Neuroradiology / Pictorial essay Jorge Docampo et al.

Pilocytic . Forms of presentation

J. Docampo*, N. González, A. Muñoz, C. Bruno y C. Morales

Fundación Científica del Sur, Lomas de Zamora, Buenos Aires, Argentina Received: december 2013; accepted: may 2014 ©SAR

Abstract Our purpose is to illustrate and describe typical and atypical imaging findings of supra- and infratentorial pilocytic astro- cytoma (PA) on computed tomography (CT) and magnetic resonance imaging (MRI). Thirty-two patients with PA were selected from our case series. Twenty-four patients had confirmed PA from histologic analysis. The remaining 8 patients presented optic pathway and PA was the most relevant presumptive diagnosis. All patients, 20 male and 12 female (age range 10 months-65 years), underwent unenhanced and enhanced MRI. Diffusion-weighted imaging and MR spectroscopy (MRS) were performed in 6 patients, and 6 patients underwent CT scan. The locations of the PA selected (n = 32) were: optic pathway and hypothalamic-chiasmatic region (n = 17), (n = 7), thalamus (n = 6) and cerebral hemisphere (n = 2). On MRI, most PA appeared as solid-cystic masses, iso- to hypointense on T1-weighted images and hyperintense on T2- weighted images and FLAIR, with post-contrast enhancement. Four patients presented atypical characteristics: 1 solid cerebellum PA with calcifications, 1 PA in a child without type 1(NF1) with no contrast enhancement and 1 cerebral hemisphere PA in a 65 year-old woman. PAs are regarded as grade I tumors by the World Health Organization (WHO) classification. The optic pathway and the hypothalamic-chiasmatic region, as well as the posterior fossa are the most frequent locations. PAs are typically well- circumscribed and solid-cystic lesions with low cellularity and slow growth. © 2013 Sociedad Argentina de Radiología. Published by Elsevier España, S.L. All rights reserved. Keywords: ; ; magnetic resonance imaging.

Introduction Materials and methods

Pilocytic (PA) are slow growth, highly vascular For this study, we selected 32 patients with PA from our case and well circumscribed tumors that infiltrate the surrounding series. Diagnosis of PA was confirmed on histological exami- tissues. According to the World Health Organization classifi- nation in all but 8 patients with optic pathway glioma who cation, PAs are classified as grade I gliomas. PAs have low cel- did not undergo biopsy because PA was the most relevant lularity and low mitotic activity, and metastatic spread is un- presumptive diagnosis. Twenty patients were males and 12 usual. Rarely, PAs may undergo malignant transformation1, 2. were females, with an age range between 10 months and PAs occur most commonly in children and young adults, as 65 years. All patients had a gadolinium-enhanced brain MRI 75% of cases manifest in the first 2 decades of life. These available, but only 6 had diffusion-weighted imaging and MR tumors show no gender predilection1. spectroscopy performed, and 6 patients also had a CT per- Most PAs occur near the midline, usually arising from the cer- formed. ebellum, the optic nerve and chiasm, around the third ven- Eight patients had neurofibromatosis (NF) type 1 as baseline tricle, and the region of the hypothalamus-thalamus1-3. disease. Seven of these patients had PA involving the optic The aim of this study is to report a case series and describe pathway and 1 had a PA located in the thalamic-midbrain typical and atypical imaging findings of supra- and infraten- region. torial PAs on computed tomography (CT) and magnetic reso- nance imaging (MRI).

