**1. Introduction**

In 2021, the latest and most recent version of the 5th edition of WHO Classification of Tumors of the Central Nervous System (CNS5) was published. One of the most important innovations is the introduction of a new classification of tumor types and subtypes, thanks to novel diagnostic technologies such as DNA methylome profiling. Focusing on mesenchymal, non-meningothelial tumors, the hybrid term solitary fibrous tumor/hemangiopericytoma (SFT/HPC), previously used in the 2016 WHO classification, has been abandoned in favor of the use of the term solitary fibrous tumor (SFT) alone to fully comply with the nomenclature of soft-tissue pathology [1].

SFTs are rare mesenchymal malignancies of soft-tissue cells with a high percentage of recurrence. In particular, intracranial SFTs constitute 2.5% of meningeal neoplasms and less than 1% of all intracranial tumors [2]. At present, magnetic resonance imaging has a wellestablished role in the identification and diagnosis of this rare tumor. However, because of SFTs' high tendency to have locoregional recurrences and distant metastases, in recent years, multimodality imaging, particularly positron emission tomography/computed tomography (PET/CT), has shown an increasingly significant role, thanks to the possibility of obtaining both metabolic and morphological information in a single scan. Furthermore, it is a non-invasive and total-body examination, with high sensitivity and specificity [3,4].

We aimed to present a brief critical overview of the role of PET/CT in diagnosing, local disease relapse, or distant metastases evaluation in the treatment response to surgery and/or radiotherapy and in the follow-up of patients affected by intracranial SFT.

**Citation:** Sardaro, A.; Mammucci, P.; Pisani, A.R.; Rubini, D.; Nappi, A.G.; Bardoscia, L.; Rubini, G. Intracranial Solitary Fibrous Tumor: A "New" Challenge for PET Radiopharmaceuticals. *J. Clin. Med.* **2022**, *11*, 4746. https://doi.org/ 10.3390/jcm11164746

Academic Editors: Arnoldo Piccardo and Francesco Fiz

Received: 18 July 2022 Accepted: 12 August 2022 Published: 14 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **2. Solitary Fibrous Tumor**

SFT is a very rare and uncommon neoplasm, accounting for only 1.6% of all central nervous system (CNS) tumors. In the 2016 World Health Organization (WHO) classification of CNS tumors, SFTs and HPCs constituted a single disease entity, known as SFT/HPC. However, in the last recent update of WHO CNS5, the term "hemangiopericytoma" was deleted to emphasize the biological similarities within tumor types and to align with the soft-tissue pathology nomenclature [1].

This tumor was firstly known by the term hemangiopericytoma, coined by Stout and Murray in 1942, to highlight the origin from the capillary and postcapillary venules of pericytes. It can occur in any anatomical body region with the presence of capillaries and a typical localization involves the meninges of the dura mater, with an incidence of 16–33% of SFTs in the head or neck [5]. From a histological point of view, these tumors are characterized by spindle mesenchymal cells with a rich vascular component. Usually, the presence of hypercellularity, hemorrhage and/or necrosis, pleomorphic nuclei, and foci of dedifferentiation suggests a malignant behavior [6]. The tumor immunophenotype can be characterized by CD34 and smooth-muscle-actin positivity and by S100 protein, CK pool, and desmin negativity [7]. In accordance with previous immunohistochemical features, the new WHO CNS5 reported that STFs show NAB2 and STAT6 gene fusion as well as STAT6 overexpression [2].

Even many years after the end of the first (line) treatment, locoregional relapses, as well as distant metastases, are very common, and a prompt diagnosis could be crucial for better patient management. However, probably due to SFTs' rarity, there are no clear recommendations on the best imaging method to evaluate them [8]. The literature is characterized by small-samplesize studies and case series, showing that complete surgical resection with clear margins is the treatment of choice. However, one year after the stand-alone surgery option, a very high percentage of disease relapse, rating from 88 to 100%, may occur as a result of tumor infiltration in adjacent vascular structures with a worsening of survival [9]. Stereotaxic radiosurgery has emerged as a promising adjuvant treatment to reduce the relapse rate of the disease [2]. In an updated overview on radiation therapy options for intracranial HPC, Ciliberti and colleagues showed that postoperative radiotherapy (RT) can lead to a consistent decrease in local recurrences, moving from 88% to 12.5%, and longer relapse-free survival [9]. In addition, when the lesion is close to critical anatomical structures, stereotactic radiotherapy can be considered. However, even after a wide surgical resection or a combined surgery–radiotherapy treatment, recurrences and metastases can subsequently occur in up to 50% of cases [10,11].

In this scenario, imaging can play a crucial role in potentially guiding the best therapeutic decision and in establishing tailored treatment.

Although the referring imaging approach in SFTs is morphologic, it may fail in differentiating between scar tissue and viable tumor in post-treatment evaluation. For this purpose, PET has demonstrated a promising role [12,13].

#### **3. Search Strategy**

A literature search was conducted on the Medline (PubMed) database including all articles published up to 30 June 2022. The following keywords were entered to rule the search: "intracranial solitary fibrous tumor" AND "positron emission tomography", AND "PET" AND "nuclear medicine", "hemangiopericytoma" AND "positron emission tomography" AND "PET" AND "nuclear medicine", "intracranial hemangiopericytoma" AND "positron emission tomography" AND "PET" AND "nuclear medicine". Only articles edited in English in the last 10 years were included in this review. After reading the abstracts, some articles were excluded because they did not meet the goal of our review in evaluating the use of PET/CT in patients affected by intracranial SFT/HPC. For the same reason, some articles were not considered in the final draft after reading the full text. To identify supplementary eligible articles, additional references were searched from the retrieved review articles. Due to the rarity of SFT, most of the articles in the literature are represented by case reports and interesting images. The main characteristics of the included studies are detailed in Table 1.


PET/CT or

> **Table 1.** Main

characteristics

 of the included studies of intracranial

 SFTs/HPCs

 with probable distant metastases

 studied using

*J. Clin. Med.* **2022**

, *11*, 4746

fluorocholine; PSMA =

inhibitor. Annotation:

 \* =

prostate-specific

semiquantitative

 parameters not known.

membrane antigen; DOTATATE = dodecane tetra-acetic acid

tyrosine-3-octreotate;

 FAPI =

fibroblast-activation-protein
