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What is Pilocytic astrocytoma and explain in detail?
Pilocytic astrocytoma (and its variant pilomyxoid astrocytoma) is a brain tumor that occurs most commonly in children and young adults (in the first 20 years of life). They usually arise in the cerebellum, near the brainstem, in the hypothalamic region, or the optic chiasm, but they may occur in any area where astrocytes are present, including the cerebral hemispheres and the spinal cord. These tumors are usually slow growing and benign, corresponding to WHO malignancy grade 1.

Signs and symptoms
Children affected by pilocytic astrocytoma can present with different symptoms that might include failure to thrive (lack of appropriate weight gain/ weight loss), headache, nausea, vomiting, irritability, torticollis (tilt neck or wry neck), difficulty to coordinate movements, and visual complaints (including nystagmus). The complaints may vary depending on the location and size of the neoplasm. The most common symptoms are associated with increased intracranial pressure due to the size of the tumor mass.

Causes
Pilocytic astrocytoma can be associated with the genetic condition neurofibromatosis type 1  (NF1), and optic nerve gliomas are among the most frequently encountered tumors in patients with this disorder.  The majority of pilocytic astrocytomas, however, arise sporadically - with no evidence of a link to an underlying hereditary predisposition or lifestyle factor. They are associated with genetic alterations in the MAPK/ERK pathway, most frequently a characteristic KIAA1549-BRAF fusion gene.

Diagnosis
Usually – depending on the interview of the patient and after a clinical exam which includes a neurological exam and an ophthalmological exam – a CT scan and/or an MRI scan will be performed to confirm the presence of a tumor.  They are usually easily distinguishable from normal brain structures using these imaging techniques.  A special dye may be injected into a vein before these scans to provide contrast and make tumors easier to identify.  Pilocytic astrocytomas are typically clearly visible on such scans, but it is often difficult to say based on imaging alone what type of tumor is present.
If a tumor is found, it will be necessary for a neurosurgeon to perform a biopsy of it. This involves the removal of a small amount of tumorous tissue, which is then sent to a (neuro)pathologist for examination and staging. The biopsy may take place before surgical removal of the tumor, or the sample may be taken during surgery to remove the bulk of the tumor.

Microscopic appearance
Pilocytic astrocytomas are often cystic tumors, and, if solid, tend to be well-circumscribed.
Under the microscope, the tumor is seen to be composed of bipolar cells with long "hair-like" GFAP-positive processes, giving the designation "pilocytic" (that is, made up of cells that look like fibers when viewed under a microscope).  Some pilocytic astrocytomas may be more fibrillary and dense in composition.  The presence of Rosenthal fibers, eosinophilic granular bodies, and microcysts can often be seen.  Myxoid foci and oligodendroglioma-like cells may also be present, though these are not specific to pilocytic astrocytoma.  Long-standing lesions may show hemosiderin-laden macrophages and calcifications.

Treatment
The most common form of treatment is having the tumor surgically removed.  Complete removal of the tumor will generally allow functional survival for many years