In our surgical series, fluorescein-guided technique is feasible and represents a useful adjunct in most of the pediatric intra-axial supratentorial tumors presenting with some degree of contrast enhancement at preoperative neuroimaging scans. Despite the heterogeneity of pediatric CNS tumors histotypes [1, 9], these neoplasms usually present with variable patterns of c.e. on their preoperative MRI [8]; the c.e. reflects the loss of endothelial cell-astrocyte foot process relationship in the altered BBB, revealing not only aggressive tumors but also such histotypes characterized by the same BBB damaging.
In particular, bright fluorescence was present in almost all cases of pilocytic astrocytomas, gangliogliomas, DNET and PXA, whereas it was less evident for choroid plexus papilloma and extraventricular neurocytoma and absent in the single reported case of polymorphous low-grade neuroepithelial tumor of the young. These findings are basically correlated to the characteristics enhancement of tumors at preoperative MRI; in particular, fluorescence was detectable in 93.9% contrasting-enhancing tumors. Moreover, the operating surgeon found SF to be useful in 84.8% of the cases for either achieving GTR or for confirming surgical fluorescent samples in tumor biopsy.
The standard of care for most pediatric tumors is surgical resection whereas adjuvant therapies are usually considered second-line treatments [5, 10]. The primary goal of resection is to maximize EOR while preserving the patient’s neurologic function and obtaining a histopathologic diagnosis. Innovative technical tools that can improve tumor visualization and borders discrimination between slight pathological peripheral neoplastic tissue and normal peritumoral brain parenchyma could have a positive influence in minimizing RV.
In this perspective, the application of new technical tools aimed at increasing the EOR could be beneficial in the management of pediatric brain tumors. Use of 5-ALA has been reported but with inconsistent results [22]: recent evidences regarding the advantages of 5-ALA in pediatric population, which is still off-label, suggest a valuable role in high-grade gliomas and in grade 3 ependymomas. Since these histotypes are more frequent in the supratentorial compartment, this last location is one of the most relevant predictors of usefulness of 5-ALA [23]; the group of Stummer, in a recent review, reported a judgement of helpfulness in less than 60% of supratentorial pediatric brain tumors scheduled for 5-ALA-guided surgery [11]. Even Preuß at al. reported a similar percentage of success of 5-ALA as a positive fluorescent dye in pediatric neuro-oncological surgery [24]. In the light of these results, many experienced surgeons in the 5-ALA field advise against the routine use of this fluorophore in pediatric brain surgery [12]. The principal drawback of 5-ALA-guided surgery is the lack of a strong predictor of usefulness in pediatric brain neoplasms: indeed, a strong fluorescence enhancement correlates neither with preoperative MRI c.e. nor with tumor aggressivity [9].
During the last years SF has emerged as intraoperative tracer able to improve brain-tumor visualization, due to its non-specific, vascular mechanism of action related to the accumulation in brain regions with BBB dysregulation, as it happens with MRI contrast enhancement [25, 26]. Additionally, the recent availability of an integrated and specific filters in the surgical microscope has contributed to the wide diffusion of fluorescein [14–16]. Previous experiences suggested that the use SF could be associated with a bright fluorescence of the tumor area in HGG [20], in metastases [27], in gangliogliomas [28], in pilocytic astrocytomas [29] and in spinal intramedullary lesions [30]. This was also associated with good results in term of extent of resection as well as progression free and overall survival.
For these reasons, we hypothesized that fluorescein could represent an ideal dye for the surgical management of CNS tumors in the pediatric population. In our preliminary analysis described in 2019 [16], we discussed that fluorescein plays a useful role during surgical resection of pilocytic astrocytomas, gangliogliomas and medulloblastomas by means of highlighting the bright fluorescent pathological tissue compared to the surrounding pinkish health brain parenchyma.