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Results was a predominantly cystic recurrence of an operated verm- ian PA (fig. 3). The locations of the PAs selected (n = 32) were: optic path- As regards PAs located in the optic pathway and in the hy- way and hypothalamic-chiasmatic region (n = 17), cerebel- pothalamic-chiasmatic region (n = 17), 2 cases showed only lum (n = 7), thalamus (n = 6) and cerebral hemisphere (n = optic nerve involvement, with increased size and signal inten- 2) (table 1). sity on T2-weighted images in the optic nerve, with no post- PAs located in the cerebellum (n = 7) included 6 solid-cystic contrast enhancement. Five cases showed only chiasmatic masses (fig. 1) and 1 solid lesion with calcifications of atypi- involvement with increased optic chiasm size (3 with no post- cal characteristics (fig. 2). In all patients, there was hetero- contrast enhancement and 2 with intense enhancement) and geneous enhancement of the solid portion of the tumor 4 cases demonstrated involvement of both the optic nerves and peripheral enhancement of the cystic portion. One case and chiasm (2 with no postcontrast enhancement (fig. 4) and

Table 1: Our case series of pilocytic astrocytomas. Patient Age Gender Location NF1 Type Postcontrast

1 20 M Cerebellum Solid cystic Enhancement 2 1 M Cerebellum Solid cystic Enhancement 3 2 M Cerebellum Solid cystic Enhancement 4 6 F Cerebellum Solid Enhancement 5 15 F Cerebellum Solid cystic Enhancement 6 21 M Cerebellum Solid cystic Enhancement 7 5 M Cerebellum Solid cystic Enhancement 8 12 M Hypothalamus Solid No enhancement 9 25 F Hypothalamus/chiasm Solid cystic Enhancement 10 24 F Hypothalamus/chiasm Solid cystic Enhancement 11 34 F Hypothalamus/chiasm Solid cystic Enhancement 12 2 M Hypothalamus/chiasm 1 Solid Enhancement 13 7 F Hypothalamus/chiasm Solid Enhancement 14 21 F Midbrain/thalamus 1 Solid cystic Enhancement 15 38 M Midbrain/thalamus Solid cystic Enhancement 16 6 M Midbrain/thalamus Solid No enhancement 17 6 F Midbrain/thalamus Solid cystic Enhancement 18 2 M Optic nerve 1 Solid Enhancement 19 10 M Optic nerve 1 Solid No enhancement 20 3 M Optic nerve and chiasm 1 Solid cystic Enhancement 21 1 M Optic nerve and chiasm Solid Enhancement 22 3 M Optic nerve and chiasm 1 Solid No enhancement 23 4 F Optic nerve and chiasm 1 Solid No enhancement 24 27 M Cerebral hemisphere Solid cystic Enhancement 25 65 F Cerebral hemisphere Solid cystic Enhancement 26 15 M Chiasm 1 Solid No enhancement 27 10 M M Chiasm Solid cystic Scan with no IV contrast 28 11 M F Chiasm Solid No enhancement 29 18 M Chiasm Solid Enhancement 30 11 M Chiasm Solid Enhancement 31 5 M Thalamus Solid cystic Enhancement 32 7 M Thalamus Solid cystic Enhancement

IV: intravenous; NF: neurofibromatosis.

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Figure 2 Solid cerebellar pilocytic astrocytoma. (a) Computed tomography: solid hypodense image, located on the left cer- ebellar hemisphere and vermis, associated with an area of peripheral calcification (arrow). (b) MRI shows a hypointense Figure 1 Cerebellar pilocytic astrocytoma. Magnetic reso- lesion on T1-weighted imaging with significant postcontrast nance imaging: solid-cystic lesion in the cerebellum, hy- enhancement. pointense on T1-weighted imaging and hyperintense on T2- weighting imaging and FLAIR in the solid portion. The lesion collapses the fourth ventricle and shows postcontrast intense enhancement of the solid component (arrow).

