Infratemporal Fossa Approach Workup

CT scan and MRI provide important and complementary information. A CT scan better depicts the remodeling or erosion of the bony skull base. An MRI better depicts the soft-tissue planes (including the interface of the tumor and soft tissues) and the presence of perineural and perivascular tumor. Images of both coronal and axial view MRI depicting a V3 neurilemoma are seen below. Both CT scan and MRI with contrast can be used to ascertain the relationship of the tumor to the ICA.

MR angiography (MRA) and CT angiography (CTA) are noninvasive tests that demonstrate the arterial anatomy of the ITF and brain. Angiography is preferred over MRA and CTA when preoperative embolization of the tumor is indicated (eg., juvenile nasopharyngeal angiofibromas [JNA], paragangliomas). Angiography identifies the vascularity of the tumor and its relationship to the ICA and demonstrates the cerebral circulation and its collateral vasculature. Neither of these anatomical tests, however, predicts the adequacy of the intracranial collateral blood supply after sacrifice of the ICA.

Collateral blood supply to the brain is better evaluated using single-photon emission computed tomography (SPECT) with balloon occlusion, transcranial Doppler, or angiography and balloon occlusion with xenon-enhanced computed tomography (ABOX-CT) scan. These tests predict the probability of cerebral ischemia when the ipsilateral ICA is sacrificed and, therefore, are indicated when the risk for injury or the need for sacrifice of the ICA is high. Although technically and logistically the ABOX-CT scan is more complex than other alternative tests, the authors prefer the ABOX-CT scan due to its superior sensitivity and specificity.

• During the ABOX-CT scan, a catheter with a non-detachable balloon is inserted in the ICA via the femoral artery. The balloon is inflated for 15 minutes, while the awake patient is monitored for any neurological deficit. Any neurological deficit warrants cessation of the test and places the patient in a high-risk category.
• If no deficits develop, the balloon is deflated, and the patient is transferred to a CT scan suite. A mixture of 32% xenon and 68% oxygen is administered via facial mask for 4 minutes. CT scan demonstrates the cerebral distribution of xenon, which reflects the blood flow and, thus, provides a quantitative assessment measured as cubic centimeters (cm³) of blood flow per minute per 100 grams of brain tissue (cm³/min/100 g).
• The process then is repeated after the ICA is occluded by inflation of the balloon. Special software calculates the differential of the xenon diffusion in the brain before and after balloon inflation. Using this information, the authors identify those patients at risk for an ischemic injury after sacrifice of the ipsilateral ICA.
• Remember, however, that patients can suffer a stroke due to embolic phenomena or the loss of collateral vessels in watershed areas that are not assessed by balloon occlusion testing.
• In addition, recognize that the ABOX-CT scan is performed under ideal and controlled circumstances and does not account for possible episodes of hypoxia, hypotension, or electrolyte- or acid-base disturbances, which may alter brain hemodynamics. Some have advocated the use of hypotension to increase the sensitivity of the test. In any event, preservation of the ICA is a preferred option.

Whenever possible, obtain a histological diagnosis before the extirpative surgery. Most tumors are amenable to punch or open biopsy. Some tumors (eg, angiofibromas, some neurilemomas) can be diagnosed based on their clinical and imaging characteristics. Tumors in the deeper planes may be sampled by FNAB. In the rare instance that an adequate biopsy cannot be obtained, a frozen section analysis may be obtained via a skull-base approach, preferably endoscopic. If the histological diagnosis can be established with a reasonable degree of confidence, the tumor may be resected within the same surgery. Consider, however, that frozen section analysis is not without limitation. Thus, sacrifice of critical neurovascular structures (eg, ICA, orbit, CNs) based solely on a frozen section analysis is not prudent. A staged procedure after reviewing permanent pathology sections with the pathology team is favored in these instances.

The staging evaluation is tailored to the histological type, extent, and site origin of the tumor. A CT scan of the neck is recommended to rule out regional lymphadenopathy. Patients with tumors that metastasize hematogenously (eg, sarcoma, melanoma, adenoid cystic, neuroendocrine) should undergo a PET/CT scan of the chest and abdomen and pelvis. A lumbar spinal tap for cerebrospinal fluid (CSF) cytology and a spinal MRI to rule out "drop metastasis" and carcinomatosis are recommended for patients who have parameningeal malignancies.