Background and Purpose: Skull base tumors are notoriously difficult to treat due to the inherent high risk
of complications from a large number of neurovascular structures within an anatomically
dense area. In the management of skull base lesions, craniofacial and endonasal approaches
have become viable options either in conjunction or in isolation. However, accurate
postoperative imaging remains an issue due to changes in anatomy from surgical defects,
reconstructions, or any grafts. Hence a solid fundamental understanding of anticipated
postoperative imaging features and expected deviations become crucial for subsequent
imaging surveillance.
Materials and Methods: We present a review of the imaging features involved in diagnosis and treatment surveillance
for skull base tumors.
Results: Each imaging modality has a unique range of practical applications. CT is often utilized
for good soft tissue resolution, excellent assessment of bone, detection of cervical
adenopathy, and rapid image acquisition for patients who are medically unstable or
unable to maintain a still supine position. Similarly, magnetic resonance imaging
(MRI) provides greater soft tissue resolution and is the modality of choice when investigating
dural invasion or perineural spread. Diffusion weighted imaging (DWI) MRI is an adjunct
to CT and/or MRI in the early post-treatment period to elucidate tumor recurrence
from normal expected changes. FDG PET/CT has limited utility in the immediate 12 week
post-treatment phase due to the higher likelihood of false positive and negatives
from inflammation and vascular compromise respectively.
The ability to detect asymptomatic recurrence is illustrated by frontal lobe edema
and differentiation of recurrent or residual tumors from retained secretions for MRI
of craniofacial resections, defect obliteration and gliosis in the adjacent cerebellar
brain parenchymal from MRI of transpetrous approaches, and hypointense facial nerve
visualization on T2-weighted MRI. On CT imaging, the tumor presents with similar attenuation
to skeletal muscle and differentiation from granulation tissue is difficult. Thus
any mass originating in the primary tumor bed, particularly those with progressive
enlargement from serial imaging, should raise suspicion of recurrence.
Following skull base reconstruction, the majority of postoperative complications occur
acutely in the immediate period, including seroma/fluid retention, fistula formation,
infection/abscess and flap necrosis. For fluid collections CSF leaks and chylous fistulas
are the primary concern. On CT and MR imaging, particularly in the inferior lower
left neck, a peripherally enhancing fluid collection may represent a chylous leak,
a hematoma, an abscess or a seroma. MR is indicated in situations where fluid or soft
tissue is subjacent to a bony defect it cannot be reliably distinguished or identified
on CT, thus T2 weighted imaging complimented by fast imaging employing steady-state
acquisition (FIESTA) sequence allows differentiation of herniation contents.
Conclusion: Knowledge of the expected appearance following open and endoscopic surgical reconstruction,
as well as radiation therapy, is necessary to successfully delineate complications
and recurrences. An understanding of the latencies involving tumor recurrence and
early versus late complications can be aligned with the clinical context to formulate
a logical approach to equivocal findings.