps. The highest regions were extracted by automatic contouring. Average Hc (DWI) and SUVmax (PET) were computed on the co-registered slices and the Spearman rank correlation (ρ) was assessed. Results: SUVmax ranges from [1.6-101.0] and Hc through [37.9 -50.6]. Regions highlighted by Hc are usually much greater than those detected by SUVmax thresholding and always include the region outlined by radiologists. 13 patients show ρ≥0.7, 3 with ρ≥0.5 and 1 only with ρ≥0.25. For 11 patients (65% of cases) the amount of PET slices pointing out malignant SUV peaks is greater than DWI slices showing high Hc values, with Hc ROIs better depicting tumour extent. For 3 patients (17.5%) PET and Hc-DWI show equivalent performance in tumour detection and in 3 cases only, Hc points out more slices with suspect tumour ROIs. However, in all cases SUV and Hc show the same trend, with the highest values around central slices and a much greater increasing rate for SUV. Conclusions: The outcomes of this study reveal a rank correlation between heterogeneity of cellularization and the expression of the membrane receptor of PSMA. The reason of such correlation requires deeper investigations and could be confirmed after examining a wider cohort. Hc result a promising compound feature for outlining malignant ROIs extent to be employed together with PET sequences for validating tumour side. A deeper analysis is required for those few cases showing weak or even poor correlation between SUV and Hc.