SUV-volume Histogram as tool for MRgFUS bone metastasis treatment monitoring

Aim: Aim of this study was to evaluate the Cumulative SUV-volume Histogram as a tool for Magnetic Resonance-guided Focused UltraSound (MRgFUS) treatment monitoring. MRgFUS is an innovative method for metastasis pain palliation and for the related cancerous tissue ablation (in the regions directly accessible to the ultrasound beam). Methods: A patient with pelvic bone metastasis, accessible to the ultrasound beam, was studied by 18F-FDG PET/CT before (I), at 3 (II) and at 6 (III) months after the MRgFUS treatment. At the three different times, the therapy outcome was assessed by: 1) patient interview to obtain the pain perception evaluation (Visual Analog Scale), ranging from 0 to 10, 2) qualitative observation of 18F-FDG PET/CT images, and 3) the target uptake Radiotracer distribution changes. This evaluation was done by using the Cumulative SUV-volume Histograms (CSH) distribution. Similarly to the Dose-Volume Histogram (DVH) distribution in Radiotherapy, the CSH represents the % of tumor metabolic volume that is higher than a SUV threshold, variable from 0 to the maximum SUV value [Van Velden et al, Eur J Nucl Med Mol Imaging,2011 vol.38(9):1636-47]. According to the literature, SUV threshold was set at 50% of maximum SUV and the area under the CSH curve was considered as a quantitative index of the heterogeneity in the FDG uptake inside the lesion volume. The partial volume effect was not considered because the lesion sphere-equivalent diameter was greater than 5 cm. In order to evaluate the ablation effect, the percentage change of CSH was obtained as follows: ?CSH(post vs pre) = 100*(CSHpost - CSHpre)/CSHpre. This value was compared with the patient interview results and with the Nuclear Medicine qualitative assessment. Results: The patient reported a quality-of-life marked improvement, with a pain reduction from 9 (I) to 2 (in II and III). CSH changes in I, II, and III times were observed: ?CSH(II vs I) = -32.7%, ?CSH(III vs II) = -20.4% and ?CSH(III vs I) = -46.4%. These CSH decreases were in agreement with qualitative observation of PET/CT images. Conclusion: In the considered patient, the treatment response along the follow-up stages indicated that MRgFUS therapy was effective in the reduction of the bone metastasis pain and in the ablation of accessible cancerous tissue. CSH showed a marked response in the regions treated with the ultrasound beam and seems to be a useful tool for evaluating the radiotracer uptake heterogeneity in the oncological lesions.