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Table of Contents
REVIEW ARTICLE
Year : 2019  |  Volume : 2  |  Issue : 1  |  Page : 3-6

Management of optic pathway gliomas: Role of magnetic resonance imaging with diffusion-tensor imaging tractography in planning surgical resection


1 Department of Neurosurgery, Krishna Institute of Medical Sciences, Secunderabad, Telangana, India
2 Department of Neurology, Krishna Institute of Medical Sciences, Secunderabad, Telangana, India

Date of Submission12-Mar-2019
Date of Acceptance17-Mar-2019
Date of Web Publication3-Jun-2019

Correspondence Address:
Dr. Manas Panigrahi
Department of Neurosurgery, Krishna Institute of Medical Sciences, 1-8-31/1, Ministers Road, Secunderabad, Telangan
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/IJNO.IJNO_4_19

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  Abstract 


Optic pathway gliomas (OPGs) are histological low grade but widely infiltrative tumors that arise along the optic nerves, optic chiasm, tracts, diencephalic structures, and white matter radiations to the visual cortex. When OPGs progress, management strategies may differ between pre-chiasmatic and chiasmatic lesions. Radiation therapy is often utilized as the primary treatment modality, but it is typically postponed till the age of 5–7 years because of the potential for late risks such as endocrinopathy, vasculopathy, and cognitive decline. No definitive studies establish the superiority of one specific chemotherapeutic agent over others. However, a favorable side effect profile is often opted for. Surgery is considered in patients with (1) single nerve is involved, (2) progressive and disfiguring proptosis, (3) blindness, and (4) significant mass effect or hydrocephalus. Encouragingly, diffusion-tensor imaging (DTI) has emerged as a useful tool for the assessment of white matter structures, including the visual pathways. We report an interim analysis of 14 patients with OPG, at our center, a tertiary referral center. We report that magnetic resonance imaging (MRI)-DTI studies provided additional information about visual fiber arrangement in relationship to the tumor that was not evident by conventional MRI methods.

Keywords: Diffusion-tensor imaging, magnetic resonance imaging, optic pathway gliomas, surgical planning, tractography


How to cite this article:
Panigrahi M, Patel C, K. Chandrasekhar Y B, Vooturi S. Management of optic pathway gliomas: Role of magnetic resonance imaging with diffusion-tensor imaging tractography in planning surgical resection. Int J Neurooncol 2019;2:3-6

How to cite this URL:
Panigrahi M, Patel C, K. Chandrasekhar Y B, Vooturi S. Management of optic pathway gliomas: Role of magnetic resonance imaging with diffusion-tensor imaging tractography in planning surgical resection. Int J Neurooncol [serial online] 2019 [cited 2019 Dec 5];2:3-6. Available from: http://www.Internationaljneurooncology.com/text.asp?2019/2/1/3/259558




  Introduction Top


Optic pathway gliomas (OPG) are histological low grade but widely infiltrative tumors that arise along the optic nerves, optic chiasm, tracts, diencephalic structures, and white matter radiations to the visual cortex.[1] In patients with neurofibromatosis type 1 (NF1), the diagnosis is usually by magnetic resonance imaging (MRI) studies alone, followed up without intervention in the absence of radiographic progression and visual or neurological deterioration.[2] When OPGs progress, management strategies may differ between prechiasmatic and chiasmatic lesions.[3] For lesion affecting the optic chiasma, while functional vision is still preserved, surgical debulking may alleviate hydrocephalus or diencephalic symptoms.[4] However, debulking is associated with morbidity if en passant fibers providing visual function are displaced.

Radiation therapy is often utilized as the primary treatment modality, but is typically postponed till the age of 5–7 years because of the potential for late risks such as endocrinopathy, vasculopathy, and cognitive decline.[5] However, in children older than age 7 years and where “tumors are not amenable to surgery and/or tumors that have progressed despite other treatments,” radiation therapy could be a potential management option, with external-beam fractionated radiation therapy being the treatment of choice.[5] Importantly, radiation therapy has been associated with 10-year relapse-free survival rates of up to 90%, with concurrent stabilization of vision.[6],[7] In fact, early introduction of radiotherapy, that is, before visual decline or chemotherapy progression has been associated with better visual outcomes.[6],[7] However, with increasing reports on long-term survival, early radiotherapy has to be counterbalanced with the development of complications later in life. Radiation therapy techniques such as proton beam radiotherapy and fractionated stereotactic radiotherapy have been shown to limit the radiation exposure to surrounding brain tissue, thus minimizing associated late risks.[6],[7],[8] Ten years post radiation therapy, increased incidence of secondary tumor development and secondary Moyamoya disease, especially in NF1 patients, have been reported.

