Morphological and Molecular Characterization of Infant-Type Hemispheric Glioma

Summary

Objectives:

To characterize the clinical, histopathological, and molecular features of infant-type hemispheric glioma (IHG) and explore any overlaps it might have with other types of pediatric gliomas.

Methods:

Data from the Children's Brain Tumor Network (CBTN) were analyzed (last accessed in January 2025), with a focus on patients aged 0–4 years with gliomas, glioneuronal tumors, or ependymomas. This analysis encompassed the clinical, histological, and molecular profiles of the cases, including those with recurrent tumors. The study scrutinized the tumor circumscription, grading features, and molecular alterations, along with the Methylation Class, using methylation profiling via the Heidelberg v12.5 and v12.8 classifiers.

Results:

IHG is predominantly well-circumscribed, but recurrence may lead to infiltrative growth. Novel findings include the acquisition of additional genetic alterations in recurrent IHG, which alters prognosis. One case exhibited progression to a pleomorphic xanthoastrocytoma-like profile, while another acquired glioblastoma-like infiltrative pattern and TP53 mutations, associated with a more aggressive course. Distinguishing IHG from desmoplastic infantile ganglioglioma/astrocytoma (DIG/A) and Pediatric High-Grade Gliomas (pHGG) remains challenging, particularly in the absence of molecular data.

Conclusion:

This study emphasizes the heterogeneity of IHG and underscores the necessity for molecular surveillance in recurrent instances. The findings aid in refining diagnostic criteria and understanding the prognostic implications of genetic alterations in IHG.

Introduction

Infant-type Hemispheric Glioma (IHG) was initially introduced as a distinct Methylation Class (MC) of tumors, originally termed infantile hemispheric glioma [1]. Further studies have since characterized IHG as a unique pediatric glioma subtype. Unlike other high-grade gliomas, it is commonly driven by Receptor Tyrosine Kinase (RTK) gene fusions, involving ALK, ROS1, NTRK, and MET. Contrary to pediatric glioblastoma, IHG generally exhibits a better prognosis [2,3]. Before IHG was identified as a separate entity, many cases were misdiagnosed as glioblastoma [central nervous system (CNS) World Health Organization (WHO) grade 4]. By contrast, others were diagnosed as desmoplastic infantile ganglioglioma/desmoplastic infantile astrocytoma (DIG/A, CNS WHO grade 1), ependymoma, embryonal tumor, or anaplastic ganglioglioma [3,4]. The differentiation of IHG from its look-alikes remains a diagnostic challenge due to overlapping histological features, especially in centers without access to advanced molecular testing. The definitive diagnosis of IHG relies heavily on extensive molecular profiling, which includes next-generation sequencing (NGS) for RNA and DNA to detect RTK fusions, and DNA methylation analysis to confirm the IHG MC [5]. Given the rarity of IHG and its mimickers, literature on their long-term development remains scarce. This study aims to enhance the histological and molecular characterization of IHG, distinguish it from histological mimics using an integrated morphological and molecular method, and offer insights into tumor progression and longitudinal changes in recurrent IHG.

Methods

This study received ethical approval through Children's Brain Tumor Network (CBTN) institutional review process (Project ID: CBTN D0155) to access and analyze these de-identified data. Informed consent was obtained from all participants or their legal representatives in line with the CBTN protocol. The analysis of the online data was carried out in Saudi Arabia. All methods strictly followed relevant guidelines and regulations.

Data were accessed from the CBTN online dataset, which included 1,029 tumor samples (https://cavatica.sbgenomics.com/). The dataset was most recently accessed on January 16, 2025. Initially, tumors from patients aged 0–4 years were selected, resulting in a preliminary cohort of 357 samples. This cohort was further refined to include gliomas, glioneuronal tumors, and ependymomas of all grades, thereby yielding a final subset of 191 samples. Methylation profiling was conducted by uploading the appropriate data to the Molecular Neuropathology website (https://www.molecularneuropathology.org/mnp). Extracted reports included MC, calibration score (CS), and copy number alterations (CNAs). All cases were first analyzed using the v12.5 Heidelberg brain tumor methylation classifier, with selected instances later reassessed using the v12.8 classifier.

