Multinodular and Vacuolating Posterior Fossa Lesions of Unknown Significance

DISCUSSION

We describe the imaging characteristics of a new entity that we referred to as MV-PLUS. This entity has never been described in the literature to the best of our knowledge.

MV-PLUS imaging features consist of the coalescence of small T1WI hypointense and T2-FLAIR hyperintense nodules in subcortical and juxtacortical areas. They are very similar to those described in MVNT, a rare and recently described brain tumor,^{8⇓⇓⇓⇓⇓–14} which was added in the World Health Organization Classification of Tumors of the Central Nervous System in 2016.^1,2 None of the case reports and small series published in the literature so far have reported posterior fossa MVNT. Among 74 patients analyzed in 24 international centers showing MR imaging features highly suggestive of MVNT, we found only 11 (15%) posterior fossa lesions, suggesting that MV-PLUS might be 10 times rarer than MVNT.

Similar to MVNT, MV-PLUS showed no sign of malignancy on MR imaging and an absence of evolutivity during follow-up.^8,9 In our study, MR imaging showed neither restriction on DWI nor intratumoral susceptibility signal on T2* or SWI. PWI showed no increase in relative CBF or CBV. MRS showed no increase in choline peaks. There was no or only a small mass effect on the surrounding posterior fossa structures. Moreover, no changes in size or signal could be observed during follow-up, and no relationship was observed between the location of lesions and clinical symptoms. Only 1 nodule in 1 patient showed enhancement after contrast injection, which is similar to the low rate of enhancement reported for MVNT in the literature.^8,9 However, this nodule had a very distinct signal compared with other nodules throughout all sequences. Enhancing MVNTs were reported in the literature to show only faint enhancement, suggesting that it might be a distinct lesion.^8,9

Differential diagnoses for posterior fossa intra-axial lesions encompass a wide range of diseases, such as ischemic lesions, inflammatory diseases like multiple sclerosis, infectious diseases, vascular malformations, neoplastic lesions, degenerative lesions like ataxias, toxic lesions, malformative lesions like dysplastic cerebellar gangliocytoma, or normal variants like enlargement of perivascular spaces.^18⇓–20 However, both the location in juxtacortical or subcortical regions and typical features such as the presence of clusters of discrete or confluent high T2-FLAIR signal intensity small nodules make the diagnosis of MV-PLUS very likely. Moreover, we reported the presence of a T2-FLAIR hypointense central dot sign for most MV-PLUS lesions. This central dot sign was present in most nodules of >4 mm and was more conspicuous on high-resolution T2WI. It was missing in images of 2 patients with low-resolution imaging. This central dot sign was already reported in the literature in MVNT, but its prevalence is not known.^8,10 It might reflect the presence of a high protein or a solid component within the vacuolated areas, which is a typical pathology feature reported in MVNT histopathologic studies.^1,7 It might be an interesting imaging criterion that could increase readers' confidence when diagnosing MV-PLUS. This sort of evidence might help rule out differential diagnoses.

Two patients had very subtle cortical involvement on imaging, which has not been reported in MVNT.^8,9 However, these 2 patients had low-resolution images, with 2D-FLAIR images with a section thickness of 3 and 5 mm, respectively. Distinguishing cortical and subcortical areas is more challenging in the posterior fossa than in supratentorial regions. Thus, this appearance might be due to partial averaging rather than true cortical involvement and should be confirmed by other observations with high-resolution imaging.

MV-PLUS should probably be considered a leave-me-alone lesion, requiring no operation to confirm the diagnosis. Its nature remains unknown, and it is not clear whether it should be considered a neoplasm or a malformation. However, the absence of changes during follow-up, the absence of or only a small mass effect or malignant pattern on MR imaging, and the absence of a relationship with clinical symptoms might suggest that MV-PLUS is a benign malformative lesion rather than a true neoplasm. However, we advise clinicians to perform a comprehensive protocol when characterizing possible MV-PLUS, including thin-millimetric T2- or T2-FLAIR sequences to detect the central dot sign, and MR spectroscopy as well as PWI to show the absence of malignant criteria. We also suggest long-term follow-up to assess any changes in size or signal.

Our study has some limitations: First, although we analyzed patients from 24 international centers, the results of the study are somewhat limited by the relatively small number of patients. Two asymptomatic patients underwent MR imaging for meningioma screening, representing 18% of our population, which is higher than expected and might be due to a selection bias. The first patient had a sibling recently diagnosed with a meningioma. The second patient had been taking cyproterone acetate for a long time, which had been reported to increase the chance of developing a meningioma.

Second, 2 centers contributed >1 case. Most of the 24 centers where clinicians were aware of MVNT had not identified any patients with similar imaging findings, suggesting that MV-PLUS might be very rare.

Third, none of our patients underwent an operation; thus, we do not have any histopathologic evidence showing that MV-PLUS might be, in fact, MVNT. The posterior fossa is a very challenging region for surgery, and none of our patients had clinical symptoms related to their lesions; thus, none of them became surgical candidates. This is why we chose to remain cautious and to refer to this entity as MV-PLUS instead of posterior fossa MVNT. Given the benign MR imaging characteristics of MV-PLUS and the absence of evolutivity with time, it might be extremely difficult to obtain pathologic details in the near future.

Fourth, the examinations were performed on various MR imaging devices from different vendors. Protocols were heterogeneous among the centers, mixing comprehensive multiparametric protocols and basic low-resolution MR imaging protocols. Therefore, our analysis of MV-PLUS might not be optimal. Some of the information we provide, such as the absence of a central dot sign or the involvement of the cortex, might be inaccurate because of low-resolution images in 2 patients. PWI and MRS were performed in only 3/11 (27%) patients; thus, the quantitative values we provided might be inaccurate. Moreover, only single-voxel spectroscopy was performed, which might have resulted in volume averaging with normal tissue.