As the reimbursement of SF as a fluorescent tracer in neuro-oncology has been approved by the Italian Drug Agency in July 2015 (determination 905/2015, Gazette n.168, 22 July, 2015), and based on our extensive experience with HGG, we decided to start a prospective observational study (i.e., the FLUOCERTUM study) on the use of SF for the resection of aggressive tumors of the CNS, applying a standardized protocol independently from patient age and tumor location, with a dosage of 5mg/kg and an i.v. injection immediately after patient intubation. From the prospective collected database, we were able to retrospectively select the pediatric population considered in this study.
Thirty patients enrolled in the study were scheduled for surgery with a previous planning of macroscopic resection but keeping in mind the philosophy of maximal safe resection: in this series, we obtained a high percentage of GTR (21/30, 70%). Minimal residual tumor (lower than 10% of preoperative tumor volume) at postoperative MRI was expected in 7 cases (23.3%), as it was involving eloquent areas and was therefore independent from the use of fluorescein-guided technique. Conversely, in only 1 case (3.3%), the residual tumor was an unexpected finding, based on the absence of clear residual intraoperative fluorescent tissue: this can be related, as for other fluorophores, to the fact that the residual tissue was not exposed during resection because hidden under the normal brain parenchyma. Finally, in the remaining other patient (3.3%), who presented a huge and hemorrhagic lesion, the surgical resection was only partial and interrupted after debulking of a large component to relieve functional structures in eloquent areas. Two patients were included in our protocol for fluorescein-guided needle biopsy; although the limitation of this case load, the diagnostic accuracy was of 100%.
We also found a good correspondence between pre-operative MRI c.e. and intraoperative fluorescence identification in several neoplastic histotypes: we hypothesized that pediatric tumors presenting a gadolinium uptake in T1 MRI sequences could be shown intraoperatively as a fluorescent mass, using SF with a dedicated filter in the surgical microscope; this consideration determined a frequent opinion of usefulness (28/33–84.8%). Intraoperative fluorescence was judged partial useful in 2 children (6.1%) and not essential in 3 patients (9.1%). Our considerations want to focus that the use of fluorescein remains questionable for tumors showing a minimal or a heterogeneous pattern of contrast medium uptake, since the high predictivity of SF role as an adjuvant tool in relation to preoperative MRI contrast enhancement. Despite the subjective intraoperative evaluation of fluorescein can constitute a major bias of our study, the clear and intense fluorescence delineating the tumor margin for such histologic entities, along with the GTR performed in almost all cases, apparently supports the great benefit of fluorescein utilization.
Nobody, between patients of our cohort, presented fluorescein-related side effects or adverse reactions to SF administration. The only visible manifestation of intraoperative fluorescein administration was the onset of transient and harmless yellowish stain of the urine, that rapidly disappears after 24 hours. We believe that the lack of any side effect, in particular any allergic reaction, was predominantly related to the low dosage used in this study, thanks to the use of a dedicated filter into the microscope, that allowed a more accurate identification of fluorescent tissue, as suggested firstly by our group [21].
Owing to the unspecified mechanism of action of SF, the use of this fluorescent tracer in pediatric neuro-oncology should be considered cautiously. However, these preliminary results seem to suggest that the properties of SF may represent a strength in this population, as most of the CNS pediatric tumors are characterized by a various degree of c.e. related to a damage in the BBB.
The main limitation of this study presented is represented by the heterogeneous histology of tumors included in this study; furthermore, the lack of a significant long-term follow-up prevents the elucidation about overall survival: in fact, the authors considered only a surrogate indicator which is the EOR. Other known pitfalls of our research are the absence of the direct comparison of surgery with and without SF aid or between the use of SF and other available fluorophores, like 5-ALA. Otherwise, we must stress that, in most of the Countries, SF is still considered off label for neuro-oncological procedures. Thus, a widespread utilization of fluorescence-guided surgery will depend on the definitive approval by the competent authorities. Future prospective studies could better address these limitations by stratifying the cohorts according specific single tumor histology: larger and more homogeneous cohort of patients should be enrolled with a longer and more systematic period of follow-up.