No definitive studies establish the superiority of one specific chemotherapeutic agent over others.[5] However, a favorable side-effect profile is often opted for. Chemotherapy is often administered either with a stand-alone drug or in combination. It usually starts with temozolomide. Depending on the standard dosage, the main interactions of chemotherapeutic drugs involve enhancing the effect of immunosuppressant drugs, ototoxic effects of aminoglycosides, and neurotoxic effects of cisplatin.[5] Whereas the main common side effects reported are myelosuppression, nausea, vomiting, hypersensitivity, Vitamin K antagonism, electrolyte abnormalities, renal impairment, hepatic impairment, and ototoxicity. Recently, Bevacizumab has been considered preferable for chemotherapy if edema associated with lesion contributes significantly to the symptoms.[5] The reported complications of Bevacizumab include hemorrhage, gastrointestinal perforation, hypertension, and wound dehiscence.

Surgery is considered in patients with (1) single nerve is involved, (2) progressive and disfiguring proptosis, (3) blindness, and (4) significant mass effect or hydrocephalus. Moreover, complete resection is considered only if the lesion is limited to one optic nerve, as it may cause blindness. In addition, ventricular shunting may be needed in the case of obstructive hydrocephalus. In conventional MRI studies, neoplastic tissue may not be distinguishable from functional visual pathway structures, making surgical planning a challenge. In fact, Bommakanti et al. have previously shown that MRI has a sensitivity of 83.33% and specificity of 50% for diagnosing optic chiasmatic-hypothalamic gliomas.[9] Perhaps, a reason a few advocate for radiation as a first step or the use of chemotherapy.[10] Encouragingly, diffusion-tensor imaging (DTI) has emerged as a useful tool for the assessment of white matter structures, including the visual pathways.[11] While DTI measures intracortical axon integrity and myelination as reflected in the fractional anisotropy (FA) of water diffusion, spatial patterns of diffusion can be discerned in three dimensions by using tractography. DTI has been previously incorporated for MRI guidance in the preoperative planning of resection of brain tumors, especially involving the optic radiations.[12],[13] Lober et al. have shown that DTI tractography is “feasible and useful for preoperative assessment, surgical planning, and resection when incorporated into image guidance.” The authors further state that seeding a regions-of-interest at the chiasm in the coronal plane allowed for maximal capture of traversing fibers, with minimal “stray” artifactual fibers unrelated to the optic apparatus. Furthermore, tracking helps reconstruct white matter connections from the optic nerve through to the chiasm, ipsilateral tract, and radiations to the occipital cortex. Moreover, monocular visual deficits corresponded to a loss or attenuation of ipsilateral prechiasmatic fibers and postchiasmatic fibers. In these cases, the loss of postchiasmatic fibers including the optic radiations may indicate anterograde transsynaptic degeneration from disruption of visual input.

In a nutshell, treatment of OPG should be a multidisciplinary approach that is based on patient's age, genetic predisposition (NF1), location of the lesion, and growth rate. A comparison of the outcome of various approaches is summarized in [Table 1]. An ideal multidisciplinary team should include ophthalmology, radiation oncology, neuro-oncology, neuroradiology, and neurosurgery.
Table 1: Overall survival and the 5- and the 10-year progression-free survival