The inclusion criteria are as follows: (1) Patients must be aged between 0–4 years. (2) Patients must have a histopathological diagnosis of glial, glioneuronal tumors, or ependymoma. (3) Patients must have a MC of IHG, DIG/A, or other high-grade gliomas. The exclusion criteria were: (1) absence of pathological report; (2) lack of representative scanned histological images; or (3) incomplete or insufficient molecular data preventing reliable classification. The morphological assessments focused on tumor circumscription and histological features relevant to glioma grading. These included mitotic activity, necrosis, and microvascular proliferation. It also considered morphological indicators of glial differentiation, such as a fibrillary background, gemistocytic-like cells (characterized by a large eosinophilic cytoplasm), and perivascular ependymal-like differentiation. The clinical and molecular data accounted for the overall and progression-free survival, genetic alterations, and copy number variations in both primary and recurrent tumors. A total of ten patients met the inclusion criteria and were categorized into three groups: (1) Six cases with IHG MC and a CS of 0.99, (2) One case involved another pediatric high-grade glioma, and (3) Three cases were initially diagnosed as DIG/A but were subsequently classified under different molecular categories.

Results

The clinical, morphological, NGS, and methylation profile data for all cases are summarized in Table 1. The first group consists of 6 cases, each initially diagnosed as high-grade astrocytomas or glioblastomas. In both versions of the methylation classifier, MC aligns with IHG, boasting a CS of 0.99. All patients were infants under one year old, with an equal distribution of sexes. Detected gene fusions included ROS1 (in 4 cases), ALK, and NTRK1, thus reinforcing the diagnosis of IHG. Scanned pathological slides were accessible for two cases, while detailed pathological reports were secured for 4 cases. This information was generally enough to assess relevant features, excluding Case 2, where the growth pattern could not be evaluated from the present slide due to the absence of the tumor border with brain parenchyma. Despite the pathological diagnosis of glioblastoma, high-grade features were not visible on the available slide, suggesting such characteristics were observed on other inaccessible slides for review. Tumor circumscription was gauged in four events. Case 1 demonstrated a well-demarcated tumor from the adjacent cortex, alongside high-grade features (Fig. 1A-C). The scanned slide for Case 2 displayed a solid growth pattern (Fig. 1D). For the remaining two cases, the pathological reports vividly described a well-demarcated, non-invasive growth pattern. Significant mitotic activity was noted in all five cases, with necrosis (pseudopalisading) identified in four out of five cases. A low-grade component was identified in Cases 5 and 6.

Table 1.

Clinical and pathological features of all cases.

No	A(Y)	Se	L	IHD	MC v12.5 & (CS)	MC v12.8 (CS)	RNA Fusion	NGS (DNA)	PFS	OS	VS	C	LG	Mi	Ne	MVP
1	0.7	M	TL	HGA	IHG (0.99)	IHG (0.99)	ZCCHC8::ROS1	NPV	NA	NA	A	C	N	S	PP	Y
2*	0.2	F	PL	GBM	IHG (0.99)	IHG (0.99)	CCDC88A::ALK	NPV	NA	NA	A	C	Y*	NS*	N*	N*
3	0.5	F	FL	GBM	IHG (0.99)	IHG (0.99)	KIF21A::ROS1	NA	NA	NA	A	ND	ND	S	PP	Y
4	0.2	F	FL & PL	HGA	IHG (0.99)	IHG (0.99)	TPM3::NTRK1	NPV	15	52	A	C	N	S	PP	N
5	0.3	M	TL	HGA	IHG (0.99)	IHG (0.99)	GOPC::ROS1	RASA slice	2	23	A	C	Y	S	PP	Y
6	0.5	M	PL & TL	HGA	IHG (0.99)	IHG (0.99)	TRIM24::ROS1	NA	5	62	D	ND	Y	S	N	N
7	3.8	M	Ce	HGA	DPHGG, RTK1 (0.98)	DPHGG, RTK1 (0.98)	NA	TP53 V173L, NF1 F1246fs* & 3 other	2	2	D	I	N	S	PP	Y
8	1.2	F	Th	DIG/A	IHG (0.82)	DIG/A (0.78)	NA	NA	124	124	A	C	Y	NS	N	Y
9	0.4	F	PL & TL	DIG/A & HGA	IHG (0.42)	IHG (0.29)	ALK::PPP1CB	NPV	4	156	A	C	Y	S	PP	Y
9R	13.1		PL & TL	GG	PXA (0.97)	PXA (0.88)	ALK::PPP1CB	CNV CDKN2A/B loss				C	Y	NS	N	N
10	0.6	F	PL & TL	DIG/A	IHG (0.99)	IHG (0.99)	CLIP1::ROS1	NPV	3	44	D	C	Y	S	PP	N
10R1	1.6		PL & TL	DIG/A	IHG (0.31)	GBM, IDH-WT, Mes (0.62)	CLIP1::ROS1	NPV, (CNV No 7+/10-)				C	Y	NS	N	Y
10R2	2.6		PL	GBM	IHG (0.99)	IHG (0.99)	CLIP1::ROS1	NPV				C	Y	S	Y	Y
10R3	2.7		PL	GBM	IHG (0.99)	IHG (0.99)	CLIP1::ROS1	TP53 R342Vfs*10 & ATRX X1520_splice				CFI	Y	S	PP	Y

^*Limited sample for examination for case 2.

