PET Scan and Brain Tumor

One of the most important uses of Positron Emission Tomography (PET) technology is its use in diagnosing and treating brain tumors. Brain tumors, if malignant, are a rare form of cancer that accounts for approximately 17,200 new cases per year in the United States of America. Despite its rarity, malignant brain tumors are very dangerous forms of cancer that is often difficult to fully remove. The American Cancer Society estimates that death rates caused by malignant brain tumors will reach a level of approximately 13,100 in 2004. Consequently, although brain tumors account for only 1.4% of all cancer cases, it is also responsible for 2.4% of all cancer related deaths. In most instances brain tumors occur when cancer from other organs such as the lung or breast has spread to the brain. Therefore malignant brain tumors are often a result of metastatic brain cancers (cancers that originated in a different area in the body but has spread to the brain). However, it is important to note that not all brain tumors are cancerous and are often benign (non-cancerous) that, for the most part, are not life threatening.

Brain tumors occurs when brain cells become abnormal and form cells in an uncontrolled manner. These extra brain cells form into a mass of tissue, also known as a tumor, which can be a classified as benign brain tumor, a mass of extra cells that are harmless and have distinct boundaries, or malignant, a mass of extra cells that are life-threatening and either cancerous or located in a vital brain area.) For the most part, malignant brain tumors are indicative of cancer and the American Cancer Society estimate that malignant brain tumors will result in approximately 13,100 deaths (1.4% of all cancer deaths) in 2004.

Although there are a number of different methods to classify brain tumors, the best way to distinguish between brain tumors is by classifying them as:

• Primary brain tumors:
  Tumors that originated from the brain. Some examples of primary brain tumors are: astrocytomas, a tumor that originates from the astroyctes (supportive tissue) of the brain that can be either benign or malignant; oligodendrogliomas, a tumor that originates from the oligodendrocytes (supportive tissue) of the brain that can be either benign or malignant; medulloblastomas, a malignant tumor that represents underdeveloped medulloblasts; and germ cell tumors.
• Secondary brain tumors:
  Tumors that originated elsewhere in the body that has metastasized (spread) to the brain. Some examples of secondary brain tumors are: breast cancer and lung cancer.

The causes of primary brain tumors are unknown but it is a condition that can occur in people of all ages. Secondary brain tumors generally occur in adults and if the secondary brain tumor is cancerous, its cause depends on where the cancer originated from in the body. If a malignant brain tumor is cancerous, there is the possibility that it will metastasize (spread) further to other locations in the brain or spinal cord, however malignant brain tumors rarely metastasize outside of the brain and spinal cord.

Symptoms of Brain Tumors

The symptoms of brain tumors are difficult to diagnose and often do not produce symptoms until it has reached an advanced stage. The size of the tumor and its location are variables that affect the type of symptoms that they produce. However, tumor growth can affect other parts of the brain that produces new symptoms. In many instances, brain tumors are difficult to detect as the symptoms they produce mimic the symptoms of other diseases. Consequently, brain tumor symptoms may be evident much earlier before they are diagnosed.

PET and Brain Tumors Staging

Brain tumors are usually detected through imaging anatomical techniques such as magnetic resonance imaging (MRI) and computed tomography (CT), and these imaging tests are usually performed if a patient displays the symptoms associated with brain tumors. If the brain tumor is malignant, often the symptoms that the patient is displaying are emblematic of degeneration of the function that the area of the brain the tumor is located in is responsible for. Although these anatomical imaging tests are vital in producing images that detail structural and anatomical changes in the brain caused by brain tumors by detecting formations of brain cell mass that suggest the presence of a tumor, these tests are limited as they are only able to detail tumor location.

In order for physicians to take the appropriate further medical actions to treat the brain tumor the level (or stage) of the brain tumors is required. At this point, Positron Emission Tomography becomes a useful tool in the physician’s arsenal to properly treat brain tumors. PET imaging is a non-invasive diagnostic imaging tool that has an advantage over anatomical imaging tools in that it is a metabolic imaging tool that is able to distinguish between benign and malignant tumors. It is often used to accurately determine the stage of the brain tumor.

