Primary Brain Neoplasms



Primary Brain Neoplasms


Todd M. Blodgett, MD

Alex Ryan, MD

Marios Papachristou, MD





Key Facts


Terminology



  • Low grade gliomas (LGG), astrocytomas, glioblastoma multiforme (GBM)


Imaging Findings



  • MR shows variable amounts of enhancement, with most low grade gliomas showing little if any enhancement


  • High grade tumors with extensive enhancement


  • FDG PET shows little activity in low grade gliomas and increasing amounts of FDG uptake in higher grade tumors


  • MR is first choice to show position and extent of tumor


  • MR: FLAIR images show extent of vasogenic edema


  • FDG PET: Increased uptake in higher grade gliomas & astrocytomas, pilocytic astrocytomas


  • GBM will typically enhance and may have necrosis centrally


  • Low grade gliomas should have no enhancement


  • CNS lymphoma, high grade glioma, and metastatic tumor show enhancement on MR


  • Low grade gliomas show mild FDG uptake, generally more than normal white matter, much less than normal cortex


Top Differential Diagnoses



  • Metastases


  • Abscess


  • Infarct


  • Radiation Necrosis


Diagnostic Checklist



  • Initial imaging evaluation should include MR with contrast ± FDG PET






Axial FLAIR MR shows abnormal signal image in a patient with a glioblastoma multiforme treated with gamma knife. Differential is radiation necrosis vs. recurrent tumor.






Axial PET shows correlative intense FDG activity image, compatible with residual/recurrent tumor rather than radiation necrosis.


TERMINOLOGY


Abbreviations and Synonyms



  • Low grade gliomas (LGG)


  • Astrocytomas


  • Glioblastoma multiforme (GBM)


  • Other primary brain neoplasms


Definitions



  • Imaging of neoplasms of glial or astrocyte origin


  • World Health Organization (WHO) grade I



    • Most are benign


    • Example: Pilocytic astrocytoma


  • WHO grade II



    • Benign to semi-benign


    • Example: Astrocytoma and oligoastrocytoma


  • WHO grade III



    • Semi-benign to malignant


    • Example: Astrocytoma


  • WHO grade IV



    • Malignant


    • Example: GBM


    • Two types



      • Primary (de novo)


      • Secondary (degeneration from lower grade tumors)


IMAGING FINDINGS


General Features



  • Best diagnostic clue



    • MR shows variable amounts of enhancement



      • Most low grade gliomas show little if any enhancement


      • High grade tumors show extensive enhancement


    • MRS shows abnormal choline-to-creatine (Cho:Cr) ratio and decreased N-acetylaspartate (NAA)


    • FDG PET shows little activity in low grade gliomas and increasing amounts of FDG uptake in higher grade tumors


  • Location



    • Intra-axial


    • 2/3 supratentorial and 1/3 infratentorial for low grade gliomas


    • Most anaplastic astrocytomas and GBMs are hemispheric


  • Size




    • Variable, from a few millimeters to several centimeters


    • Smaller tumors < 6 mm often not visualized by FDG PET


  • Morphology



    • Variably sized intra-axial tumor ± enhancement with surrounding vasogenic edema


    • Gliomas tend to be diffuse without sharp border, especially WHO grade II tumors


Imaging Recommendations



  • Best imaging tool



    • MR is first choice to show location and extent of tumor


    • MR



      • Primary tumor usually demonstrates extent of enhancement


      • Low grade tumors may not show abnormal enhancement


      • FLAIR images show extent of vasogenic edema


      • May show mass effect, hemorrhage, necrosis, and signs of increased intracranial pressure


    • FDG PET



      • Increased uptake in pilocytic astrocytomas and higher grade gliomas & astrocytomas


      • Can estimate grade and malignancy of tumor before operation, as well as show tumor extent and heterogeneity


  • Protocol advice



    • FDG PET



      • Minimize auditory and visual stimulation during FDG uptake phase


      • Dynamic acquisition


  • Additional nuclear medicine imaging options



    • SPECT


  • Other PET tracers for neurooncology (investigational)



    • C-11 Methionine: Amino acid transport


    • C-11 Tyrosine: Amino acid transport


    • C-11 Choline: Membrane synthesis, proliferation


    • F-18 Fluorothymidine: Proliferation


  • Correlative imaging features



    • CT: Variable appearance, difficult to see without contrast unless large


    • MR: Most primary CNS tumors will show some enhancement, except low grade gliomas (grade I-II); usually accompanied by vasogenic edema


CT Findings



  • General



    • NECT



      • Ill-defined mass occasionally with calcifications (up to 20% in LGG, rare in anaplastic and GBM)


