Expanding the Imaging Spectrum of Polymorphous Low-Grade Neuroepithelial Tumor of the Young in Children

DISCUSSION

To our knowledge, this is the largest series of imaging features of the rare tumor entity PLNTY in children. In our study, all tumors were in the supratentorial compartment, and most were in the temporal lobe in a cortical-subcortical location, in keeping with prior literature.^5,7,9 Kurokawa et al,⁹ in a systematic review, reported 2 main imaging patterns for PLNTY: 1) a well-circumscribed, solid, and cystic mass with scarce mass effect surrounded by normal parenchyma; and 2) an ill-defined, small mass with tiny cysts and TLS. Although the first pattern was reported as the most common in that article, in this study, the second pattern was predominant.

Prominent coarse and central calcifications have been suggested as characteristic features of PLNTY.³ However, extensive calcifications were not a common imaging feature in this series. Calcifications showed, predominately, a punctate pattern, also called “salt and pepper pattern.”¹² Despite these differences, 1 case gradually developed a sizeable central calcification during a follow-up period of 10 years, exhibiting the classic prominent central calcification. This finding suggests that there may be an evolving imaging appearance of these tumors, in which small calcifications develop in younger patients and grow or coalesce with time to become dense coarse calcifications in older patients. Due to the small number of cases, we cannot assess a correlation between the age of presentation or the duration of symptoms with the presence or development of prominent central calcifications. For example, our case 6 with a 4-year history of seizures had a prominent calcification on the initial MR imaging, while our case 10 did not demonstrate calcifications on the initial MR imaging or develop calcifications during 7 years.

Kurokawa et al⁹ associated TLS with a longer duration of symptoms and found that cortical dysplasia was more frequent in patients with TLS. Other reports have also described this association.^10,13 In our study, the duration of seizures for cases that showed TLS was variable, and pathology-proved cortical dysplasia in association with TLS was found in only 25% (1 of 4) of patients. These findings may be due to pathologic sampling confounding. It is also possible that cortical dysplasia may be seen without a clear TLS, or alternatively, not all TLSs are truly indicative of cortical dysplasia. The proximity of temporal lobe tumors to the margins of the ventricles may also cause difficulties in appreciating the TLS. A transmantle appearance may cause diagnostic confusion on imaging, with the possibility of assigning simple focal cortical dysplasia type IIb as the diagnosis, or in cases in which a mass is readily apparent, the tumor may be diagnosed as dysembryoplastic neuroepithelial tumor (DNET). However, the features of PLNTY as described here can indicate a correct diagnosis or a more targeted differential consideration of PLNTY in children. Close attention to the presence of small tumoral features on multiple planes can prevent this potential misdiagnosis.

Isolated reports describe PLNTY cases with difficult therapeutic control or disease recurrence.^11,14 Armocida et al⁸ found that contrast enhancement in PLNTY was associated with recurrence or poor control of epileptic symptoms. We found a recurrence of seizures after surgery in 3 cases; none showed postcontrast enhancement. We suspect that the presence of contrast enhancement is not necessarily the main predictor of poor seizure control.

Nearly all the PLNTYs have either BRAF p.V600E mutations or fusion events involving FGFR2/FGFR3.¹ The BRAF p.V600E mutation is the most common.⁵ We had an equal proportion of BRAF p.V600E mutations and FGFR2 fusions. There are no clear associations between genetic mutations and clinical outcomes in patients with PLNTY.⁸ We found that recurrence of symptoms after surgery occurred similarly in patients with both mutations. The FGFR3-TACC3 fusion gene in PLNTY has shown malignant transformation in some reports.¹⁵ This mutation was not found in our series.

The differential diagnosis of PLNTY in children includes DNET, ganglioglioma, and pleomorphic xanthoastrocytoma, all of which are commonly seen in the temporal lobe and can commonly present with seizures.^6,9,10 Cystic changes and calcifications have been reported to be more common in PLNTY than in gangliogliomas,¹⁶ though we do not find that this distinction helped guide a differential diagnosis. Nevertheless, larger unilocular cysts are not typical of PLNTY and were not seen in our series. Vasogenic edema is uncommon in all these tumors, and they may be seen in cortical/subcortical locations. PLNTY may show mild or no contrast enhancement, differing from the typical moderate-to-marked enhancement in parts of ganglioglioma or pleomorphic xanthoastrocytoma. Intratumoral T2 FLAIR hyperintense septations with multiple cystlike central T2 FLAIR hyperintense areas (FLAIR rim sign) are much more frequent in DNET.⁹ Both DNET and ganglioglioma may have associated cortical dysplasia on histopathology. Neurocysticercosis also has cystic areas with punctate calcifications but typically does not have a clear associated mass.