## 4 Discussion
In detail, we observed 11 solid tumors classified as typical, while the other nine tumors showed unusual MRI features and were classified as atypical. The majority (73%) of typical pheos (8/11) showed a more or less homogeneous hyperintensity to the spleen on T2-WI images, while the remaining three lesions showed the bright signal intensity, isointense to the cerebrospinal fluid (CSF), on T2-WI images confirming the low occurrence (27%) of the CSF lightbulb bright signal intensity as previously reported in literature [11; 15; 22]. All the typical pheos showed no signal loss on T1-WI out-of-phase CS sequence and significant contrast enhancement on T1-WI FS post-contrast DCE sequence. Conversely, the majority (78%) of atypical pheos (7/9) were cystic suggesting that cystic changes represent the most frequent tissue degeneration in such tumors; of note, five of these tumors were totally or predominantly cystic surrounded by residual viable tissue in peripheral borders or the remnant solid component, respectively; only two lesions appeared as partially cystic with contextual areas of solid tissue. The remaining two atypical pheos showed areas of high signal intensity on T1-WI images suggesting hemorrhagic degeneration. All atypical pheos showed no signal loss on T1-WI out-of-phase CS sequence confirming the absence of intracytoplasmic lipid content, as well as showed heterogenous enhancement on T1-WI FS post-contrast DCE sequence according to degrees of degeneration. In our series, the results of radionuclide imaging studies showed that in typical pheos MIBG concentration was observed in the majority (75%) of cases as focal increased uptake allowing tumor characterization and diagnosis. However, in the two remaining cases the result of MIBG scan was false negative since tracer uptake was not observed in the tumor masses of which one was necrotic and the other of small size. These latter findings are not surprising since false negative MIBG cases may occur [7; 23]. FDG scan was available only in two histologically proven large (>5 cm) pheos in which was performed for the clinical suspicion of malignancy; in these lesions FDG was heterogeneously increased supporting the solid tumor structure. On the other hand, the results of nuclear medicine imaging scans in atypical pheos were peculiar for tumor characterization. In particular, in these lesions the results of MRI were not diagnostic for pheo since the specific imaging criteria suggestive for this tumor-type were not present. Conversely, the majority (78%)of these tumors were cystic (totally, predominantly or partially) or hemorrhagic on MRI. In these atypical pheos the concentration of the used radiotracers, both of MIBG and/or FDG, was able to characterize the residual solid tissue or the peripheral rim of these lesions allowing tumor diagnosis. This finding occurred in four lesions (one totally cystic, one predominantly cystic, one partially cystic and one hemorrhagic) using MIBG, even though in the remaining lesion no abnormal residual tracer uptake was found. Similarly, this finding occurred in eight lesions (two totally cystic, two predominantly cystic, two partially cystic and two hemorrhagic) using FDG. However, according to the specific mechanisms of MIBG and FDG tissue concentration, residual MIBG uptake allows the characterization and tumor diagnosis of pheos, while residual FDG accumulation generically reflects the presence of viable solid component of such tumor lesions. Similar findings have been previously reported either with iodine-123 MIBG single photon emission tomography (SPECT) integrated with CT [15; 16; 17; 21] or with FDG hybrid imaging [13]. Therefore, according to our experience the radionuclide imaging features of such atypical pheos should be rationalized in clinical practice. In particular, when a large cystic adrenal mass may be suspected for pheo, imaging characterization is required. For this purpose, MIBG should be preferred to FDG as radiotracer for nuclear scanning.