The prognostic value of a novel irregularity measure of FDG uptake in sarcomas

Ziel/Aim:

In sarcoma patients it is important to estimate the prognosis at the time of diagnosis, particularly to define patients at high risk. FDG PET is widely used for this purpose. Heterogeneity of FDG uptake in the tumor has been shown to be a useful characteristic for grading and prediction of outcome in patients with various sarcoma types, independent of the SUV. Here we propose and evaluate a novel measure for the irregularity of the tumor`s shape in the FDG image with respect to its predictive power in sarcoma patients.

Methodik/Methods:

The retrospective analysis included 56 sarcoma patients with different histological background: 17 patients with osteosarcoma (OS), 22 with Ewing sarcoma (EW) and 17 with various different sarcoma types (VS). Whole-body FDG-PET had been performed prior to therapy in all patients. The tumors were segmented fully automatically using the ROVER 3D segmentation tool which is based on thresholding at the volume-reproducible intensity threshold after subtraction of the local background. SUVmax, SUVmean were determined in addition to the novel measure of spatial irregularity (IRREG) of the primary tumor. IRREG is defined as the deviation of the tumor's shape from sphere symmetry, which can be computed as the third power of the tumor's surface devided by the second power of its volume. IRREG is normalized to 1 for a spherical lesion. Kaplan-Meier curves were obtained for IRREG, SUVmax and SUVmean with respect to both overall (OAS) and progression-free survival (PFS). Survival and PFS curves were separated by the best discrimination threshold according to ROC analysis and then compared by log-rank tests.

Ergebnisse/Results:

Median follow-up in all patients was 37.2 months. Sixteen patients died, 24 further patients experienced tumor progression. Median PFS was 29.5 months. SUVmax / SUVmean were higher in OS (median 12.5/4.9) than in EW (7.5/3.6, Wilcoxon p=0.002/0.096, Holm-adjusted) and VS (7.8/3.5, p=0.001/0.066). Median IRREG was smaller in OS (1.7) than in EW (2.9, p=0.002), it was intermediate in VS (2.3). Kaplan-Meier analysis revealed prognostic power of IRREG with respect to OAS in all patient groups with highest statistical significance in OS (p=0.0001, 0.049 and 0.031 in OS, EW and VS, respectively). Higher pretherapeutic IRREG was associated with shorter OAS, i.e. high risk, in OS, whereas it was associated with low risk in EW and VS. SUVmax and SUVmean were not predictive for OAS in OS and VS, but in EW only (high risk at SUVmax≤7.0, p=0.012, and SUVmean≤3.3, p=0.004). In OS, IRREG provided prognostic power also with respect to PFS (p=0.015). SUVmean was predictive for PFS in both OS (p=0.016) and EW (p=0.019), SUVmax in EW only (p=0.019).

Schlussfolgerungen/Conclusions:

The novel irregularity measure is promising for the identification of high risk sarcomas. It appears to provide better prognostic power than SUV, particularly in osteosarcomas.