191Metastatic lesions of the posterior fossa are
often well suited for SRS intervention, yet SRS is
not without complications. Brain edema after
SRS is a well-described phenomenon, and the
small space of the posterior fossa amplifies these
effects. Judicious use of steroid therapy and a
slow wean over a couple of weeks can overcome
most brain edema issues. Post-SRS contrast-
enhanced MRI often demonstrates an apparent
increase in tumor size and surrounding edema,
but this increase often does not truly represent
tumor progression [ 85 ]. Tumor “pseudoprogres-
sion” on radiographic imaging is a complicating
factor for predicting patient prognosis and plan-
ning future therapies after initial intervention. We
tend to ignore early small increases in enhancing
volume, especially those clearly within the
treated field. Persistent increasing volume or
development of symptoms not relieved with ste-
roids leads to surgical resection if possible or ste-
reotactic biopsy if the lesion is not amenable to
open resection. Despite these complicating fac-
tors, SRS is used to treat small metastatic lesions
to the posterior fossa, especially those that are
deemed unresectable because of their proximity
to vital structures. We propose that SRS can be
used for metastatic lesions to the posterior fossa
in the following situations:
- Lesions deemed unresectable due to their
proximity to eloquent brain - Lesions <3 cm or without any mass effect
- Up to three lesions in the posterior fossa (or
five total including lesions in supratentorial
compartment) in the absence of mass effect
Relative contraindications for SRS include
space-occupying lesions and symptoms of brain-
stem or aqueductal compression and hydrocepha-
lus. Resection of the offending lesions in these
cases should be expedited, and SRS may be con-
sidered as an adjunct therapy or for locally recur-
rent lesions. When multiple lesions are present,
WBRT should always be considered. If the
known cancer is melanoma, renal cell carcinoma,
or sarcoma, we tend to favor SRS for up to five or
six lesions. For other cancers, our threshold is
typically three lesions but can be up to five
lesions. Radiosurgical treatments have become a
mainstay in the neurosurgical oncologist’s prac-
tice. The indications for these therapies appear to
be increasing, and future clinical analyses teasing
apart optimal integration of SRS with other che-
motherapeutics will undoubtedly advance the
field of neuro-oncology.Conclusions
Metastatic lesions to the posterior fossa present
complex clinical and surgical challenges for the
neurosurgical oncologist. With a ratio of metasta-
ses to primary brain tumors of approximately
10:1, these lesions represent a large volume of
the neurosurgeon’s caseload. Despite the prog-
ress made in the past decades, devising optimal
treatment strategy paradigms that incorporate a
patient’s overall disease state, the tumor’s loca-
tion, and tumor biology remains a central
problem.
Surgical approaches to the posterior fossa
necessitate specialized care and perioperative
consideration for hydrocephalus, cerebellar mut-
ism, and leptomeningeal spread of metastatic
lesions. The neurosurgical oncologist needs to be
well versed in posterior fossa anatomy, standard
cranial base approaches, and indications and
contraindications for these procedures in the con-
text of metastatic neoplasms. Tumors should be
removed en bloc whenever possible, paying
respect to the varying tumor biologies that are
encountered. Many lesions can be targeted with
SRS, and often SRS and surgical approaches are
combined for optimal therapy. Moving forward,
the optimization of SRS promises to influence
therapeutic decision making well into the future.
In the past, surgical intervention of lesions in
the posterior fossa was often deemed futile,
owing to the often-devastating outcomes of sur-
gery. Today, techniques marrying MRI and real-
time neuronavigational software allow tumors to
be removed in previously inoperable regions of
the brain. SRS has further reduced the invasive-
ness of treating brain metastases, yet evidence
supports the superiority of surgical excision in
many instances. Detailed knowledge of the clinical13 Metastasis to the Posterior Fossa