Objective To investigate the ¹⁸F-fluorodeoxyglucose (FDG) PET/CT imaging characteristics of pulmonary cryptococcosis (PC).

Methods A retrospective study was performed in 22 patients with PC (17 male and 5 female; aged 54.77±7.93 years), confirmed through etiology or pathological examination from January 2011 to January 2017 in Fujian Provincial Cancer Hospital. ¹⁸F-FDG PET/CT was performed. The number, size, distribution, maximum standardized uptake value (SUV_max), and nodule sign of the nodules were analyzed. T test or Pearson correlation analysis was used in the comparison in measurement data. The differences in nodule signs among the different groups were determined by χ² test, continuous correction χ² test, or Fisher exact probability method.

Results ① A total of 235 nodules were found by ¹⁸F-FDG PET/CT imaging in the 22 cases of PC patients with the diameter of (0.7±0.5) cm. Of these nodules, 130 showed high ¹⁸F-FDG metabolism with an SUV_max of (3.5±2.9) and a diameter of (0.9±0.5) cm. A positive correlation was found between the two (r=0.702, P=0.000). Meanwhile, 105 nodules showed no ¹⁸F-FDG metabolism with a diameter of (0.3±0.1) cm. The diameter in the group with high ¹⁸F-FDG metabolism was greater than that in the group without ¹⁸F-FDG metabolism, and the difference was statistically significant (t=13.621, P=0.000). ② The nodules were mainly found in the right lung (57.4%, 135/235), lower lobe (64.3%, 151/235), and outer zone or subpleura (80.0%, 188/235). ③ The SUV_max in the nine cases of immunocompromised host with high ¹⁸F-FDG metabolism was 5.7±4.7 higher than that in the 13 cases of non-immunocompromised host (SUV_max was 3.0±2.0), and the difference was statistically significant (t=2.731, P=0.011). The ratio of wide base posted to the pleural sign was 17.5 (10/57) of the former and higher the 6.2% (11/178) of the latter. The ratio of halo sign was 25.3% (45/178) in the latter and higher than the former of 10.5% (6/57). All the differences were statistically significant (χ²=7.911, 4.628; P=0.005, 0.031). ④ The nodules of the group with high ¹⁸F-FDG metabolism accounted for 80% (16/20) and 53% (114/215), respectively, in the each of 11 cases of one lobe or multiple lobar involvement patients. The former SUV_max was 5.6±3.4 and higher than the latter of 3.2±2.7. The difference was statistically significant (t=2.652, P=0.016). The lobulation sign (35%, 7/20), spicule sign (30%, 6/20), and pleural indentation sign (15%, 3/20) in the former were higher than the 3.3% (7/215), 0.9% (2/215) and 0.9% (2/215) in the latter, respectively. The difference was statistically significant (χ²=32.911, 47.022, 17.395, all P<0.01). ⑤ The SUV_max of the 21 nodules with high ¹⁸F-FDG metabolism in the 44 nodules found in the 11 misdiagnosed cases was 5.0±4.6, which was higher than 3.2±2.4 in the 11 other correctly diagnosed cases where 191 nodules were found, including 109 nodules with high ¹⁸F-FDG metabolism. The difference was not statistically significant (t=16.825, P=0.106). The lobulation sign, spicule sign and pleural indentation sign in the former were 15.9% (7/44), 18.2% (8/44), 6.8% (3/44) respectively, which are higher than the 3.7% (7/191), 0.0% (0/191), and 1.0% (2/191) in the latter. The difference was statistically significant (χ²=9.570, 35.951, 5.720; all P<0.05). The halo sign was 26.2% (50/191) in the latter, and higher than the 2.3% (1/44) in the former. The difference was statistically significant (χ²=12.027, P=0.000).

Conclusions The SUV_max of the nodules is positively correlated with diameter in cases of PC by ¹⁸F-FDG PET/CT imaging. Halo sign is a reliable sign of diagnosis. The resembling tumor signs and the high ¹⁸F-FDG metabolism, caused the ¹⁸F-FDG PET/CT imaging to misdiagnose easily in patients of PC with single lobe involvment.