A: alive; A(Y): age in years; C: circumscription; Ce: cerebellum; CFI: circumscribed with focal infiltration; CS: calibration score; D: deceased; DIA/G: desmoplastic infantile astrocytoma / desmoplastic infantile ganglioglioma; FL: frontal lobe; GBM: glioblastoma multiforme; HGA: high-grade astrocytoma; IHG: infant-type hemispheric glioma; IHD: initial histological diagnosis; LG: low-grade area distinct from the high-grade component; MC: methylation class; Mi: mitosis; MVP: microvascular proliferation; N: No; No: number; NA: not available; ND: not described; Ne: necrosis; NGS: next generation sequencing; NPV: no pathological variant; OP: optic pathway; OS: overall survival in months; PFS: progression-free survival in months; PL: parietal lobe; PP: pseudopalisading; S: significant; Se: sex; Th: thalamus; TL: temporal lobe; VS: vital status; Y: yes.

Fig. 1.

Infant hemispheric glioma (IHG) & high-grade infiltrative astrocytoma. (A-C) Case 1: Infant hemispheric glioma. (A) Relatively well-demarcated from adjacent cortex. (B) High cellularity with three visible mitoses (circled). (C) Pseudopalisading necrosis. (D) Case 2: IHG showing a solid growth pattern. (E,F) Case 7: High-grade infiltrative astrocytoma. (E) Infiltrative growth pattern with high cellularity. (F) Pleomorphic neoplastic glial cells. Scale bar: A-F: 200 μm; D-G: 100 μm.

The second group consisted of cases diagnosed with high-grade gliomas other than IHGs. This group included only one case (case 7). The cerebellar tumor in this 3.8-year-old male demonstrated evident tumor cell infiltration into the granular, molecular, and subpial layers of the cerebellum (Fig. 1E, F). An overall survival duration of only 2 months highlighted the poor prognosis associated with this tumor.

The third group included 3 cases that were initially diagnosed with DIG/A at the time of their first resection. Case 8 involved the resection of a thalamic tumor from a 14-month-old girl. For this case, the tumor predominantly exhibited classic histological features of DIG/A (Fig. 2A-F), despite its unusual midline location. These features included a well-circumscribed low-grade glial component, focal areas with increased cellularity, glomeruloid microvascular proliferation, and dysplastic ganglion cells. The MC results for this case conveyed contradictory information: v12.5 pointed to an IHG with a CS of 0.82, while v12.8 indicated DIG/A with a CS of 0.78. Owing to the lack of NGS tests undertaken, molecular data were unavailable. With a progression-free survival (PFS) of 124 months at the final follow-up, the diagnosis appeared to be aligned more closely with DIG/A.

Fig. 2.

Desmoplastic infantile ganglioglioma/astrocytoma. (A-C) Case 8: Desmoplastic infantile ganglioglioma/astrocytoma. (A) Glioma with numerous gemistocytic neoplastic cells and desmoplastic stroma. (B) GFAP immunostaining shows positivity in most neoplastic cells. (C) Peripheral tumor areas with higher cellularity and smaller cells. (D) Blood vessels with a glomeruloid appearance. (E) Dysplastic ganglion cells. (F) Neurofilament protein immunopositive in dysplastic ganglion cells. Scale bar: A-D: 200 μm; E,F: 100 μm.

