Microcystic meningioma with late-phase accumulation on thallium-201 single-photon emission computed tomography: case report.

Microcystic meningiomas are rare but benign brain tumors. Previous reports have shown that Thallium-201 single-photon emission computed tomography ((201)Tl SPECT) demonstrated a higher late-phase accumulation of (201)Tl in malignant or recurrent meningiomas than in nonaggressive meningiomas. No study has reported (201)Tl SPECT findings in microcystic meningiomas. We here describe a case of a microcystic meningioma with a high (201)Tl SPECT retention rate in a 62-year-old woman who complained of headache. Computed tomography revealed an intracranial tumor in the right frontal lobe. Moreover, (201)Tl SPECT revealed a high uptake of (201)Tl in the tumor, which was particularly prominent in the delayed phase. The uptake index on an early image was 1.46 and that on a delayed image was 1.35. Therefore, the retention index was 0.92. After 2 years of tumor growth, we performed successful radical resection, and histological examination revealed the presence of a microcystic meningioma. Therefore, we concluded that (201)Tl SPECT may be useful for the preoperative diagnosis of microcystic meningiomas and that late-phase accumulation of (201)Tl is not a specific finding of malignant brain tumors. Therefore, we need to be careful in the evaluation and judgment of high retention in a delayed image of (201)Tl SPECT.

Introduction

Thallium-201 single-photon emission computed tomography (201Tl SPECT) exhibits early and relatively high accumulation in all types of meningiomas, but its retention rate varies according to the histological type.[1,2)] 201Tl SPECT has demonstrated higher late-phase accumulation of 201Tl in malignant or recurrent meningiomas than in nonaggressive meningiomas.[3–6)] To our knowledge, no study has reported 201Tl SPECT findings in microcystic meningiomas. We here describe a case of microcystic meningioma with a high 201Tl SPECT retention rate and review of the literature.

Case Report

A 62-year-old woman presented with headache. Computed tomography (CT) revealed an intracranial tumor in the right frontal lobe. The tumor had been growing for 2 years, and the patient was referred to our hospital for a detailed examination. Neurological examination results were normal. CT revealed a low-density lesion with edema (Fig. 1A), whereas angiography did not demonstrate any tumor staining (data not shown). The tumor appeared as a low-density lesion on T1-weighted magnetic resonance imaging (MRI) (Fig. 1B) and as a high-density lesion on T2-weighted MRI (Fig. 1C). On enchanced T1-weighted MRI, the tumor appeared to have an enhanced wall (Fig. 1D). Early phase 201Tl SPECT images showed high tumor uptake (Fig. 1E), which was particularly prominent in the delayed phase (Fig. 1F). The uptake index on an early image was 1.46 and that on a delayed image was 1.35. Therefore, the retention index was 0.92.

Fig. 1.

A: CT scan revealing a low-density lesion. B: T1-weighted MRI showing a low-density lesion. C: T2-weighted MRI showing a high-density lesion. D: Tumor showing an enhanced wall on enhanced T1-weighted MRI. E: Early 201Tl SPECT image showing high tumor uptake. F: Delayed 201Tl SPECT image showing high tumor uptake. CT: computed tomography, MRI: magnetic resonance imaging, 201Tl SPECT: Thallium-201 single-photon emission computed tomography.

Surgical treatment (radical resection) was successfully carried out. The tumor, with a soft and moist consistency similar to that of a jellyfish, was easily separated from the brain surface and underwent Simpson Grade 1 resection. Following radical resection, duraplasty was performed using artificial dura. Histological examination revealed a tumor characterized by vacuolated cells with abundant cytoplasmic processes forming variable-sized microcysts (Fig. 2A, B). Immunohistochemically, the tumor cells were diffusely positive for vimentin (Fig. 2C) and focally positive for epithelial membrane antigen (Fig. 2D). The mindbomb homolog 1 (MIB-1) index, detected by Ki67 staining, ranged from 3% to 5% (Fig. 2E). These histological and immunohistochemical findings were diagnostic of a microcystic meningioma.

Fig. 2.

A, B: Histological examination, indicative of a microcystic meningioma, showing variable-sized tumor cells with vacuolated cytoplasm and elongated cell processes forming microcystic spaces (hematoxylin and eosin stain) (A ×40, B ×200). C: Vimentin positivity on immunostaining (×200), D: Epithelial membrane antigen positivity on immunostaining (×100). E: MIB-1 index ranged from 3% to 5% (×100). MIB-1: mindbomb homolog 1.

The patient's postoperative recovery was favorable, and she was discharged with no signs of neurological deficit.

Discussion

We reported that late-phase accumulation of 201Tl may occur in microcystic meningiomas. We speculated that 201Tl SPECT may be useful for the preoperative diagnosis of microcystic meningiomas and that late-phase accumulation of 201Tl is not a specific finding of malignant brain tumors.

Microcystic meningiomas are rare but benign brain tumors, accounts for only 1.6% of all intracranial meningiomas.[7)] In 1993, the World Health Organization classified this variant as a distinct subgroup of meningiomas.[8,9)] On CT and MRI, microcystic meningiomas present characteristics different from other meningiomas.[10–12)] In our case, the tumor appeared as a low-density lesion on CT and T1-weighted MRI and as a high-density lesion on T2-weighted MRI. On gadolinium diethylenetriaminepentaacetate (Gd-DTPA)-enhanced T1-weighted imaging, the tumor did not show intense enhancement. Peritumoral cerebral edema is a common MRI finding of microcystic meningiomas.[13–16)]

In the present case, the CT and MRI results were typical of microcystic meningiomas, but 201Tl SPECT has not been reported previously and was found to be relatively atypical for a benign meningioma. 201Tl SPECT has been reported to be useful for detecting tumor viability in cases of malignant tumors[3,4,6,17,18)] and exhibits early and relatively high accumulation in all types of meningiomas. However, the retention rates differ according to histological types. Jinnouchi et al.[1)] reported that early and delayed 201Tl SPECT studies of meningiomas revealed meningothelial, transitional, fibroblastic, and malignant tumors. They concluded that retention was lower for meningothelial meningiomas than for malignant meningiomas, and the transitional and fibroblastic meningiomas showed relatively high retention in the delayed image of 201Tl SPECT. Based on the case of our microcystic meningiomas and previous reports, high retention in a delayed image of 201Tl SPECT always does not imply the detection of a malignant brain tumor. Therefore, we need to be careful in our evaluation and judgment of high retention in a delayed image of 201Tl SPECT.

There have been no previous reports focusing on 201Tl SPECT findings in microcystic meningiomas. Therefore, it would be helpful to study more patients presenting with microcystic meningiomas in relation to 201Tl SPECT results to obtain further evidence.