Subtle magnetic resonance imaging differences in tegmental pilocytic astrocytomas as a caution against attempting gross-total resection: illustrative cases

Tariq Al-Saadi Departments of Pediatric Surgery, Division of Neurosurgery, and

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Steffen Albrecht Pathology, and

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Jean-Pierre Farmer Departments of Pediatric Surgery, Division of Neurosurgery, and

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Daniela Toffoli Department of Ophthalmology, McGill University, Montreal, Quebec, Canada; and

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Christine Saint-Martin Medical Imaging, Montreal Children’s Hospital, McGill University Health Center, Montreal, Quebec, Canada

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Nada Jabado Department of Pediatrics, McGill University and McGill University Heath Centre Research Institute, Montreal, Quebec, Canada

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Roy W. R. Dudley Departments of Pediatric Surgery, Division of Neurosurgery, and

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BACKGROUND

Although surgery within the tegmentum of the midbrain is challenging, resection of tegmental pilocytic astrocytomas (PAs) is a standard treatment because this has been shown to outperform chemotherapy and radiotherapy in terms of long-term tumor control. Gross total resection (GTR) assisted by intraoperative neuroelectrophysiological monitoring can be achieved with a reasonable risk-to-benefit ratio, especially for well-circumscribed tumors, but careful scrutiny of magnetic resonance imaging (MRI) is critical to surgical decision making. The authors present two cases of tegmental PAs, which appeared grossly similar on MRI and were operated on via the same surgical approach using the same intraoperative adjuncts.

OBSERVATIONS

The tumors had identical histopathological and molecular diagnoses but drastically different functional outcomes for the patients, with significant long-term complications for one of the children, which the authors believe was due to a slightly more invasive nature of this tumor. The authors demonstrate subtle preoperative MRI findings that might be potential clues to a more infiltrative nature of one PA versus another and present pathological findings supporting this argument.

LESSONS

This report serves as a reminder that not all tegmental PAs can be managed by the same surgical approach. Subtle signs of infiltration may indicate that GTR should not be attempted.

ABBREVIATIONS

CUSA = Cavitron ultrasonic aspiration; FLAIR = fluid-attenuated inversion recovery; GTR = gross total resection; iMRI = intraoperative magnetic resonance imaging; IONM = intraoperative neuroelectrophysiological monitoring; MRI = magnetic resonance imaging; PA = pilocytic astrocytoma; POD = postoperative day; SSEP = somatosensory evoked potential; WHO = World Health Organization

BACKGROUND

Although surgery within the tegmentum of the midbrain is challenging, resection of tegmental pilocytic astrocytomas (PAs) is a standard treatment because this has been shown to outperform chemotherapy and radiotherapy in terms of long-term tumor control. Gross total resection (GTR) assisted by intraoperative neuroelectrophysiological monitoring can be achieved with a reasonable risk-to-benefit ratio, especially for well-circumscribed tumors, but careful scrutiny of magnetic resonance imaging (MRI) is critical to surgical decision making. The authors present two cases of tegmental PAs, which appeared grossly similar on MRI and were operated on via the same surgical approach using the same intraoperative adjuncts.

OBSERVATIONS

The tumors had identical histopathological and molecular diagnoses but drastically different functional outcomes for the patients, with significant long-term complications for one of the children, which the authors believe was due to a slightly more invasive nature of this tumor. The authors demonstrate subtle preoperative MRI findings that might be potential clues to a more infiltrative nature of one PA versus another and present pathological findings supporting this argument.

LESSONS

This report serves as a reminder that not all tegmental PAs can be managed by the same surgical approach. Subtle signs of infiltration may indicate that GTR should not be attempted.

