Diffuse (G2) astrocytomas typically arise from deep, midline regions (ie, the thalamus) and are ill-defined with blurred anatomical boundaries from tissue infiltration (hence their categorization as "infiltrative astrocytomas").
Anaplastic astrocytomas (G3) are better circumscribed than diffuse astrocytomas and have a higher proclivity for contrast enhancement.
Histopathology
Moderate cellularity, overall: anaplastic astrocytomas will have more cellularity than diffuse astrocytomas.
The term "diffuse" helps us remember the lack of cellular concentration in a grade 2 astrocytoma.
Diffuse astrocytomas comprise oval, hyperchromatic nuclei with rare mitoses.
Anaplastic astrocytomas comprise greater degrees of pleomorphic (variable-shaped) atypical nuclei and mitoses than diffuse astrocytomas, a greater tendency for hemorrhage, and a greater likelihood of having multinucleated cells.
Astrocytomas bear a threadlike, filamentous background from fibrillary processes.
GFAP (Glial fibrillary acidic protein) staining highlights these processes.
This can be remembered because astrocytomas are glial tumors.
Consider that reactive astrocytes, instead, have star-like, elongated processes.
Variants
Fibrillary:
Minimal cytoplasm. The sparse cytoplasm gives the cells a "naked nuclei" appearance.
Gemistocytic:
Gemistocytes - tumor cells with bulging (plump), eosiniphilic cytoplasmic bellies and eccentric nuclei.
Protoplasmic:
Astrocytes have sparse processes, oval-shaped nuclei, minimal cytoplasm, and a microcystic background.
Genetic Mutations
Anaplastic Astrocytoma
Common Mutations
TP53
IDH1
ATRX
Note that 1p19q deletion is common in oligodendrogliomas but not in anaplastic astrocytoma.
Prognosis
In diffuse astroctyomas, the prognosis is far better in children than in adults: the majority of diffuse astrocytomas in adults will undergo malignant transformation; whereas, in children, the minority of diffuse astrocytomas will do so.
In anaplastic astrocytomas, survival is typically poor (often < 1 year), regardless of the patient's age.
Mixed oligoastrocytomas have a better prognosis than pure astrocytomas (but worse prognosis than pure oligodendrogliomas).
Radiation
Note that radiation is an established risk factor for the development of glioma.
Typically, there is radiation for a CNS tumor that later leads to development of an aggressive glioma, later in life.
Also note that although radiation is one of the few actual well-established risk factors for development of glioma (chromosomal abnormalities being another notable risk factor).
Thus, although it's common for tobacco use and immunosuppression to be associated with development of malignancy, they are NOT associated with development of a glioma.
And although infection (such as EBV with lymphoma) can be a trigger for malignancy, they are NOT for glioma.
Also, although there has been great media attention given to cell phone use and cancer risk, it has NOT been shown to be associated with glioma.
References
Adesina, Adekunle M., Tarik Tihan, Christine E. Fuller, and Tina Young Poussaint. Atlas of Pediatric Brain Tumors. Springer, 2016.
Chaichana, Kaisorn L., Thomas Kosztowski, Ashwini Niranjan, Alessandro Olivi, Jon D. Weingart, John Laterra, Henry Brem, and Alfredo Quiñones-Hinojosa. “Prognostic Significance of Contrast-Enhancing Anaplastic Astrocytomas in Adults.” Journal of Neurosurgery 113, no. 2 (August 2010): 286–92. https://doi.org/10.3171/2010.2.JNS091010.
“File: 405663-PLEOMORPHIC XANTHOASTROCYTOMA.Jpg - Wikipedia.” Accessed June 18, 2018. https://it.m.wikipedia.org/wiki/File:405663-PLEOMORPHIC_XANTHOASTROCYTOMA.jpg.
Jensflorian. English: Histopathology Specimen (FFPE) of a Diffuse Astrocytoma, WHO Grade II, Fibrillary Type. October 22, 2015. Own work. https://commons.wikimedia.org/wiki/File:Diffuse_astrocytoma_HE_stain.jpg.
———. English: Histopathology Specimen of Diffuse Astrocytoma, Gemistocytic Appearance (H&E Stain, High Magnification). November 2, 2015. Own work. https://commons.wikimedia.org/wiki/File:Gemistocytic_astrocytoma.jpg.
Killela, Patrick J., Christopher J. Pirozzi, Zachary J. Reitman, Sian Jones, B. Ahmed Rasheed, Eric Lipp, Henry Friedman, et al. “The Genetic Landscape of Anaplastic Astrocytoma.” Oncotarget 5, no. 6 (October 16, 2013): 1452–57.
Louis, David N., Arie Perry, Guido Reifenberger, Andreas von Deimling, Dominique Figarella-Branger, Webster K. Cavenee, Hiroko Ohgaki, Otmar D. Wiestler, Paul Kleihues, and David W. Ellison. “The 2016 World Health Organization Classification of Tumors of the Central Nervous System: A Summary.” Acta Neuropathologica 131, no. 6 (June 1, 2016): 803–20. https://doi.org/10.1007/s00401-016-1545-1.