2 with chiasmal enhancement (fig. 5). Five patients had solid- gion (n = 6), 3 extended to the ipsilateral lentiform nucleus. cystic tumors in the hypothalamic-chiasmatic region, with in- As regards the cases studied by MR spectroscopy (n = 6), tense postcontrast enhancement (figs. 6 and 7). In only one we identified a decrease in the N-acetylaspartate (NAA) peak case (a 3-year-old child), a solid PA of atypical characteris- and an elevation of the choline peak in the solid portion of tics was identified in the left hypothalamic region, protrud- the tumor along with elevated lipid and lactate peak (1.3-1.5 ing over the left ventricle with no chiasmal involvement and ppm). In the case of the PA of the left hypothalamus, as it was appearing hyperintense on T2-weighted images and FLAIR, a solid lesion, there was no elevation of the lipid and lactic with no post intravenous contrast enhancement (fig. 8). The acid peak, but there was an increase in the choline peak (with atypical feature of this case was the tumor location and the a choline/creatine ratio of 1.3) and a decrease in the NAA lack of postcontrast enhancement. peak (fig. 8). We identified 2 cases of PA of the cerebral hemisphere: one In the patients who had diffusion-weighted imaging per- in a 20-year-old female patient, with a PA located in the left formed, facilitated diffusion was observed in the cystic por- parietotemporal region, which had no vasogenic edema and tion of the tumor, while in the solid portion diffusion was was predominantly cystic, with a postcontrast enhancing mu- similar to that of the brain parenchyma. In the case of the left ral nodule (fig. 9); the other in a 65-year-old patient was a hypothalamic PA protruding over the third ventricle, facilitat- solid-cystic PA with calcification and involvement of the left ed diffusion was observed, suggesting low cellularity (fig. 8). thalamus (fig. 10). The atypical feature of this case was, in addition to the location of the tumor, the age of the patient at the time of diagnosis, as PAs are more common in children Discussion and young adults. Six cases of PA were identified in the thalamic-midbrain re- PAs generally appear on CT as well-demarcated lesions with gion: 3 had thalamic and midbrain involvement (figs. 11 and a round or oval shape, solid-cystlike characteristics and oc- 12), 1 was a predominantly cystic lesion with a mural nodule casional calcifications. In 94% of these tumors, there is an originating from the midbrain and involving the right thala- intense enhancement of the solid component after contrast mus (fig. 13), and the remaining 2 were circumscribed to the administration2,4. thalami, with no midbrain involvement. As regards the form At MRI, PAs appear isointense to hypointense on T1-weight- of presentation, 5 were solid-cystic lesions with post intrave- ed images, and hyperintense on T2-weighted images. These nous contrast enhancement and 1 was a solid lesion without lesions present scarce peritumor edema in comparison with postcontrast enhancement. Of all tumors located in this re- high-grade 1 glial neoplasms, and usually show intense en-

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Figure 3 Cerebellar astrocytoma (recurrence after ). (a) MRI shows in the left cerebellar hemisphere and vermis a heterogeneous, predominantly cystic, bilobed lesion that ap- pears hypointense on T1-weighted images and hyperintense on T2-weighted and FLAIR images (high protein content), with postcontrast peripheral enhancement. (b) MR Spectros- copy (short and long echo time) shows elevation in choline (red arrow) and lactate and lipid (white arrow) peaks. b

Figure 4 Pilocytic astrocytoma of the optic pathway in a 7-year-old patient with neurofibromatosis type 1: involve- ment of both optic nerves (white arrows), with increased thickness. In addition, there is involvement of the optic chi- asm (white circle), with an increase in its volume and signal intensity on T2-weighted images.

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hancement of the solid portion and peripheral enhancement Cerebellar PAs are typically cystlike masses with a mural nod- of the cystic portion. ule, composed of one or more and occasional calcifi- Histologically, PA manifests in a biphasic pattern, composed cations. The solid portion is of variable size and proportion, of a combination of glial tissue (with numerous vacuoles and ranging from a mural nodule at the periphery of the to a occasional microcysts) and compacted piloid tissue. The pi- completely solid lesion (the latter being less common). loid tissue is composed of dense sheets of elongated bipolar The association between cerebellar PA and hemorrhage is cells that demonstrate fine fibrillary processes and abundant extremely rare, but the presence of hemorrhage might be Rosenthal fibers1,4. related to abnormal vasculature within the tumor5. Complete resection is the objective of the surgical manage- MR spectroscopy shows in the solid portion of the tumor an ment of PAs, as the prognosis is excellent when PAs are totally elevated choline peak and a decrease in the NAA peak. The resected. However, PAs of the hypothalamic-optic chiasm re- choline/NAA ratio is usually elevated (ranging between 1.80 gion are not amenable to total resection because of the high and 3.40), and the choline/creatine ratio is also elevated (with rate of morbidity and mortality associated with surgery in this values similar to those reported above). Furthermore, the lac- region (including damage to the pituitary gland, optic path- tate peak may also be elevated in these tumors (evidencing way, hypothalamic structures and carotid arteries). a lactate doublet at 1.33 ppm), and there may be a minimal elevation of lipids, although not as high as that noted in glio- blastoma and metastasis1,6. Cerebellar pilocytic astrocytoma Diffusion-weighted sequences show facilitated diffusion in the cystic portion while, according to various authors, in the In pediatric patients, cerebellar astrocytomas are the most solid portion of PAs the apparent diffusion coefficient (ADC) common astrocytomas (85%)1. is within the range of 1.13 to 1.92 x 10-3 mm2/s. Rumbold