      • Often mass may not be visible on a noncontrast study


    • CECT



      • Low grade gliomas should have no enhancement


      • GBM will typically enhance and may have necrosis centrally


  • GBM



    • NECT



      • Irregular isodense or hypodense mass with central hypodensity representing necrosis


      • Marked mass effect and surrounding edema/tumor infiltration


      • Hemorrhage not uncommon


      • Calcification rare (related to low grade tumor degeneration)


    • CECT



      • 95% have strong heterogeneous irregular rim enhancement


  • Low grade astrocytoma



    • NECT



      • Ill-defined homogeneous hypo-/isodense mass


      • 20% calcified; cysts are rare


      • Calvarial erosion in cortical masses (rare)


  • CECT



    • No enhancement or very minimal



      • Enhancement should raise suspicion of focal malignant degeneration


MR Findings



  • FLAIR




    • Typically will show a larger area of involvement representing edema


  • T1 C+



    • No enhancement with low grade tumor


    • Variable amounts of enhancement, mass effect, and central necrosis with GBM


  • MRS



    • Elevated Cho:Cr ratio and decreased NAA


  • CNS lymphoma, high grade glioma, and metastatic tumor show enhancement on MR


Nuclear Medicine Findings



  • Metabolic activity in primary glial and astrocytic tumors correlates with tumor grade and prognosis


  • Low grade gliomas show mild FDG uptake



    • Generally more than normal white matter, much less than normal cortex


  • Higher grade tumors have increasing FDG activity, with GBM being very FDG avid, as much or more than normal cortex


  • PET/CT neuronavigation-guided surgery can achieve total tumor resection in 31% of cases vs. 19% in conventional operation


  • High grade gliomas show significantly higher SUV average and SUV maximum than metastatic tumors



    • However, considerable overlap between these two tumors exists


    • FDG accumulation alone is unlikely to be adequate in clinical setting


  • SUV max of 15 used for cutoff of high grade glioma and lymphoma


  • Using cutoff SUV of 15, lymphoma can be excluded, and differential can be narrowed to high grade glioma vs. metastatic brain tumor


  • SUV max most accurate parameter for distinguishing CNS lymphoma from other brain tumors



    • CNS lymphoma typically has highest uptake of primary brain tumors


  • SUV in primary brain tumor dependent on variety of factors



    • Plasma glucose level, steroid treatment, tumor size and heterogeneity, time after injection, and previous irradiation


    • Steroid treatment may decrease FDG uptake in CNS lymphoma


DIFFERENTIAL DIAGNOSIS


Metastases



  • Primary cannot be differentiated from a metastatic lesion by imaging



    • Whole-body FDG PET can help identify primary lesion if outside the brain


  • Multiple ring-enhancing lesions


  • History of malignancy makes this diagnosis the most likely


Abscess



  • Usually cannot be differentiated by imaging


  • Patients usually have other infectious symptoms such as fever and elevated white blood cell count


Infarct



  • Little or no FDG uptake


  • May conform to a vascular distribution


Multiple Sclerosis (MS)



  • Tumefactive MS can look similar to an intracranial neoplasm


Radiation Necrosis



  • PET negative in most cases


  • In contrast, high grade tumors tend to have increased levels of FDG uptake


Vasculitis



  • Usually multiple smaller areas of involvement


  • Usually no enhancement


PATHOLOGY


General Features



  • Genetics: Loss, mutation, or hypermethylation of tumor suppressor gene TP53


  • Etiology: Variable


  • Epidemiology



    • Gliomas



      • Incidence is 6-8/100,000, with 50% being malignant subtypes


    • GBM



      • 3-4/100,000; ≈ 50% of all gliomas are GBM


    • Younger patients tend to have lower grade gliomas with grade increasing in older age groups


Gross Pathologic & Surgical Features



  • Tumor is difficult to distinguish from normal or edematous brain tissue at operation



    • Percentage of complete removal by routine surgery is disappointing, leading to poor prognosis


    • Genuine total removal of glioma probably impossible because of diffuse growth and location


Microscopic Features



  • Grade depends on



    • Degree of cellularity


    • Cellular pleomorphism


    • Mitotic figures


    • Necrosis


    • Vascular proliferation


CLINICAL ISSUES


Presentation



  • Most common signs/symptoms



    • Various neurologic symptoms



      • Headaches


      • Seizures


      • Visual disturbances


  • Other signs/symptoms: Other symptoms related to mass effect or hemorrhage


Demographics

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Sep 22, 2016 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Primary Brain Neoplasms

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