ABBREVIATIONS

CUSA = Cavitron ultrasonic aspiration; FLAIR = fluid-attenuated inversion recovery; GTR = gross total resection; iMRI = intraoperative magnetic resonance imaging; IONM = intraoperative neuroelectrophysiological monitoring; MRI = magnetic resonance imaging; PA = pilocytic astrocytoma; POD = postoperative day; SSEP = somatosensory evoked potential; WHO = World Health Organization

Pilocytic astrocytoma (PA) of the brainstem accounts for only 2% of all central nervous system tumors in children,1 and those specifically located in the tegmentum of the midbrain are extremely rare. In a 21-year review of all brainstem PAs, Kestle et al.2 found that only 4 (14.2%) of 28 cases were in the tegmentum. As such, these tumors are less well understood than their more common brainstem counterparts, such as tectal gliomas.3,4 In addition to ophthalmoplegia, hemiparesis, and ataxia, these patients often present with hydrocephalus due to aqueduct compression.5–7 Once some form of cerebrospinal fluid diversion has been performed, unlike in tectal tumors, resection of the mass is usually considered if the diagnosis of a symptomatic, well-circumscribed tegmental PA is highly suspected and if the tumor comes to one of the surfaces of the midbrain, especially if progressive neurological deficits are encountered. Indeed, gross total resection (GTR) can be curative, and even partial resection can improve neurological deficits7 and lead to better tumor control than chemotherapy or radiotherapy.5,6 Even authors who have emphasized the “indolent natural history” of these tumors and cautioned against resection in many cases have recommended surgery when there is clinically significant mass effect.8

With the assistance of intraoperative neuroelectrophysiological monitoring (IONM) and neuronavigation, very well-circumscribed tumors can be resected completely with low morbidity.9,10 However, operating in the critical and sensitive confines of the midbrain cannot be taken lightly due to the obvious risks to motor and cranial nerve function, not to mention concerns for other “en passage” tracts and the reticular activating system, which is critical for arousal and consciousness, especially for what are almost always histopathologically benign lesions.11 For each individual case, it may not always be easy to decide if GTR should be attempted. Even PAs that grossly appear to be well circumscribed may have subtle findings on magnetic resonance imaging (MRI) that could negatively impact surgical success and increase the risk of complications and poor functional outcomes. Here we report two cases of tegmental PAs that appeared to be grossly similar on MRI and underwent resection by the same experienced neurosurgical team via the same surgical approach but had drastically different outcomes. Despite equivalent histopathological and molecular diagnoses, in retrospect, subtle imaging differences were seen that may explain the contrasts in outcome, thus providing the valuable lesson that not all tegmental PAs are the same and not all can be resected safely even in experienced hands.

Illustrative Cases

Case 1

A previously healthy 6-year-old boy presented with a 6-month history of right-handed clumsiness. While on vacation abroad, he slowly developed right hemiparesis, dysconjugate gaze, and headaches. Locally performed MRI revealed acute hydrocephalus and a large mass lesion in the midbrain. A ventriculoperitoneal (VP) shunt was inserted, his headache completely resolved, and his hemiparesis improved somewhat. On returning, 3-T MRI (Fig. 1A–I) at our center demonstrated a 3.4 × 2.3 × 3.8–cm tumor in the posterior left aspect of the midbrain tegmentum with complete obliteration of the aqueduct. It was hypointense on T1-weighted imaging, hyperintense on T2-weighted and fluid-attenuated inversion recovery (FLAIR) imaging, had significant but heterogeneous enhancement, and did not show restricted diffusion. Importantly, it was very well circumscribed with smooth borders and no signs of infiltration into the surrounding structures. Ophthalmology diagnosed bilateral fourth-nerve palsies. After multidisciplinary discussions, it was concluded that this was likely a PA and that resection offered the best chance of symptom improvement and tumor control.

FIG. 1.
FIG. 1.

Case 1. Preoperative MRI with axial (A) and sagittal (B) T1-weighted sequences; axial T2-weighted (C), FLAIR (D), diffusion-weighted imaging (E), and apparent diffusion coefficient (F) sequences; and axial (G), sagittal (H), and coronal (I) postgadolinium contrast injection sequences. MRI during the first surgery consisted of axial (J), sagittal (K), and coronal (L) T1-weighted sequences. Postoperative MRI following the second surgery consisted of axial (M), sagittal (N), and coronal (O) postgadolinium contrast injection sequences.

