CNS Malignancy (Brain & Spinal Cord)

BRAIN:

• [Imaging of the Sella is covered separately, here]

Fast Facts:

• 2nd leading cause of death for men, ages 20-39
• 5th leading cause of death for women, ages 20-39
• Metastatic lesions are >10 times more common than primary lesions.

Tumor Grade:  

While there are many types and subtypes of primary CNS malignant lesions, the PET/CT radiologist can address them as 2 groups:

• “Low-Grade”: Grades I & II (20-80% 5-year survival rate)
• “High-Grade”: Grades III & IV (4-17% 5-year survival rate)

Utility of PET/CT Scan for Primary Brain Tumors: 

PET/CT is generally not used to decide whether or not a patient has a primary brain tumor (a diagnosis of brain tumor has nearly always already been made by CT or MRI prior to PET/CT scanning).  The true value of PET/CT is determining whether or not a known tumor is “high-grade” or not.

• Characterizing lesions as “low-grade” or “high-grade” (with dramatic implications on management & prognosis):
  • Low-Grade: Uptake ≈ white matter (may use contralateral centrum semiovale for comparison)
  • High-Grade: Uptake > white matter (≈ grey matter)
  • As the brain lesion may not clearly identifiable on the non-contrast CT images, it is essential to view the prior MRI (or CT) that lead to the initial diagnosis of “brain tumor”.
• Diagnosis of residual or recurrent tumor.
• Assess potential malignant degeneration of a low-grade lesion to a high-grade lesion.
• Assess response to therapy.
• Direct targeted biopsy of lesion (to its most hypermetabolic area).
• Defining target volumes for radiotherapy.
• Differentiate radiation necrosis (or chemonecrosis) from active malignancy.

Radiation Necrosis (or Chemonecrosis):

• Necrosis presents months to years after radiation therapy (rarely < 3 months post radiation).
• Difficult to differentiate necrosis from malignancy on MRI, as both demonstrate enhancement.
• Necrosis is typically non-avid on PET images. 
• Recurrent malignancy will demonstrate FDG uptake, often focal and involving only a small portion of a surgical cavity. The uptake may be intense (an easy diagnosis) or may be minimally above adjacent background uptake (a more difficult diagnosis, which may require follow-up). 
• FDG-avid recurrence should correspond to the area of enhancement noted on MRI (MRI should be done first, with images available at time of PET/CT interpretation).
• If no evidence of pathologic uptake, we report, “There is no evidence of high-grade tumor recurrence.”

Dual Time Point Imaging:

• Some advocate an additional delayed set of images at 2 hours, as tumor should continue to increase FDG uptake, while normal tissue should plateau. 
• The additional time, money, effort, radiation (and questionable utility) dissuade most from employing this technique.

Brain Metastases:

• Far more common than primary lesions (> 10 times).
• If small and at grey-white junction, can be easily overlooked.
• Commonly from lung, breast, melanoma and GI primaries.

CNS Lymphoma:

• Usually very hypermetabolic (FDG uptake is often greater than high-grade gliomas or metastatic lesions).
• Association with immunocompromised status.

False Negatives:

• Low-grade lesions
• Cystic lesions with only minimal soft-tissue component
• Cortical lesions obscured by normal intense brain uptake
• Edema may decrease uptake in a region of tumor
• Lesions < 8.0 mm (beneath the resolution of PET)

False Positives:

• Some benign lesions are FDG-avid (e.g. meningioma, pituitary adenoma)
• Seizure focus (if ictal @ time of FDG injection)
• Some low-grade lesions can be intensely avid (e.g. pilocytic astrocytoma, pleomorphic xanthoastrocytoma)

 Crossed Cerebellar Diaschisis:

• May see generalized decreased FDG uptake in the cerebellar hemisphere opposite of a supratentorial abnormality (e.g. tumor, edema, infarct, gliosis).

SPINAL CORD:

• Primary lesions are rarely evaluated with PET/CT.
• Metastatic lesions are not uncommon, appearing very focal, with uptake > liver activity.