## 4 Discussion  
Pheos consist of catecholamine-secreting tumor lesions originating from chromaffin cells in the medullary of adrenals [18; 19]. Tumor diagnosis depends on imaging detection of an adrenal lesion associated with specific clinical symptoms and laboratory demonstration of increased catecholamine production. In the majority of cases, pheos show typical imaging features as solid, hyper-vascular tumors with increased CT density (>10 UH) and significantly high signal intensity on T2-WI MRI [2; 14; 20]; furthermore, pheos on nuclear imaging studies are mainly reflected by increased tracer uptake using MIBG and/or \({}^{18}\)F-dihydroxyphenylalanine (DOPA) as well as also by \({}^{18}\)FDG [4; 5; 6; 7]. Of note, the role of \({}^{18}\)F FDG is limited in pheos since this radiocompound is unspecific for this tumor type; however, \({}^{18}\)F FDG is able to reflect the residual viable/solid tissue in case of lesion degeneration; in fact, pheos usually undergo different types of lesion degeneration such as necrosis, calcification, fibrosis, cystic or intracellular lipid changes, misleading the accurate  
Figure 2: Right adrenal partially cystic pheochromocytoma. MR coronal images represented by T1-WI (**A**) and T2-WI (**B**) show a partially cystic adrenal mass measuring 5 cm (arrows). Planar adrenal scintigraphy with \({}^{131}\)MIBG in anterior view at 48 h after tracer injection (**C**) shows inhomogeneous tracer uptake in the adrenal lesion (arrow). \({}^{18}\)FDG PET-MRI coronal image shows a heterogeneously hypointense adrenal mass with regular margins (arrow) on enhanced MRI scan (**D**); hybrid fused PET-MRI image (**E**) shows heterogeneous abnormal FDG accumulation in the residual solid component of the adrenal mass (arrows), as also confirmed by the extracted FDG image (**F**). tumor diagnosis and thus the epithet of \"an imaging chameleon\" has been formulated in its atypical appearances [2]. Furthermore, SR may be overexpressed in pheo so that this feature may be detected with SR nuclear imaging; originally, indium-111 pentetreotide has been used to image SR positive pheos [8; 9]; recently, gallium-68 DOTA peptides, as DOTATOC, DOTATATE and DOTANOC, have been proposed as PET/CT radiotracer to detect SR in pheo showing high diagnostic accuracy [10]. However, atypical pheo have been described with different imaging characteristics compared to the typical appearance of such tumor [2; 11; 12; 13; 14; 15; 16; 17; 18; 19]. In particular, these atypical lesions may be misdiagnosed since occur more frequently in asymptomatic patients without biochemical abnormalities compared to typical solid tumors [18]. MRI signal intensity heterogeneity has been reported reflecting tumor changes in atypical pheos [11]; of note, tumor cystic degeneration has been classified as total, predominant or partial with different imaging findings [15]. In addition, MIBG scintigraphy has been described to be useful in this type of pheo in some case reports showing increased tracer uptake confined only to the solid viable peripheral portion of tumors [15; 16; 17]. Similarly, MIBG uptake has been reported in a case of cystic malignant pheo with liver infiltration demonstrating also the efficacy of iodine-131 MIBG therapy in such patient who had excellent clinical and imaging responses to the radioactive treatment [21]. In the present preliminary study, we investigated the role of planar/hybrid nuclear imaging using radiolabeled MIBG and/or FDG agents to characterize atypical pheos systematically correlating radionuclide and MRI images. In our experience the incidence of pheos in a series of adrenal tumors observed in a period of twelve years was 25% (n = 18/73 patients); in particular, in such cases a total of 20 pheos were summed since three tumor lesions were detected in the same patient. An interesting imaging finding was that almost the half of pheos consisted of atypical tumor lesions on the basis of MRI appearances.