The patient underwent an uneventful surgery via an occipital transtentorial approach using Cavitron ultrasonic aspiration (CUSA), IONM, and intraoperative MRI (iMRI). For the most part, under microscopic visualization, the tumor looked quite different and had a different tactile consistency from the normal brainstem. Tumor was gently peeled off normal-appearing tissue in almost all directions; however, in the most inferolateral aspect, we believed it was more difficult to discern the tumor from the normal brain. Hence, we stopped the resection. The IONM remained at normal baseline throughout the case. Postdissection iMRI (not shown) revealed a small residual where we had difficulty distinguishing the tumor from normal, but because we had already removed the IONM electrodes to perform the MRI, we did not believe it was safe to go after this remaining tumor. The patient did well postoperatively, except for the expected transient Parinaud syndrome. His hemiparesis improved, and he left the hospital after 3 days. Histopathology revealed a PA (World Health Organization [WHO] grade I) with a BRAF fusion, negative for BRAF V600E mutations and H3 K27M mutations. Ki-67 labeling was 3%–5%.

One month after his surgery, his eye movements had improved, but he was experiencing worse right hemiparesis. Repeat MRI (Fig. 1J–L) showed interval growth of the residual tumor with compression of the left cerebral peduncle. Upon discussing the case at the tumor board and with the child’s mother, the decision was made to proceed with a second surgery for resection of the rapidly reexpanding tumor. His second surgery via the same approach and IONM was uneventful; this time the postdissection iMRI revealed GTR, and his symptoms resolved. More than 5 years after his surgeries, he remains well with complete resolution of his hemiparesis and near-complete resolution of ophthalmoplegia. His MRI has shown no residual (Fig. 1M–O), and he received no adjuvant treatments.

Case 2

A 25-month-old boy presented with a 1-year history of progressive abnormal eye movements and a recent history of irritability and headaches. MRI (Fig. 2A–I) revealed acute hydrocephalus and a 2.5 × 2.4 × 2.7–cm tumor centered on the right midbrain tegmentum and completely obscuring the aqueduct of Sylvius. It was hypointense on T1-weighted sequences and hyperintense on the T2-weighted and FLAIR sequences, and it showed very faint and patchy gadolinium enhancement. There was no restricted diffusion. His neurological examination result was normal, except for his subtle eye movement abnormalities with partial right third-nerve palsy. An uneventful endoscopic third ventriculostomy was performed the same day. It was believed that this tumor was most likely a PA. Upon discussion with our neuro-oncology group, surgery was recommended as the best option for long-term tumor control and to address his progressive symptoms.

FIG. 2.
FIG. 2.

Case 2. Preoperative MRI with axial (A) and sagittal (B) T1-weighted sequences; axial T2-weighted (C), FLAIR (D), diffusion-weighted imaging (E), and apparent diffusion coefficient (F) sequences; axial (G), sagittal (H), and coronal (I) postgadolinium contrast injection sequences. MRI during the first surgery consisted of axial (J), sagittal (K), and coronal (L) T2-weighted sequences. Five-year postoperative follow-up imaging consisted of axial (M) and sagittal (N) T2-weighted sequences and coronal T1-weighted sequences (O).

An occipital transtentorial approach was performed with IONM and iMRI, and CUSA was used to resect the tumor. Once within the tumor, IONM was checked every 5 minutes. In this case, it was difficult to discern tumor from normal brainstem, and neuronavigation was relied on greatly to regularly confirm that we were inside the abnormal tissue. Late in the case, when it was believed that approximately three-fourths of the tumor had been resected, there was a >80% drop in the somatosensory evoked potentials (SSEPs) on the left side; the brainstem auditory evoked responses and motor evoked potentials did not change. Because of concern that normal tissue had been transgressed, the resection was halted. It was confirmed that the blood pressure was adequate and that there was no change in the anesthesia agents. After more than 20 minutes, the SSEPs did not recover. With this, the surgical site was closed, the IONM electrodes were removed, and postdissection iMRI performed (Fig. 2J–L), which showed no obvious complication or other unexpected findings, with a sizable residual. Importantly, MRI confirmed that the areas of resection had stayed within the lesion, but in some areas, particularly anteromedially, the surgery had come very close to the normal tegmentum. Thus, it was suspected that the surrounding midbrain structures, including at least the medial lemniscus, had been irritated and hence the decrease in SSEP signals.

Upon showing early signs of breathing on his own, and without any MRI signs of midbrain injury, the patient was extubated. However, he did not wake up further at that time, could not protect his airway, and was reintubated almost immediately upon arrival in the pediatric intensive care unit. An urgent postoperative CT scan (Fig. 3A–C) showed no bleeding, edema, or other complications. Despite this reassuring imaging, the patient did not wake up fully for another 3 weeks, with a consistent neurological examination in the first few postoperative days (PODs) showing minimal reactivity to pain, no spontaneous eye opening, and a Glasgow Coma Scale score of 6T.

