An unusual case of fatty posterior mediastinal ganglioneuroma

Ganglioneuromas, which arise from neural crest cells, are typically seen in adolescent and young adults. We describe an unusual case of posterior mediastinal ganglioneuroma with a large fatty component in a middle-aged male. This imaging feature has only been reported in five published manuscripts in the English literature.


INVESTIGATION
The scout film obtained during the CT scan of the chest showed a lesion that obscured the normally seen left supralateral contour of the aortic arch ( Figure 1). An unenhanced CT scan of the chest demonstrated a wellcircumscribed left paraspinal mass measuring 3.3 Â 5.6 Â 9.2 cm in the transverse, anterioposterior and craniocaudal diemnsions respectively, abutting the descending thoracic aorta and the posterior left fifth through seventh ribs ( Figure 2). The mass had heterogeneous attenuation. The relative density of the central portion of the mass was consistent with that of fat. A few punctate foci of calcifications were present within the peripheral soft tissue component. A pre-and postcontrast MRI of the thoracic spine showed a mass abutting the posterior surface of the descending thoracic aorta in the left paravertebral groove, extending from T 4 to T 7 without expansion of or extension into the neural foramina. The inherent T 1 shortening of the central portion of the lesion was suppressed with fat suppression techniques, confirming the central fatty component (Figure 3). The peripheral portion demonstrated mainly intermediate-to-low signal intensity on T 1 weighted images and intermediate signal intensity on T 2 weighted images. The peripheral soft tissue components of the mass showed heterogeneous enhancement after the administration of intravenous gadolinium-based contrast. There was no evidence of bony erosion, reactive oedema or remodelling in either CT or MRI scan ( Figure 4).

DIFFERENTIAL DIAGNOSIS
The well-defined margins and absence of aggressive features narrowed the differential diagnosis to entities such as ganglioneuroma, schwannoma, angiolipoma and lowgrade liposarcoma.

TREATMENT
Given the large size of the lesion and the presence of associated chest wall pain, a robotic-assisted thoracoscopy and excision of the mass was performed. After the pleura was incised, a predominantly fatty consistency lesion became apparent. The mass was dissected off the chest wall and completely resected. No difficulties were reported by the surgeon in resecting the mass. OUTCOME AND FOLLOW-UP Histological sections of the 36 g mass (2.5 Â 7.1 Â 7.4 cm) revealed a pseudoencapsulated subpleural heterogeneous lesion. Based on our experience as well as prior reports, the degree of the overestimation of the size of the mass by MRI scan as compared with the measurement of the resected mass stated in the pathology report is customary. 1 The lesion was composed of neural and fibrous nodules with focal mucoid areas and clusters of ganglion cells intermixed with mature adipose tissue ( Figure 5). The neural tissue occasionally surrounded the mature fat in a nodular fashion ( Figure 5b). Numerous clusters of ganglion cells were seen in the background of nerve fibres as highlighted by immunohistochemical stain S100 (Figure 5e).
The post-operative course was uneventful and the patient left the hospital on the next day after the procedure. At a six month follow-up, the patient was doing well, except for hyperesthesia at the operative site.

DISCUSSION
Ganglioneuromas are slow growing tumours of autonomic ganglia. They are typically asymptomatic and often an incidental finding. Clinical manifestations are usually secondary to the location of the neoplasm. Ganglioneuromas most commonly occur in the posterior mediastinum (60-80%); other sites include the retroperitoneum and less commonly the adrenal medulla. 2,3 Ganglioneuromas are responsible for up to 35% of the intrathoracic neurogenic tumours. 4,5 Posterior mediastinal ganglioneuromas with fatty components as described here are rare. A review of the English literature revealed only five reported cases (Table 1). [6][7][8][9][10] In addition to these case reports, two different retrospective studies described four 11 and two 12 cases of mediastinal and thoracic ganglioneuroma, respectively, containing variable amounts of fatty tissue. Two potential aetiologies have been proposed to explain the presence of fatty component in this type of ganglioneuromas. One theory suggests that the fatty component arises from involvement of paravertebral fat. 11 Alternatively, ganglioneuroma may undergo fatty degeneration. The latter explanation can  also account for the older mean age of patients presenting with fat containing ganglioneuromas. The average age of these patients is typically reported to be in the mid-forties, which is higher than the typical age for ganglioneuromas. 4,6,8,10,12 Limited reported cases of lipomatous ganglioneuromas make generalisations about the imaging findings of these rarely reported tumours difficult. Furthermore, excluding low-grade liposarcoma from the differential diagnosis of mediastinal fatty  mass without aggressive features can also be challenging. Independent of the fatty components, several cases of the gangalioneuroma reported whorled appearance on both T 1 and T 2 weighted images, a feature which we also observed. 13 Punctate calcifications, as seen in our case, have also been reported with these types of tumours. 14 The oblong shape of this mass with craniocaudal orientation (Figure 2) is another potential clue to the diagnosis and the benign nature of these tumours. This craniocaudal orientation was observed in three of the other reported cases. 6,8,9 Radiological-pathological comparisons by Forsythe et al 2 demonstrates that the degree and heterogeneity of enhancement corresponds to the proportion of components such as myxoid stroma, cellular components and collagen fibres.
The case we describe here, along with the other case reports, justify the inclusion of ganglioneuromas in the differential diagnosis of posterior mediastinal masses with fatty component. Features such as the craniocauadal orientation, punctate calcification and whorled appearance should further narrow the differential consideration to this entity. Continued study and reporting of additional lipomatous ganglioneuromas may help further characterize lipomatous ganglioneuromas and guide treatment plans.

TREATMENT AND PROGNOSIS
The prognosis for ganglioneuromas is favourable. Surgical removal is the treatment of the choice, as the diagnosis of ganglioneuroma cannot be ascertained before the removal of the mass. Although rare, spontaneous development of malignant peripheral sheath tumours in a benign ganglioneuroma has been reported. 15,16 Figure 5. (a) Low power view of a solid nodule under the pleura (black arrow), surrounded by mature adipose tissue and a pseudo capsule (arrow head) (H&E , magnification Â20). (b) Neural bundles (arrows) encasing mature adipose tissue (H&E, magnification Â20). (c) Perivascular mucoid hypocellular bluish material (arrows) was seen within the solid areas adjacent to large mature looking neurons with abundant eosinophilic cytoplasm and eccentrically located nucleus consistent with ganglion cells (arrowheads) (H&E, magnification Â200; insert Â400). (d) Clusters of ganglion cells (arrows) in neural background (H&E, magnification Â400). (e) S100 immunohistochemical stain highlights the presence of ganglion cells (arrow) and neural tissue in the background (arrow heads) (H&E, magnification Â400). H&E, hematoxylin and eosine. LEARNING POINTS 1. Ganlioneuromas should be included in the differential diagnosis of fat containing posterior mediastinal masses. 2. Craniocaudal orientation, intrinsic whorled appearance and punctate calcification should favour the diagnosis of ganglioneuroma. 3. The ganglioneuromas with fat typically present in middle-aged adults, a mean age that is older than the typical age of presentation for more common forms of ganglioneuroma.
CONSENT Written informed consent for the case to be published (incl. images, case history and data) was obtained from the patient for publication of this case report, including accompanying images.