Melanoma is the third most common form of skin cancer and the most serious. Melanoma begins in melanocytes, the cells throughout the skin that produce melanin, the pigment that gives skin its color.
According to the American Cancer Society, approximately 100,350 new cases of melanoma will be diagnosed and more than 6,850 people will die from the disease in 2020. Cancer of the skin is by far the most common of all cancers. Melanoma accounts for only about 1% of skin cancers but causes a large majority of skin cancer deaths.
Treatment of melanoma includes surgery, electrodessication and curettage, cryosurgery, laser surgery, radiation therapy, photodynamic therapy, chemotherapy and immunotherapy.
New developments in molecular imaging technologies are dramatically improving the ways in which melanoma is diagnosed and treated. Research in molecular imaging is also contributing to our understanding of the disease and directing more effective care of patients with melanoma.
What is molecular imaging and how does it help people with melanoma?
Molecular imaging is a type of medical imaging that provides detailed pictures of what is happening inside the body at the molecular and cellular level. Where other diagnostic imaging procedures—such as x-rays, computed tomography (CT) and ultrasound—predominantly offer anatomical pictures, molecular imaging allows physicians to see how the body is functioning and to measure its chemical and biological processes.
Molecular imaging offers unique insights into the human body that enable physicians to personalize patient care. In terms of diagnosis, molecular imaging is able to:
As a tool for evaluating and managing the care of patients, molecular imaging studies help physicians:
Molecular imaging procedures are noninvasive, safe and painless.
How does molecular imaging work?
When disease occurs, the biochemical activity of cells begins to change. For example, cancer cells multiply at a much faster rate and are more active than normal cells. Brain cells affected by dementia consume less energy than normal brain cells. Heart cells deprived of adequate blood flow begin to die.
As disease progresses, this abnormal cellular activity begins to affect body tissue and structures, causing anatomical changes that may be seen on CT or MRI scans. For example, cancer cells may form a mass or tumor. With the loss of brain cells, overall brain volume may decrease or affected parts of the brain may appear different in density than the normal areas. Similarly, the heart muscle cells that are affected stop contracting and the overall heart function deteriorates.
Molecular imaging excels at detecting the cellular changes that occur early in the course of disease, often well before structural changes can be seen on CT and MR images.
Most molecular imaging procedures involve an imaging device and an imaging agent, or probe. A variety of imaging agents are used to visualize cellular activity, such as the chemical processes involved in metabolism, oxygen use or blood flow. In nuclear medicine, which is a branch of molecular imaging, the imaging agent is a radiotracer, a compound that includes a radioactive atom, or isotope. Other molecular imaging modalities, such as optical imaging and molecular ultrasound, use a variety of different agents. Magnetic resonance (MR) spectroscopy is able to measure chemical levels in the body without the use of an imaging agent.
Once the imaging agent is introduced into the body, it accumulates in a target organ or attaches to specific cells. The imaging device detects the imaging agent and creates pictures that show how it is distributed in the body. This distribution pattern helps physicians discern how well organs and tissues are functioning.
What molecular imaging technologies are used for melanoma?
The most commonly used molecular imaging procedure for diagnosing and guiding the treatment of melanoma are, lymphoscintigraphy and positron emission tomography (PET) scanning, which is often used in conjunction with computed tomography (CT) scanning.
What is lymphoscintigraphy and how is it performed?
Lymphoscintigraphy is a procedure that provides images of the lymphatic system, a network of small channels similar to blood vessels that circulate fluid (called lymph) and cells (lymphocytes) of the immune system throughout the body. Lymph nodes, which act like a filter for foreign bodies such as germs, viruses and pollen, are located along this network.
Lymphoscintigraphy helps identify the first few, or sentinel, lymph nodes that filter lymph fluid from the melanoma site and those most likely to be affected by cancer.
The procedure involves injecting small amounts of the radiotracer Technetium-99m sulfur-colloid in proximity to the melanoma site. Depending on the site of the melanoma, images are then taken with a gamma camera of the patient’s arms and underarms, legs and groins, head, neck and chest, or other areas.
The radiotracer gives off energy in the form of gamma rays that can be detected by the gamma camera. The camera produces images that reflect the amount of radiotracer absorbed throughout the lymphatic system. Using these images, a hand-held probe as a guide, a physician removes only the lymph nodes that have absorbed the radiotracer.
What are the advantages of lymphoscintigraphy?
Lymphoscintigraphy is a valuable tool for assessing whether melanoma has spread to other parts of the body. Because melanoma can travel throughout the lymphatic system in unexpected ways, lymphoscintigraphy is helpful in directing the surgeon to potentially cancerous areas that may have otherwise been overlooked. Images generated by the procedure allow physicians to remove only sentinel lymph nodes, sparing patients from undergoing unnecessary removal of normal lymph nodes.
What is PET?
PET involves the use of an imaging device (PET scanner) and a radiotracer that is injected into the patient’s bloodstream. A frequently used PET radiotracer is 18F-fluorodeoxyglucose (FDG), a compound derived from a simple sugar and a small amount of radioactive fluorine.
Once the FDG radiotracer accumulates in the body’s tissues and organs, its natural decay includes emission of tiny particles called positrons that react with electrons in the body. This reaction, known as annihilation, produces energy in the form of a pair of photons. The PET scanner, which is able to detect these photons, creates three-dimensional images that show how the FDG is distributed in the area of the body being studied.
Areas where a large amount of FDG accumulates, called ‘hot spots’ because they appear more intense than surrounding tissue, indicate that a high level of chemical activity or metabolism is occurring there. Areas of low metabolic activity appear less intense and are sometimes referred to as ‘cold spots.’ Using these images and the information they provide, physicians are able to evaluate how well organs and tissues are working and to detect abnormalities.
PET-CT is a combination of PET and computed tomography (CT) that produces highly detailed views of the body. The combination of two imaging techniques—called co-registration, fusion imaging or hybrid imaging—allows information from two different types of scans to be viewed in a single set of images. CT imaging uses advanced x-ray equipment and in some cases a contrast-enhancing material to produce three-dimensional images.
A combined PET-CT study is able to provide detail on both the anatomy and function of organs and tissues. This is accomplished by superimposing the precise location of abnormal metabolic activity (from PET) against the detailed anatomic image (from CT).
How is PET performed?
The procedure begins with an intravenous (IV) injection of a radiotracer, such as FDG, which usually takes between 30 and 60 minutes to distribute throughout the body. The patient is then placed in the PET scanner where special detectors are used to create a three dimensional image of the FDG distribution.
Scans are reviewed and interpreted by a qualified imaging professional such as a nuclear medicine physician or radiologist who shares the results with the patient’s physician.
How is PET used for melanoma?
Physicians use PET and PET-CT studies to:
Are molecular imaging procedures covered by insurance?
Medicare and private insurance companies cover the cost of most PET-CT scans. Check with your insurance company for specific information on your plan.
What is the future of molecular imaging and melanoma?
There are many new and emerging molecular imaging technologies that can benefit melanoma patients. Other molecular imaging procedures under development often combine imaging technologies to form hybrid imaging systems to improve accuracy and allow physicians to see how cancer may be affecting other systems in the body. One of the more promising research areas is in investigational PET imaging biomarkers. Another exciting area of study is radioimmunotherapy, a form of treatment that targets cancer-killing radiation directly to cancer cells.