FIG. 3.
FIG. 3.

Case 2. Immediate postoperative CT scanning consisted of axial (A), coronal (B), and sagittal (C) sequences. POD 4 brain MRI included axial (D) and coronal (E) T2-weighted sequences; sagittal postgadolinium injection (F); and axial FLAIR (G), diffusion-weighted imaging (H), and apparent diffusion coefficient (I) sequences.

A 24-hour electroencephalography showed no epileptiform activity. Repeat MRI on POD 4 (Fig. 3D–I) again confirmed no evidence of complications. Toward the end of the first postoperative week, he began having some minimal movements and a spontaneous cough. The following week, he started to have spontaneous eye opening and proximal limb movements. He remained on weaning dexamethasone for 10 days with the hope that edema was playing some role in his condition, and on POD 11, he was started on amantadine due to its potential to enhance arousal from coma via its dopaminergic effects.12–14 He continued to improve, and, when he was more awake, it was clear that he had a right-sided third-nerve palsy and mild left-sided hemiparesis.

He was extubated 23 days after the surgery and was consistently very irritable and was mute, findings that were most consistent with posterior fossa syndrome in his context, as the dentatothalamocortical tract, which passes through the midbrain, is believed to be involved in this entity. He stayed in the hospital for almost 3 months, during which time we worked regularly with physiotherapy, occupational therapy, and speech therapy to help the patient relearn how to walk, eat, and speak, and eventually he was transferred to a rehabilitation center. About 2 years later, he had surgery for the strabismus and ptosis resulting from his right third-nerve palsy, and he continued to improve in terms of his speech and gait. Five years after the surgery, he was active and going to school and was quite independently functional, but he continued to have a speech delay, ataxic gait, and a very mild left-sided hemiparesis.

The histopathological diagnosis of his tumor was a PA (WHO grade I), with a BRAF tandem fusion, but negative for the BRAF V600E and H3 K27M mutations. Ki-67 labeling was 3%–5%. Interestingly, there were scattered entrapped normal neurons. During the patient’s more than 5-year follow-up, the residual tumor slowly progressed on MRI (Fig. 2M–O), particularly in the first 3 years following his surgery, gradually filling in the surgical cavity and extending somewhat into the right superior cerebellar peduncle. Despite this radiological progression, he continued to improve neurologically. Therefore, no adjuvant treatment was started, and the last two MRI scans obtained 1 year apart revealed no interval progression.

Discussion

The tegmentum, the central part of the midbrain posterior to the cerebral peduncles and anterior to the tectum, contains a vast array of crucial bilateral structures specific to this area, as well as multiple en passage motor and sensory conduits that allow signals to travel from the cerebrum to spinal cord and vice versa.15 These include the substantia nigra, the red nuclei, the central tegmental tract, the medial longitudinal fasciculi, the bilateral oculomotor nerve nuclei complexes and trochlear nerve nuclei, the medial lemniscus, the spinothalamic tract, and the reticular formation, among others.15–17 The only way that tumor surgery can be performed safely in this area is if both (1) the tumor comes close to an approachable surface of the midbrain and (2) the tumor has pushed the aforementioned structures away and has not simply invaded through them; this is most critical. Among brainstem and midbrain tumors, tegmental tumors themselves are quite rare, and few studies have detailed outcomes after resection specifically for the tegmentum tumor cases.2,6,8,18 Miyamoto et al.19 described the GTR of two PAs involving the midbrain tegmentum via the subtemporal approach. In one case, the tumor was resected completely without neurological deficit, but for the other the patient, complete removal of the tumor led to worsening of his hemiparesis, which was said to be “persistent.”19 Tsuboi et al.20 reported GTR of a tegmental PA following radiotherapy. The patient had persistent mild hemiparesis, which was present even before the surgery, but her facial weakness, eye movements, and auditory function improved following the surgery. Although rare reports such as this one have described good functional outcomes following the resection of tegmental tumors, we speculate that there could be a reporting bias where surgical cases complicated by postoperative deficits may not be described.