In case 9, the original resection of a hemispheric tumor from a 5-month-old girl exhibited histological features of DIG/A, along with pseudopalisading necrosis, significant mitotic activity, and proliferative characteristics (Fig. 3A-C). The primary histological diagnosis was DIG/A, with sectors of high-grade astrocytoma. The MC results did not offer a definitive match; still, the highest score for both versions implied IHG. NGS unveiled ALK fusion, which meets the essential CNS WHO diagnostic criteria for IHG. Yet, the tumor reappeared in the identical location 12 years later as a well-defined low-grade glial tumor with dispersed dysplastic ganglion cells and pleomorphic neoplastic glial cells, but without high-grade features (Fig. 3D-F). NGS of the recurring tumor demonstrated the same ALK fusion, along with a methylation report for CNA suggesting CDKN2A/B loss. The MC at this stage was a pleomorphic xanthoastrocytoma (PXA) with a CS of 0.97 (v12.5) and 0.88 (v12.8). Although the histological traits of the recurrent tumor were categorized as ganglioglioma, the possibility of PXA of CNS WHO grade 2 could not be ruled out. This introduces the possibility that the tumor might have been a PXA from the outset. The absence of CDKN2A/B loss in the initial tumor could account for the low CS of the MC in the first specimen. This case underscores the challenges in differentiating between DIG/A, IHG, ganglioglioma, and PXA, even with progressive molecular techniques.

Fig. 3.

Overlapping Histological Features. (A-F) Case 9: IHG vs. pleomorphic xanthoastrocytoma (PXA). (A-C) First resection: (A) Pseudopalisading necrosis. (B) High cellularity with three mitoses (circled). (C) Ki67 immunostaining showing a high proliferative index. (D) Another area with low-grade histological features. (E,F) Recurrent tumor showing 2 distinct areas: one with prominent Rosenthal fibers, the other with pleomorphic neoplastic glial cells. (G-I) Case 11: DIG/A vs. IHG. (G) Multinucleated cells and eosinophilic granular bodies (EGBs). (H) Recurrent tumor with necrosis and endothelial proliferation. (I) Numerous multinucleated cells. Scale bar: A-I: 200 μm.

In Case 10, four separate resections were subjected to tests for molecular alterations. Each one displayed the CLIP1-ROS1 fusion. All resections were clearly outlined tumors; however, the fourth resection also revealed focal infiltration. The initial diagnosis was DIG/A, with the presence of high-grade features considered within normal limits. Nevertheless, in light of the molecular results showcasing both ROS1 fusion and an MC of IHG with a CS of 0.99, the diagnosis of IHG seems more fitting. While the first resection showed high-grade features, the cancer cells progressively became more pleomorphic, hyperchromatic, and cellular as shown in Fig. 3G-I. The third and fourth resections, in fact, were diagnosed histologically as glioblastoma. NGS from the fourth resection brought to light mutations in P53 and ATRX, signaling tumor progression. Forty-four months post the initial resection, the patient passed away, which suggests a progression to a tumor with increasingly aggressive behavior.

Discussion

This study underlines the significant clinical and diagnostic features of IHG, contributing to the understanding of elements yet not fully defined in the literature. IHG primarily affects infants under one year of age. However, cases in older children are documented, with the oldest in our series being 4.4 years old [6].

A key novel finding of this study is the potential for genetic alterations to emerge in recurrent IHG cases, leading to distinct clinical behaviors. Case 9 demonstrates the progression to a PXA-like molecular profile; while Case 10 developed additional genetic modifications associated with more aggressive gliomas and exhibited a clinical course reminiscent of high-grade glioma. These findings underscore the necessity for long-term molecular surveillance in IHG patients, as recurrence may be connected with additional genetic alterations that could influence tumor behavior.

Histologically, the presence of infiltration seems to differentiate pediatric high-grade gliomas (pHGG) – which usually have a poor prognosis – from IHGs, which characteristically exhibit well-defined growth and a generally more favorable prognosis. The distinction from desmoplastic infantile ganglioglioma/astrocytoma (DIG/A) remains a challenge, as both entities share histological features that overlap. However, IHGs often display predominantly high-grade characteristics, with sporadic low-grade areas, contrasting DIG/A, which typically appears as a predominantly low-grade tumor with isolated high-grade components.

The IHG was initially identified as a unique methylation subclass of supratentorial tumors, typically diagnosed as glioblastomas. However, the foundational study by Capper et al. did not provide any specific morphological descriptions [1]. The categorization was later refined by Stucklin et al, who suggested that infant gliomas could be classified based on their location and molecular alterations. It was shown that RTK-driven hemispheric infant gliomas had a moderate prognosis, whereas RAS/MAPK-driven cases revealed an excellent prognosis post-surgery. In that study, out of the 29 RTK-driven tumors, 24 were high-grade while five were low-grade. However, detailed histopathological data was rather limited, especially regarding tumor circumscription and instances of mixed low- and high-grade areas [2].