Although tissue samples may indicate the same information they are often less accurate than PET imaging. With PET scans, a physician is able to ascertain information about the patient’s metabolic state as PET images produce visual images detailing biochemical changes caused by brain tumors. This function differentiates PET imaging from such anatomical imaging tools as x-rays, computed tomography (CT), and magnetic resonance imaging (MRI), which are imaging tools that produce images detailing body structure changes caused by disease.

Brain tumor PET imaging involves the administration of a radioactive tracer that is a combination of a radioisotope (a radioactive compound whose movements are detectable by a PET scanner) with a natural body compound. When used in brain tumor scanning, the radioactive tracer used in PET is Fluorodeoxyglucose (FDG), which combines the natural body compound glucose with the radioisotope Fluorine-18. This radioactive tracer, or radiopharmaceutical, is used in brain PET imaging as the radioactive compound that it uses has a short half-life that will disappear from the body within hours. Although it is reasonable for individuals to be concerned about the radiation used in PET imaging, this procedure has been shown to be highly safe. Consequently, patients should free themselves of any worry about the radiation content of this procedure.

Brain tumor PET scanning uses FDG as its radioactive tracer because it contains the body compound glucose. The use of FDG, which shares a similar structure to glucose, is important, as the absorption of glucose is effective in determining whether a cell is alive or non-cancerous. PET imaging traces the absorption rate of FDG by cells and can determine whether cancerous cells are present in the brain and other organs or tissues, as glucose (which FDG shares a similar structure) is absorbed at a faster rate by cancerous cells compared to healthy cells. By tracing the movement of FDG in the patient’s brain, the physician is able to determine whether the lung mass detected through anatomic imaging is benign (non-cancerous) or malignant (cancerous). From the images produced by the PET scan, a physician will be able to determine the stage of the brain tumor and also the speed in which tumor cells are growing.

Brain tumors are staged using the World Health Organization (WHO) system of grading. Based on the images produced by Positron Emission Tomography, a physician is able to determine the aggressiveness of the tumor spread, which is used to determine the appropriate further medical action for brain tumor treatment. The WHO system of brain tumor grading is a four grade system, in which Grade I represents the slowest growing, least aggressive tumors and Grade IV representing the fastest growing, most aggressive tumors.

PET and Brain Tumor Follow-Up

Besides brain tumor staging, Positron Emission Tomography is a valuable tool in brain tumor treatment when it is used as a follow-up to brain tumor treatment. Brain tumors are a difficult to fully treat due to the sensitivity of its location. Therefore, brain tumor recurrence is often possible. Imaging tests such as magnetic resonance imaging (MRI) and computed tomography (CT) are able to be used to detail structural changes, such as tumor formation, in the brain. However, these imaging tests often detect structural changes caused by side effects from previous brain tumor treatments, such as surgery or radiation therapy.

In cases where structural change has been detected through MRI or CT, a physician often uses PET imaging to determine the nature of the detected structural changes. A non-invasive, diagnostic imaging tool, PET scanning is a metabolic imaging procedure that has an advantage over anatomical imaging tools as it details biochemical changes within the body caused by disease. Through the administration of the radioactive tracer, Fluorodeoxyglucose (FDG), which combines the radioisotope (a radioactive compound whose movements are detectable by a PET scanner) Flourine-18 with the natural body compound glucose, PET scanning is able to produce images detailing the biochemical functioning of the brain.

The use of FDG is important in PET brain tumor follow-up as FDG shares a similar structure to glucose. Glucose absorption is a medical method to determine whether a cell is alive or non-cancerous. PET imaging traces the absorption rate of FDG by cells and can determine whether cancerous cells have recurred in the brain or whether the mass of tissue detected by anatomical imaging are either benign or side effects of previous treatment, such as scar tissue.

Besides being a valuable imaging tool in detecting brain tumor recurrence, PET imaging is also helpful for physicians to assess the response of the brain tumor to treatments such as radiation therapy or chemotherapy. Prior to the clinical use of PET scans, physicians applied radiation therapy and chemotherapy according to standard rules. However, with PET imaging, it is now possible for physicians to specifically cater brain tumor treatment to a patient’s particular situation. This is because Positron Emission Tomography allows a physician to view the location, extent, and resilience of a patient’s brain tumor.