Observations

Here, we described two pediatric PAs in almost the exact same location of the midbrain tegmentum with identical histopathological and molecular diagnoses and that were operated on by the same neurosurgical team using the same surgical approach and intraoperative adjuncts. Yet, one patient had no unexpected complications, whereas the second remained in a coma for 3 weeks and experienced a third-nerve palsy and severe posterior fossa syndrome. The intraoperative decrease in SSEPs and the combination of postoperative symptoms, including coma, posterior fossa syndrome, and third-nerve palsy suggest that normal neural tracts and nuclei that populate the tegmentum (discussed above) were at least partially within the perceived borders of the tumor. In retrospect, there were subtle differences in the imaging characteristics of these two tumors that may support this hypothesis, thus explaining the patients’ greatly divergent outcomes.

In the first case, the tumor was not only very well-circumscribed but also almost perfectly spheroid in shape, without any irregularities or signs of nodularity along its borders (Fig. 4A). However, in the second case, the tumor was less spherical and more irregular or nodular at its periphery, a finding that now makes us speculate that these areas of irregular contour are areas where the tumor bends around or even infiltrates through normal neural tracts (Fig. 4B). Furthermore, at some borders, there was a subtle blurring that could easily be mistaken for edema, but this might in fact be an area of tumor infiltration. Finally, the first case had more avid and extensive gadolinium enhancement, whereas the second case had very faint and limited gadolinium uptake. Indeed, avid enhancement has been associated with benign pathology for brainstem tumors, specifically suggesting PA. However, enhancement in these tumors is known to be highly variable,2,4,6,8,18–21 and, to our knowledge, there is no study indicating that greater or more extensive enhancement impacts tumor invasiveness. Intraoperatively, under the microscope, the first tumor, for the most part, looked nothing like normal brain; it was dark gray or brown and somewhat gelatinous and was easily aspirated using the CUSA. However, the second tumor was not so obviously different from the normal brain; it did not have the same frank grayish-brown color; it was only slightly off-white in color; and it was not at all gelatinous and took more work to aspirate it. We never found a clear interface that we could confidently say was a tumor border. Finally, although the histopathological analysis of these two tumors provided the identical diagnosis and molecular genetic characteristics, the pathology specimen in the second case contained what were most consistent with entrapped normal neurons, suggesting the tumor was able to infiltrate between such cells as opposed to pushing them away.

FIG. 4.
FIG. 4.

Comparison of axial T2 images of the tegmental PAs from both patients. Note that the tumor in case 1 (A) shows a very sharp and distinct border with almost no blurring between the tumor margins and the surrounding midbrain, whereas the tumor in case 2 (B) has a less distinct interface with the surrounding tegmentum, including a more irregular margin, and a subtle blurring of tumor into what appears to be normal midbrain.

Lessons

The lesson from these two cases is that not all grossly well-circumscribed tegmental PAs are the same, despite even having the same histopathological and molecular diagnosis. Some have perfectly clear and smooth borders between the tumor and normal brain, whereas some appear to be slightly more irregular and/or blurred at this interface, which may suggest that they are either more infiltrative or more readily wrap around tracts and fascicles, such that normal brain could be incorporated into the periphery of the tumor. In such a case, extra care would have to be taken as one approaches the tumor borders, and it would be safer not to attempt GTR but to aim for a maximum safe resection, conceding that a thin rim of tumor will be left behind at the interface with normal tissue. This distinction has been reported for PAs of the medulla3 and is akin to the way one would approach a spinal cord astrocytoma or ganglioglioma (i.e., usually leaving a thin tumor rim behind), as opposed to a spinal cord ependymoma, which can usually be resected completely thanks to its distinct, noninfiltrative tumor margin.22,23 As stated above, in retrospect, subtle clues could be seen on MRI for the second case to suggest a distinct margin would likely not be found, as such GTR would not be possible. Perhaps the faint and heterogeneous gadolinium enhancement and the microscopic appearance of the tumor were additional hints of its more infiltrative nature.