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Figure 5 Pilocytic astrocytoma of the optic pathway. MRI: (a). FLAIR images show hyperintensity in the optic chiasm and op- tic tracts; (b) T2-weighted images show enlargement of both optic nerves, predominantly on the right side (red arrows) with enlargement of the optic chiasm, which appears hyperintense (white arrow); (c) T2- and T1 post-contrast weighted images show a large mass that appears hyperintense on T2-weighted images, located in the hypothalamic-chiasmatic region, with ex- tension to the sellar region and interpeduncular cistern, which shows heterogeneous postcontrast enhancement, with a central area of low uptake, probably related to the presence of necrosis.

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et al found in posterior fossa PAs values of ADC within the cisterns. It is estimated that 5-10% of low-grade gliomas range of 1.4 to 2.09 x 10-3 mm2/s. Thus, on diffusion- may show leptomeningeal dissemination. Dissemination is weighted sequence, PAs usually have higher ADC values than believed to be part of the natural history of an untreated pi- and , in correlation with locytic astrocytoma.11 In our series, there was a case of re- tumor cellularity (which is higher in the latter two cases)7-10. currence of operated cerebellar PA, with dissemination to the Complete resection is the objective of the surgical manage- vermis and the left cerebellar hemisphere (fig. 3). ment of PAs, as the prognosis is excellent when PAs are totally The main differential diagnosis that should be considered is resected, with a 10-year survival rate of over 94% and a 20- , as PA usually appears as a cystic lesion year survival rate of 79%1. If partial resection is performed, with a mural nodule that enhances after intravenous contrast PAs may recur or even extend to the ventricular system and administration.

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Figure 6 Pilocytic astrocytoma of the optic pathway. (a and b) MRI shows a heterogeneous lesion involving the optic chiasm, with an increase in its volume associated with a cystic lesion located in the third ventricle (yellow arrow); the presence of small cysts in the optic chiasm is confirmed on T2-weighted sequence (red arrows); (c) no restriction is observed on diffusion-weight- ed images (a finding that suggests low cellularity).

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Figure 7 Hypothalamic-chiasmatic pilocytic astrocytoma. e On the previous page, MRI: (a) T2-weighted images, (b) T1- weighted images and (c) gadolinium-enhanced T1-weighted images, showing a heterogeneous and solid lesion with a cystic/necrotic centre, located in the hypothalamic/chiasmatic region, showing heterogeneous postcontrast enhancement. Small hypointense areas are observed on T2-weighted im- ages, which appear hyperintense on T1-weighted images, related to hemorrhagic areas (red arrows). On this page, (d) Diffusion-weighted imaging shows no water diffusion restric- tion; (e) MR spectroscopy demonstrates an increase in the lipid peak, in correlation with the presence of necrosis.