Molavi, Diana Weedman. The Practice of Surgical Pathology: A Beginner’s Guide to the Diagnostic Process. Springer Science & Business Media, 2008.
Nephron. English: High Magnification Micrograph of an Anaplastic Astrocytoma. H&E Stain. [object HTMLTableCellElement]. Own work. https://commons.wikimedia.org/wiki/File:Anaplastic_astrocytoma_-_high_mag.jpg.
———. English: High Magnification Micrograph Showing Reactive Astrocytes in Massive Cerebral Ischemia Leading to Pseudolaminar Necrosis. H&E-LFB Stain. [object HTMLTableCellElement]. Own work. https://commons.wikimedia.org/wiki/File:Reactive_astrocytes_-_lfb_-_high_mag.jpg.
———. English: Very High Magnification Micrograph of an Anaplastic Astrocytoma. H&E Stain. [object HTMLTableCellElement]. Own work. https://commons.wikimedia.org/wiki/File:Anaplastic_astrocytoma_-_very_high_mag.jpg.
Newton, Herbert B. Handbook of Brain Tumor Chemotherapy, Molecular Therapeutics, and Immunotherapy. Academic Press, 2018.
Orkin, Stuart H., David E. Fisher, A. Thomas Look, Samuel Lux, David Ginsburg, and David G. Nathan. Oncology of Infancy and Childhood E-Book. Elsevier Health Sciences, 2009.
Perry, Arie. “WHO’s Arrived in 2016! An Updated Weather Forecast for Integrated Brain Tumor Diagnosis.” Brain Tumor Pathology 33, no. 3 (July 1, 2016): 157–60. https://doi.org/10.1007/s10014-016-0266-4.
Prayson, R. A., and M. L. Estes. “Protoplasmic Astrocytoma. A Clinicopathologic Study of 16 Tumors.” American Journal of Clinical Pathology 103, no. 6 (June 1995): 705–9.
Prayson, Richard A., and Mark L. Cohen. Practical Differential Diagnosis in Surgical Neuropathology. Springer Science & Business Media, 2000.
Reni, Michele, Elena Mazza, Silvia Zanon, Gemma Gatta, and Charles J. Vecht. “Central Nervous System Gliomas.” Critical Reviews in Oncology/Hematology 113 (May 2017): 213–34. https://doi.org/10.1016/j.critrevonc.2017.03.021.
Ryall, Scott, Uri Tabori, and Cynthia Hawkins. “A Comprehensive Review of Paediatric Low-Grade Diffuse Glioma: Pathology, Molecular Genetics and Treatment.” Brain Tumor Pathology 34, no. 2 (April 1, 2017): 51–61. https://doi.org/10.1007/s10014-017-0282-z.
Samuels, Martin A., Allan H. Ropper, and Joshua Klein. Adams and Victor’s Principles of Neurology 10th Edition. McGraw-Hill Education, 2014.
Sharma, Suash, and Prabal Deb. “Intraoperative Neurocytology of Primary Central Nervous System Neoplasia: A Simplified and Practical Diagnostic Approach.” Journal of Cytology / Indian Academy of Cytologists 28, no. 4 (2011): 147–58. https://doi.org/10.4103/0970-9371.86339.
Wang, Y.Y., K. Wang, S.W. Li, J.F. Wang, J. Ma, T. Jiang, and J.P. Dai. “Patterns of Tumor Contrast Enhancement Predict the Prognosis of Anaplastic Gliomas with IDH1 Mutation.” American Journal of Neuroradiology 36, no. 11 (November 2015): 2023–29. https://doi.org/10.3174/ajnr.A4407.
Wippold, F. J., A. Perry, and J. Lennerz. “Neuropathology for the Neuroradiologist: Rosenthal Fibers.” American Journal of Neuroradiology 27, no. 5 (May 1, 2006): 958–61.
Image References:
Diffuse Astrocytoma: Fibrillary Pattern
Jensflorian. English: Histopathology Specimen (FFPE) of a Diffuse Astrocytoma, WHO Grade II, Fibrillary Type. October 22, 2015. Own work. https://commons.wikimedia.org/wiki/File:Diffuse_astrocytoma_HE_stain.jpg.
Diffuse Astrocytoma: Gemistocytic Pattern
Jensflorian. English: Histopathology Specimen of Diffuse Astrocytoma, Gemistocytic Appearance (H&E Stain, High Magnification). November 2, 2015. Own work. https://commons.wikimedia.org/wiki/File:Gemistocytic_astrocytoma.jpg.
Anaplastic Astrocytoma
Nephron. English: High Magnification Micrograph of an Anaplastic Astrocytoma. H&E Stain. [object HTMLTableCellElement]. Own work. https://commons.wikimedia.org/wiki/File:Anaplastic_astrocytoma_-_high_mag.jpg.
Nephron. English: Very High Magnification Micrograph of an Anaplastic Astrocytoma. H&E Stain. [object HTMLTableCellElement]. Own work. https://commons.wikimedia.org/wiki/File:Anaplastic_astrocytoma_-_very_high_mag.jpg.