Are volume-dependent parameters in positron emission tomography predictive of postoperative recurrence after resection in patients with thymic carcinoma?

Usefulness of positron-emission tomography for predicting the World Health Organization grade of thymic epithelial tumors

Background

It is often difficult to distinguish between thymoma and thymic carcinoma by preoperative radiological tests. While there have been some reports that the maximum standardized uptake value (SUV_max) in positron emission tomography‐computed tomography (PET‐CT) is useful to this end, no large‐scale analysis has been performed. We therefore analyzed the usefulness of the SUV_max and tumor size (TS) for differentiating thymic epithelial tumors.

Methods

From 2011 to 2019, 129 patients with thymic epithelial tumor who underwent PET‐CT before surgical treatment were enrolled. The relevance of the SUV_max to the World Health Organization (WHO) histological type was assessed. To reduce the impact of the TS, the ratio of the SUV_max to the TS was also investigated.

Results

A total of 99 thymoma cases and 30 thymic carcinoma cases were enrolled into the study. The SUV_max and SUV_max/TS of thymic carcinoma were significantly higher than those of thymoma (SUV_max: 7.7 ± 3.4 vs. 3.3 ± 1.3, p < 0.01; SUV_max/TS: 1.5 ± 0.7 vs. 0.6 ± 0.4, p < 0.01). Focusing on the patients with a moderate SUV_max of ≤5 (84 thymoma and 4 thymic carcinoma), the SUV_max/TS values of thymic carcinoma were still significantly higher than those of thymoma (1.6 ± 0.8 vs. 0.6 ± 0.4, p < 0.01).

Conclusions

PET‐CT might provide significant information for differentiating images of thymoma and thymic carcinoma. We experienced several cases of thymic carcinoma with a moderate SUV_max of ≤5, and SUV_max/TS was considered a useful parameter for differentiating such cases.

Diagnostic and prognostic values of 2-[18F]FDG PET/CT in resectable thymic epithelial tumour

OBJECTIVES

We aimed to evaluate the diagnostic ability for the prediction of histologic grades and prognostic values on recurrence and death of pretreatment 2-[¹⁸F]FDG PET/CT in patients with resectable thymic epithelial tumours (TETs).

METHODS

One hundred and fourteen patients with TETs who underwent pretreatment 2-[¹⁸F]FDG PET/CT between 2012 and 2018 were retrospectively evaluated. TETs were classified into three histologic subtypes: low-risk thymoma (LRT, WHO classification A/AB/B1), high-risk thymoma (HRT, B2/B3), and thymic carcinoma (TC). Area under the receiver operating characteristics curve (AUC) was used to assess the diagnostic performance of PET/CT variables (maximum standardised uptake value [SUVmax], metabolic tumour volume [MTV], total lesion glycolysis [TLG], maximum diameter). Cox proportional hazards models were built using PET/CT and clinical variables.

RESULTS

The tumours included 52 LRT, 33 HRT, and 29 TC. SUVmax showed good diagnostic ability for differentiating HRT/TC from LRT (AUC 0.84, 95% confidence interval [CI] 0.76 - 0.92) and excellent ability for differentiating TC from LRT/HRT (AUC 0.94, 95% CI 0.90 - 0.98), with significantly higher values than MTV, TLG, and maximum diameter. With an optimal cut-off value of 6.4, the sensitivity, specificity, and accuracy for differentiating TC from LRT/HRT were 69%, 96%, and 89%, respectively. In the multivariable Cox proportional hazards analyses for freedom-from-recurrence, SUVmax was an independent prognostic factor (p < 0.001), whereas MTV and TLG were not. SUVmax was a significant predictor for overall survival in conjunction with clinical stage and resection margin.

CONCLUSION

SUVmax showed excellent diagnostic performance for prediction of TC and significant prognostic value in terms of recurrence and survival.

KEY POINTS

• Maximum standardised uptake value (SUVmax) shows excellent performance in the differentiation of thymic carcinoma from low- and high-risk thymoma. • SUVmax is an independent prognostic factor for freedom-from-recurrence in the multivariable Cox proportional hazard model and a significant predictor for overall survival. • 2-[¹⁸F]FDG PET/CT can provide a useful diagnostic and prognostic imaging biomarker in conjunction with histologic classification and stage and help choose appropriate management for thymic epithelial tumours.