Supratentorial pilocytic astrocytomas in the optic nerve, whereas in adolescents and young adults they are more commonly located in the optic chiasm1. Supratentorial PAs may be located in the optic nerves, in the The association of PA with NF1 is well known. PA is the most region of the hypothalamus and chiasm, basal ganglia (thal- common tumor seen in this population (15-21%) and typi- 1,2,12 amus) or the cerebral hemisphere. In adults, PAs are more cally involves the optic nerve or chiasm . common in the supratentorial region. There are differences between optic pathway gliomas in pa- Pilocytic astrocytoma of the optic pathway and hypothalam- tients with and without NF1. In patients with NF1, optic path- ic-chiasmatic region way gliomas are characterized by enlargement and increased When PA arises within the optic nerve, it infiltrates the nerve tortuosity of the optic nerve, because of a circumferential and causes fusiform enlargement and kinking of that struc- perineural infiltration extending to the subarachnoid space. ture. In children, optic pathway PAs are more likely to arise The central portion of the optic nerve is generally preserved. At MRI, these tumors typically appear isointense on T1-

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Figure 8 Hypothalamic pilocytic astrocytoma adjacent to the third ventricle. (a) A heterogeneous lesion is observed on the left lateral wall of the third ventricle (red arrows); the lesion appears hypointense on T1-weighted images, and hyperintense on T2-weighted images, showing no enhancement after intravenous contrast administration; (b) no water diffusion restriction is observed; (c) MR Spectroscopy shows a slight increase in the choline peak and a decrease in the N-acetylaspartate peak on long echo time PRESS sequence.

weighted images and hyperintense on T2-weighted images not postcontrast enhancement. because of gliomatosis. In the optic chiasm, these tumors In patients without NF1, the characteristics of these tumors are small and homogeneous, they have no cystic component are similar to those of PAs located in other sites, as they ap- and enhancement is observed in some cases13. In our series, pear as well-demarcated masses with a solid-cystic compo- there were 8 patients with NF1: in 7 patients, the PA involved nent and postcontrast enhancement. Histologically, these the optic pathway and in 1 patient the PA was located in tumors display intraneural growth13. the thalamus-midbrain. As regards optic pathway PAs, 6 were PAs may also involve the hypothalamus. PAs of the hypo- solid lesions and 1 was a solid-cystic lesion; 4 cases showed thalamus appear as well-circumscribed tumors with a solid

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Figure 9 Cerebral hemisphere pilocytic astrocytoma. MRI. shows a cystic lesion on the left cerebral hemisphere, in the temporoparietal region; the lesion appears hyperintense on T2-weighted and FLAIR images, and hypointense on T1- weighted images, with a postcontrast enhancing mural nod- ule (arrows). a c

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Figure 10 Cerebral hemisphere pilocytic astrocytoma. (a) Computed tomography shows a cystic/solid lesion on the left tempo- roparietal region, with calcified areas (arrows) and a slight peritumor edema and mass effect with midline shift and collapse of the posterior horn of the ipsilateral lateral ventricle; (b) MRI shows hypointensity on T1-weighted images and hyperintensity on T2-weighted and FLAIR images, with heterogeneous postcontrast enhancement of the solid portion; (c) Images of the surgical procedure, showing drainage of the cystic portion and excision of the solid portion.

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Figure 11 Thalamo-lenticular pilocytic astrocytoma. CT shows a solid-cystic heterogeneous lesion on the left thalamo-len- ticular region, with intense postcontrast enhancement of the solid portion (arrow). The lesion is well circumscribed, with no associated vasogenic edema. It collapses the third ven- tricle, causing dilatation of the lateral ventricles.

