Which of the following is NOT one of the four common brain tumors?

Adenocarcinoma is primarily a type of cancer that originates in glandular cells and is most commonly found in organs such as the breast, prostate, and colon. While it can metastasize to the brain, it is not classified as a brain tumor in itself. Instead, it refers to a type of cancer that can spread to various body parts, including the brain, where it forms a secondary tumor.

In contrast, meningiomas, glioblastomas, and metastatic tumors are categorized specifically as types of brain tumors. Meningiomas arise from the meninges, the protective layers covering the brain and spinal cord. Glioblastomas are aggressive brain tumors that originate from glial cells, which support and protect neurons. Metastatic tumors refer to cancer that has spread to the brain from another part of the body, making it part of the brain tumor classification.

Thus, choosing adenocarcinoma clarifies that it is not classified as a brain tumor in the same sense as the others listed.