Clarke et al [3] extended these findings via extensive methylation profiling and genetic analysis in children under 4 years old. They found that the majority of RTK-driven tumors were associated with the IHG MC, while a few correlated more closely with DIG/A. Histological analysis showed that IHG usually presents with clearly defined borders and high-grade features, with the majority of cases initially identified as high-grade astrocytomas or congenital glioblastomas.

Given the findings of these studies, the WHO classification now recognizes IHG as a new entity within the category of diffuse pediatric high-grade gliomas. Although the WHO criteria underline the presence of RTK pathway-activating gene fusions or an IHG MC, they do not lay out specific histological features or grade prerequisites.

Since most IHG cases are diagnosed within the first 6 months of life, there is a significant overlap with what was previously classified as congenital glioblastoma (cGBM). Gelani et al. reported that seven out of eight cGBM cases were IHG, while the remaining case showed no correspondence and was suggested to represent a high-grade neuroepithelial tumor [7]. In another study of congenital brain tumors, 4 of 11 cGBM cases with available molecular testing results were classified as IHG, while the remaining cases lacked sufficient molecular data for definitive classification [8]. Although the number of cases was small, the available molecular data suggest that most cGBM cases are indeed IHG, with none confirmed to represent other high-grade glioma (HGG) classes in this age group.

Case 9 highlights the challenge of distinguishing IHG from PXA, especially its anaplastic variants. The rarity of PXA in patients under 4 years old complicates this differentiation. Although PXAs are associated with alterations in the mitogen-activated protein (MAP) kinase pathway [9], the few reported cases in this age group have suggested that PXA may be associated with RTK fusions such as NTRK1 and ALK [3,6,10,11] (the defining alteration of IHG). Nevertheless, a helpful feature is that all of these cases showcased a characteristic copy number alterations (CNA) of PXA, specifically, the homozygous deletion (HD) of CDKN2A/B. Case 9 was initially classified as IHG due to the presence of ALK fusion, despite the absence of a matching MC. After 12 years, the tumor recurred and was reclassified as PXA, maintaining the ALK fusion but now displaying both a CDKN2A/B deletion (absent in the initial tumor) and a MC consistent with PXA.

Case 10 represents the most novel finding in this series. This case draws attention to the potential for IHG to acquire additional molecular alterations associated with worsening prognosis, a characteristic not previously reported. The initial tumor met all crucial diagnostic criteria for IHG. However, the final recurrence demonstrated additional genetic mutations in TP53 (often associated with tumor progression). An overall survival of 44 months is not typical for IHG and aligns more closely with other HGGs, suggesting these additional molecular alterations were associated with a less favorable prognosis. This discovery is crucial as we are dealing with a rare tumor and an undecided WHO grade; this case suggests that higher-grade IHGs exist and correlate with acquired genetic alterations. In the recurrent tumors, the margins began to be more infiltrative than the initial well-circumscribed tumor, suggesting that infiltration is typically associated with high-grade tumors.

A more critical yet less frequently discussed differential diagnosis for IHG involves other pediatric HGG, which are significant due to their varied prognoses. These tumors often follow the typical behavior of high-grade astrocytomas [3,6]. While advanced molecular tests such as DNA and RNA sequencing or DNA methylation profiling can provide a clear differentiation between these entities, access to such testing is still limited in many laboratories worldwide. This highlights the essential role of morphological features in distinguishing IHG from other HGG. Based on the findings from this and two other studies – where histological data is available from supplementary materials [3,6] – several histological features could suggest a diagnosis of HGG rather than IHG. In tumors with glial differentiation and uniformly high-grade features, the following characteristics can aid in differentiation: (1) IHG predominantly affects infants younger than 6 months, whereas most HGG cases occur in patients older than one year. The IHG can, however, rarely present at this age; (2) Non-hemispheric tumor locations (i.e., infratentorial or midline) strongly suggest an HGG rather than IHG diagnosis; and (3) Well-demarcated tumors are more commonly associated with IHG, whereas HGGs tend to be more infiltrative.

The WHO classification does not currently assign a grade to IHG, primarily due to its paradoxical clinical behavior. Many instances exhibit high-grade histological characteristics (e.g., mitotic activity, endothelial proliferation), yet their prognosis is aligned with low-grade gliomas. Some low-grade tumors, such as DIG/A, can show heightened cellularity and mitotic activity, yet they remain at WHO grade 1.