component. These tumors may contain cysts and show het- erogeneous postcontrast enhancement. The typical clinical manifestations of PAs of this location are delayed pubertal development and symptoms associated with the mass effect of the tumor14. Figure 12 Thalamo-lenticular pilocytic astrocytoma. (a) Com- We report a rare case of atypical characteristics, located in puted tomography shows a solid-cystic lesion in the left the left hypothalamic region, with protrusion over the third thalamo-lenticular region. The solid portion is isodense to hy- podense, and collapses the third ventricle, with dilatation of ventricle. This tumor was solid, with no cystic component and the ventricular system; (b) MRI shows a solid-cystic lesion; the no enhancement after intravenous contrast administration. solid portion appears hypointense on T1-weighted images ADC map showed facilitated diffusion and on MR spectros- and hyperintense on T2-weighted images, with postcontrast copy, the choline/creatine ratio was 1.3. Both findings were heterogeneous enhancement of the solid portion (arrow). consistent with the low cellularity of the lesion (even lower than that of a PA of typical characteristics) (fig. 8). Pathology was confirmed by biopsy. Even if several cases of involution of optic pathway PA with NF1 have been reported, the mechanism behind it is not well known15. Tumor growth deceleration has also been report- Pilocytic astrocytomas of ed, being directly proportional to the age of the patient, and the cerebral hemispheres cases of PA involution have been reported in patients without and basal ganglia NF1. Apoptosis and a humoral and cellular immune response to the lesion are believed to play a role in the mechanism of Cerebral hemispheres and basal ganglia are rare locations PA involution16. In our series, a 3-year-old patient with NF1 for PAs, but those tumors arising in the cerebral hemisphere and PA in the hypothalamic-chiasmatic region showed partial are more likely to occur in the temporal lobes1. These are involution 3 years after initial diagnosis, having received no solid lesions with a cystic component and may have calcifi- treatment (fig. 14). cations; after intravenous contrast administration, the solid portion usually exhibits heterogeneous enhancement, while the cystic portion shows peripheral enhancement. These tu- mors show low celullarity and mitotic activity, and have slow

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Figure 13 Midbrain-thalamic pilocytic astrocytoma. (a and b) MRI shows a heterogeneous, predominantly cystic lesion, with a solid, heterogeneous nodule (red arrow) originating from the midbrain and extending cephalically, involving the right thalamus. The cystic portion is slightly hyperintense on T1-weighted images and markedly hyperintense on T2-weighted and FLAIR images due to the presence of a high protein-hemorrhagic content (blue arrow). The solid portion shows postcontrast heterogeneous enhancement; (c) Diffusion-weighted imaging shows mild water diffusion restriction in the solid portion. growth; therefore they are typically not associated with vaso- cal because of the supratentorial parenchymal location of the genic edema17. tumor and because of the patient’s age, as these tumors are Differential diagnoses to be considered include pleomorphic usually seen in children and young adults (fig. 10). xanthoastrocytoma and ganglioma, because of their similar PAs may also involve the thalamus and midbrain. According imaging characteristics. to some authors’ reports, these tumors arise at the interface In our series, a 65-year-old patient had a solid-cystic left tem- of the thalamus and midbrain, involving both. Clinical presen- poroparietal supratentorial PA with calcifications in the solid tation consists in slow and progressive spastic hemiparesis. portion and without vasogenic edema. This case was atypi- Because of their location, these lesions are considered to be

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Figure 14 Hypothalamic-chiasmal pilocytic astrocytoma with partial involution. Male patient with neurofibromatosis type 1: (a) at the age of 3 the patient was diagnosed with a glioma of the hypothalamic-chiasmatic region with increased size and hetero- geneous enhancement after intravenous contrast administration; (b) in a follow-up, 3 years later, partial involution is observed with a decrease in the lesion size and no enhancement after intravenous contrast administration

unresectable. Surgery is associated with high morbidity and At MRI, these lesions appear isointense to hypointense on T1- mortality; therefore these tumors are treated with chemo- weighted images, and hyperintense on T2-weighted images, therapy and radiotherapy18. with scarce peritumor edema. The solid portion of the lesion shows intense enhancement, while the cystic portion exhibits peripheral enhancement. MR spectroscopy demonstrates an Conclusion elevated choline peak, a decrease in the NAA peak in the solid portion of the tumor and, in some cases, an increase PAs are low-grade gliomas (classified as grade I by the World in lactate. Health Organization) more commonly located in the optic pathway, the hypothalamic-chiasmatic region and the poste- rior fossa. These tumors are typically well-circumscribed and Conflicts of interest solid-cystic lesions with low cellularity and slow growth. If total The authors declare no conflicts of interest resection is performed, patients have a high rate of survival.

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