18-Fluorine fluorodeoxyglucose positron emission tomography in the pretreatment evaluation of thymic epithelial neoplasms: a metabolic biopsy confirmed by Ki-67 expression

To investigate the usefulness of 18-fluorine fluorodeoxyglucose (F-18-FDG) positron emission tomography/computed tomography (PET-CT) in the pretreatment evaluation of thymic epithelial neoplasms (TENs). We previously demonstrated that the ratio between standardized uptake value of the tumour and aortic arch (SUV T/M) correlates with World Health Organization (WHO) classification. We now focused our evaluation on thymomas only, excluding carcinomas. We also searched for the expression of a pathological biomarker, Ki-67, that gained both diagnostic and prognostic relevance for various solid tumours. Its correlation with SUV T/M and WHO classification was evaluated. We performed a retrospective dynamic cohort study of data from January 2006 to December 2012, on 23 consecutive patients with pathologically proven TEN, excluding thymic carcinomas, evaluated with PET-CT. For each patient, SUV T/M was calculated. The patients were then categorized, according to WHO classification, into two groups (low-risk: 3 A, 9 AB, 5 B1; high-risk: 5 B2, 1 B3) and Ki-67 labelling index (LI) was defined. We employed the Spearman rank non-linear correlation coefficient (rho) to estimate the correlations between variables. SUV T/M proved to be significantly higher for high-positive Ki-67 samples, indicating a strong correlation between SUV T/M and Ki-67 LI (rho = 0.8). Furthermore, high Ki-67 LI samples correlate with the higher-risk WHO subgroup (rho = 0.9). FDG PET-CT can provide a useful tool in the preoperative work-up of TEN, reflecting its proliferation capacity, as described also by the Ki-67 expression. In particular, SUV T/M could provide a 'metabolic biopsy' to divide TEN into high-risk and low-risk neoplasms.