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  Our Experience Top


As part of the current review, we report an interim analysis of 14 patients with OPG, at our center, a tertiary referral center, included eight patients where DTI was not used in comparison to six patients where DTI was used. In this preliminary study, we observed that among the eight patients where DTI was not used, while postoperatively, vision deteriorated in these four patients, the other four patients had vision similar to preoperative. Among the six patients where DTI was used, while in three patients' vision remained the same postoperatively, vision improved in the remaining three. In addition, DTI helped us to choose the surgical approach, where if the fibers were posterosuperior, fronto-orbital (pteronial) craniotomy [Figure 1] with optic canal deroofing was chosen. If the fibers were anteroinferiorly placed, midline interhemispheric transcallosal was opted for [Figure 2]. Moreover, we observed that FA value on DTI was high in the three patients whose vision improved after surgery and all three patients whose vision remained the same, had low FA values. Radiation therapy was used as the tumor progressed at latest follow-up in two of the six patients with DTI. While one patient of the two, had a WHO Grade II tumor, the second patient had an infiltrative tumor with low FA values; therefore, suboptimal resection of the glioma was preferred. The complications observed in our case series are summarized in [Table 2]. Although limited by insufficient sample size, this study demonstrates the feasibility and potential utility of applying DT tractography to the evaluation of visual fiber pathways in patients with OPG. When used, MRI-DTI studies provided additional information about visual fiber arrangement in relationship to the tumor that was not evident by conventional MRI methods.
Figure 1: (a) A 21-year-old female, preoperative scan showing optochiasmatic gliomas. (b) Postoperative follow-up at 1 year shows no tumor progression. No postoperative chemotherapy or radiotherapy. (c and d) Preoperative magnetic resonance imaging diffusion tensor imaging optic pathway showing anterior and lateral displacement of optic pathway fiber, so interhemispheric transcallosal approach used for tumor resection

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Figure 2: (a) A 4‑year‑old female, operative scan showing optochiasmatic gliomas. (b) Preoperative magnetic resonance imaging diffusion tensor imaging optic pathway showing superior displacement of optic pathway fiber, so pterional craniotomy and sub frontal approach used for tumor resection. (c) Postoperative follow‑up at 1 year shows no tumor progression. No postoperative chemotherapy or radiotherapy

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Table 2: Reported rates of complications

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Studies have previously reported that in patients with NF-1 quantification of white matter disease reveals a reduction in FA and an increase in mean diffusivity among visual pathways, probably due to reflecting microstructural aberrations of myelin.[13] However, the tractography employed by Lober et al. relied on similarities in diffusion characteristics between voxels, making the technique robust for clinical use. The authors further add that, “because DTI tractography is feasible at these sites despite grossly abnormal diffusion characteristics, lesions might be better described for surgical planning and more accurately classified based on arrangement of native fibers.”

Moreover, MRI-DTI has been shown to characterize histopathological differences between sporadic and NF1-associated OPG. Moreover, expansile intraneural growth of sporadic OPG can also be distinguished from perineural arachnoidal gliomatosis in NF1-associated tumors. Further studies will be required to determine if the various tractography fiber arrangements correspond to histopathological features. Early research suggests that diffusion characteristics may differentiate indolent lesions from those more likely to progress and require therapy. It will be interesting to see whether MRI-DTI and DT tractography will have predictive value in these lesions.

For surgical planning, the ideal technique would trace functionally relevant fibers in the vicinity of the tumor. At this point, it is unclear if tractography will be sensitive or specific for detecting vision loss. Controlling for hydrocephalus, nonneoplastic abnormalities, amblyopia, and other confounding factors would be necessary for the design of a sufficiently powered prospective study before this technique could be validated for monitoring of vision in therapeutic decisions.


  Conclusions Top


In patients with OPG, our experience reveals that DTI tractography not only helps to determine the surgical approach but also helps plan the extent of resection. In prechiasmatic lesions, DTI reveals two arrangements of visual fibers: either the fibers stop abruptly at well-defined bulbous enlargements of the optic nerve, or they traversed areas of nerve thickening. Lesions of the optic chiasma have an additional third arrangement, in which fibers of the optic nerves or tracts diverge around tumor. Importantly, significant splaying of visual fibers occurs in patients with chiasmatic lesions without measurable changes in vision. MRI-DTI and tractography provide information that could be incorporated into surgical navigation for tumor biopsy, debulking, or resection procedures. This information obtained from DTI may help design future studies to evaluate that the role of progression-free survival and overall survival of patients undergoing surgical resection of OPG either independently or in combination with radiation therapy and/or chemotherapy. Perhaps, with emergence of DTI, surgery for OPG may be considered more often than not.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

[18]



 
  References Top

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