The most common genetic alteration in PAs is the tandem duplication resulting in KIAA1549-BRAF fusion proteins, which occur in 70% of PAs, including our two cases. Indeed, PA may be diagnosed by means of classical histopathology or by a low-grade piloid astrocytic neoplasm with a solitary mitogen-activated protein kinase alteration, such as KIAA1549-BRAF tandem duplication/fusion. Other genetic changes include loss of NF1 wild type, BRAF V600E mutation, FGFR1 mutations, fusion, and duplications, among others.24 We speculate that yet unknown genetic differences in our two tumors cause one PA to be more invasive than another, which is likely influenced by the surrounding microenvironment; perhaps younger patients, such as our second case, have immature neural tracts25–27 that cannot resist the slow, indolent infiltration of even the most benign tumors.

Fortunately, IONM prevented us from further encroachment into normal tissue, which could have led to an even worse outcome. Nonetheless, the patient had a severe postoperative neurological injury with a 3-week-long coma, third-nerve palsy, posterior fossa syndrome, and hemiparesis, Thankfully, he has made a tremendous recovery following several years of rehabilitation, but, even with this improvement, his neurodevelopment and quality of life were severely affected, which cannot be dismissed. As such, this single case has changed our practice: Each time a grossly well-circumscribed tegmental tumor is seen, we look more closely for (1) any subtle blurring of the tumor-normal interface, (2) even the slightest irregularity of the tumor borders, and (3) a faint and/or limited gadolinium enhancement pattern. In patients with progressive neurological deficits, if these characteristics are noted, an IONM-guided biopsy or very conservative debulking is recommended, which is even more justified because of the benign pathology and potential long-term stability of these lesions.8 For truly well-circumscribed tumors without these MRI characteristics, an attempted GTR aiming to cure the patient is recommended, but again very close adherence to IONM (i.e., checking every 2–3 minutes) is performed once the brainstem is entered.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: Dudley, Al-Saaadi, Farmer. Acquisition of data: Dudley, Al-Saaadi, Albrecht, Farmer, Saint-Martin, Jabado. Analysis and interpretation of data: Dudley, Farmer, Toffoli, Saint-Martin, Jabado. Drafting the article: Dudley, Al-Saaadi, Albrecht, Jabado. Critically revising the article: all authors. Reviewed submitted version of manuscript: Dudley, Farmer, Toffoli, Saint-Martin, Jabado. Approved the final version of the manuscript on behalf of all authors: Dudley. Administrative/technical/material support: Farmer. Study supervision: Dudley, Farmer. Figure composition: Saint-Martin.

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  • FIG. 1.

    Case 1. Preoperative MRI with axial (A) and sagittal (B) T1-weighted sequences; axial T2-weighted (C), FLAIR (D), diffusion-weighted imaging (E), and apparent diffusion coefficient (F) sequences; and axial (G), sagittal (H), and coronal (I) postgadolinium contrast injection sequences. MRI during the first surgery consisted of axial (J), sagittal (K), and coronal (L) T1-weighted sequences. Postoperative MRI following the second surgery consisted of axial (M), sagittal (N), and coronal (O) postgadolinium contrast injection sequences.

  • FIG. 2.

    Case 2. Preoperative MRI with axial (A) and sagittal (B) T1-weighted sequences; axial T2-weighted (C), FLAIR (D), diffusion-weighted imaging (E), and apparent diffusion coefficient (F) sequences; axial (G), sagittal (H), and coronal (I) postgadolinium contrast injection sequences. MRI during the first surgery consisted of axial (J), sagittal (K), and coronal (L) T2-weighted sequences. Five-year postoperative follow-up imaging consisted of axial (M) and sagittal (N) T2-weighted sequences and coronal T1-weighted sequences (O).

  • FIG. 3.

    Case 2. Immediate postoperative CT scanning consisted of axial (A), coronal (B), and sagittal (C) sequences. POD 4 brain MRI included axial (D) and coronal (E) T2-weighted sequences; sagittal postgadolinium injection (F); and axial FLAIR (G), diffusion-weighted imaging (H), and apparent diffusion coefficient (I) sequences.

  • FIG. 4.

    Comparison of axial T2 images of the tegmental PAs from both patients. Note that the tumor in case 1 (A) shows a very sharp and distinct border with almost no blurring between the tumor margins and the surrounding midbrain, whereas the tumor in case 2 (B) has a less distinct interface with the surrounding tegmentum, including a more irregular margin, and a subtle blurring of tumor into what appears to be normal midbrain.

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