A review of the literature indicates that high-grade features were documented in 120 cases, [2,3,4,6,7,8,11,12,13,14,15,16], while eight cases primarily displayed low-grade features [2,7,11]. Seven cases exhibited a mix of low- and high-grade areas [2,7,15], and 2 cases revealed only significant mitotic activity [4,16]. The prognosis for cases with high-grade features is comparable to that of other low-grade gliomas [2,3,6], thereby highlighting a paradox and raising questions about the reliability of traditional grading criteria for IHG. Even though IHG cases with predominantly low-grade features are rare, they tend to carry a better prognosis than those with high-grade features [2]. However, the presence of consistent high-grade features in the IHG can be a valuable distinguishing characteristic of its mimicker.

The differential diagnosis of IHG depends on the studies available. Morphological studies should encompass astrocytomas with high-grade features, DIG/A, and less commonly, ependymomas, glioneuronal tumors, and SEGA. Advanced molecular studies can offer more accurate diagnoses, although some cases can still pose challenges, as we will discuss.

Most studies have concentrated on distinguishing IHG from DIG/A [7,11]. Gilani et al. discovered that only one case diagnosed histologically as DIG/A aligned with the DIG/A MC, whereas two cases matched the IHG MC, one of which had a ROS1 fusion. They surmised that using histology alone is not sufficient to reliably distinguish between IHG and DIG/A. Meanwhile, another study confirmed that nine out of ten cases initially diagnosed as DIG/A were indeed DIG/A when determined by molecular studies, with the remaining cases matching the IHG MC. Nevertheless, the final case harbored a PRKAR2A::RAF1 fusion, and the integrated diagnosis stayed as DIG/A, which presented a challenge when interpreting IHG MC [11].

Tauziède-Espariat and colleagues discovered that desmoplasia and the existence of ganglion cells were statistically significant in DIG/A as compared to IHG. On the other hand, microvascular proliferation, previous hemorrhage traces, and necrosis were more commonly linked with IHG. Evidence of previous hemorrhage was found more often in IHG, and these tumors frequently displayed cyst wall enhancement, a feature not seen in DIG/A [11].

Chiang et al selected 12 DIG/A cases that exhibited high-grade features, which include mitoses, necrosis, and microvascular proliferation, to conduct a molecular comparison between high-grade and low-grade areas. While their findings showed no notable molecular differences between the two areas, the study highlighted the frequent occurrence of high-grade features in DIG/A. This raises concerns regarding the use of these features to distinguish between DIG/A and IHG [17].

This study aligns with literature that emphasizes the significant overlap between DIG/A and IHG in terms of histology, molecular characteristics, and prognosis. This similarity often makes distinguishing between the cases particularly challenging. However, the ensuing guidelines can aid in suggesting one entity over the other: DIG/A generally exhibits uniformly low-grade histological attributes, accompanied by a pronounced desmoplastic reaction. If high-grade components are present, they are typically confined to smaller areas within a predominantly low-grade tumor. On imaging, DIG/A tumors often lack cyst wall enhancement and are not associated with intratumoral bleeding – features more consistent with IHG. Pathologically, IHG predominantly presents with high-grade attributes, although smaller sections featuring low-grade attributes may also exist. In some instances, low-grade components may even be the predominant feature.

This study had several limitations, such as its retrospective design, reliance on an online database, and the small sample size of this uncommon tumor. However, these limitations should not impact the novel findings of additional genetic alterations in this rare tumor. Moreover, our findings regarding the similarity between IHG and DIG/A align with existing literature.

In conclusion, this study offers valuable insights into the clinical and morphological characteristics of IHG. Despite the primary aim being to clarify its histological features, especially its circumscription, an unanticipated observation emerged from the progression of one IHG case due to the introduction of additional molecular alterations. This discovery underscores the prognostic implications of secondary genetic changes, highlighting the necessity for consistent monitoring and sparking potential evidence that such shifts may influence IHG grading. Lastly, the report of another possible pediatric PXA case associated with ALK fusion strengthens the case for including PXA in the differential diagnosis of IHG. This case underscores the diagnostic challenges posed by the overlap of histological and molecular features among pediatric tumors. Overall, these findings increase our understanding of IHG's unique histological and molecular attributes and provide significantly needed evidence to refine its diagnostic and prognostic considerations.

Acknowledgments

We would like to thank American Manuscript Editors (https://americanmanuscripteditors.com) for English language editing. Also, this study was made possible in part due to the Children's Brain Tumor Network (CBTN).

Disclosure

Author has no conflict of interests and the work was not supported or funded by any drug company