Abstract
2053
Background: Schwannomas are the most common benign peripheral nerve sheath tumor (BPNST). Differentiating benign from malignant peripheral nerve sheath tumors (MPNSTs) can be challenging given overlapping clinical and imaging features but is vital given the differing treatment and patient outcomes of BPNSTs and MPNSTs. Prior studies have shown that FDG PET is useful in differentiating BPNSTs from MPNSTs; in general, MPNSTs tend to be more FDG avid. In a previous study at our institution1, all peripheral nerve tumors with SUVmax > 8.1 were malignant, and all those with SUVmax < 4.3 were benign (both schwannomas and neurofibromas). We also found that malignant lesions were more likely to have heterogeneous FDG activity1. However, since then, we have encountered benign schwannomas with SUVmax > 8.1 and heterogeneous FDG activity, mimicking MPNSTs. The primary aim of this study was to examine the 18F-FDG PET/CT imaging features of biopsy-proven schwannomas, noting multiple characteristics, including size, presence of calcifications, SUVmax, metabolic tumor volume (MTV), and total lesion glycolysis (TLG).
Methods: This IRB-compliant retrospective study included patients from our institutional database with pathologically-proven schwannomas who underwent 18F-FDG PET/CT between 1/1/2002 and 4/1/2018. Analyzed imaging features included size, SUVmax, MTV, TLG, presence of calcification, and pattern of FDG activity. A chart review was performed for clinical and pathological correlation.
Results: There were 75 patients with a mean age of 56.8 ± 14.8 years (range 42-72 years); 38 M, 37 F. The average maximal lesion dimension was 5.0 ± 2.7 cm and average volume was 136.7 ± 268.9 cm. Seven schwannomas (9.3%) contained calcifications. The average SUVmax was 5.4 ± 2.7, MTV was 64.6 ± 114.5 cm³, TLG was 224.2 ± 428.3 cm³·g/ml, and lesion SUVmax/liver SUVmean was 2.2 ± 1.2. Thirty lesions (40%) demonstrated heterogeneous metabolic activity. Eleven schwannomas (15%) had a SUVmax > 8.1; twenty-six (35%) had a SUVmax > 6.1; and fourteen (19%) had a lesion SUVmax/liver SUVmean > 3.0. Lesions with a SUVmax > 6.1 were more likely to contain calcifications than those with lower metabolic activity (p= 0.045), and were more likely to exhibit heterogeneous FDG activity (p= 0.0004). Lesions with SUVmax > 8.1 had larger average maximal diameter (6.6 ± 2.2 vs. 4.7 ± 2.7 cm, p = 0.0286). Lesions with calcifications had larger maximal dimension than those without (8.8 ± 3.3 cm vs. 4.6 ± 2.4 cm, p < 0.0001), greater volume (501.5 ± 554.5 cm3 vs. 99.2 ± 192.2 cm3, p < 0.0001), and higher SUVmax (7.4 ± 1.0 vs. 5.2 ± 0.3, p = 0.0334). Lesions with heterogeneous FDG activity had higher average SUVmax/liver SUVmean (2.7 ± 0.97 vs. 1.9 ± 1.2, p= 0.0035), and had a higher SUVmax (6.5 ± 0.5 vs. 4.7 ± 0.4, p= 0.0031). There was a trend toward significance of lesions with SUVmax > 8.1 to be more likely associated with numbness and tingling (p = 0.07) and a palpable mass (p = 0.0622), but no significant association with pain (p = 0.228) or weakness (p = 0.1872). Conclusion: Benign schwannomas may be large and demonstrate intense heterogeneous FDG activity, mimicking MPNSTs on ¹⁸F-FDG PET/CT. A significant proportion exhibit FDG activity above levels previously thought to be useful in differentiating BPNSTs from MPNSTs. Those interpreting PET/CT should be aware of these highly metabolic variants, and have low threshold to recommend biopsy in these cases. Reference:1Broski, S.M., Johnson, G.B., Howe, B.M., et al. (2016). Evaluation of (18)F-FDG PET and MRI in differentiating benign and malignant peripheral nerve sheath tumors. Skeletal Radiology, 45(8), 1097-1105.
