A new method for segmentation of FDG PET metabolic tumour volume using the peritumoural halo layer and a 10-step colour scale. A study in patients with papillary thyroid carcinoma.

AIM

We observed a layer between tumour activity and background on FDG PET/CT with the 10-step colour scale and the window level set properly. We named the layer peritumoral halo layer (PHL). We performed this study to establish the reliability of metabolic tumor volume (MTV) segmentation using PHL (MTV(PHL)) in patients with papillary thyroid carcinoma.

PATIENTS, METHODS: Of a total of 140 papillary thyroid carcinoma (PTC) patients, 70 (50.0%) had FDG-avid PTC. In these patients, MTV(PHL), MTV segmented according to fixed 50% SUVmax (MTV(50%)), and fixed SUV with 2.5 to 4.0 (MTV(2.5) to MTV(4.0)) were compared with pathologic tumour volume (PTV). The absolute percentage difference between MTV(PHL) and PTV was compared in micropapillary carcinoma (MPTC) and non-micropapillary carcinoma (non-MPTC) subgroups. The % SUVmax and SUV thresholds of MTV(PHL) were compared with tumour SUVmax.

RESULTS

Among the MTVs, MTV(50%) was not correlated with PTV (r = -0.16, p = 0.182) and was not reliable according to the Bland-Altman plot. Although MTV(2.5), MTV(3.0), MTV(3.5), and MTV(4.0) correlated with PTV (r = 0.85, 0.86, 0.87, and 0.87, respectively; p < 0.001), these MTVs were not reliable on Bland-Altman analyses. MTV(PHL) was significantly correlated with PTV (r = 0.80, p < 0.001), and the Bland-Altman plot did not show systemic error. The MTV(PHL) was more accurate in non-MPTC than in MPTC (p < 0.001), and the absolute % difference was smaller as PTV became larger (σ = -0.65, p < 0.001). The MTV(PHL) thresholds had correlations with SUVmax (% SUVmax threshold: σ = -0.87, p < 0.001; SUV threshold: r = 0.88, p < 0.001).

CONCLUSIONS

MTV(PHL) was more reliable than MTV(%SUVmax) or MTV(SUV). The reliability of MTV(PHL) improved with larger PTVs. The threshold of the MTV(PHL) was naturally altered by PHL according to SUVmax.