Glossary
Imaging - Microscopic Metastasis
Certain types of scans function better depending on which area of the body is being evaluated. For the prostate gland itself Color Doppler Ultrasound or Endorectal MRI is best. For lymph node evaluation, a CT scan or MRI is best. For evaluation of the bones, a Fluoride PET scan or MRI with gadolinium is best.
Metastatic spread of prostate cancer is less common than other types of cancers. However, when metastases are visible on scans, it is a sign that in this particular individual the prostate cancer is behaving aggressively. Early spread of cancer is invisible to the naked eye and therefore cannot be seen on scans because the metastases are microscopic. However, experts can estimate the likelihood of microscopic metastases based on characteristics such as Gleason score, PSA and extent of disease on biopsy. When microscopic metastases are strongly suspected, preemptive radiation treatment to the most likely area of spread, the pelvic lymph nodes, should be considered.
Color Doppler Ultrasound
High resolution Color Doppler Ultrasound is really two scans in one: A grey scale image in black and white and a color image that detects areas of increased blood flow. Prostate cancer shows up as a dark area on grey scale imaging or as an area of increased flow on a Color Doppler Ultrasound. When both a dark spot and increased blood flow are seen in the same location in the prostate, cancer is more likely. Color Doppler can be used to perform lesion-directed biopsies as opposed to random biopsies as are commonly performed by most urologists. Color Doppler Ultrasound imaging requires skilled and experienced physicians with special experience and training to obtain accurate results.
Endorectal MRI
Recent imaging enhancements, including the capacity to perform lesion-directed biopsies, have enabled Endorectal MRI to gain mainstream acceptance in the urologic community. Better image quality results from using 3.0 Tesla magnetic field. In addition, two new imaging sequences, diffusion and contrast enhancement have been added, transforming state-of-the-art MRI into three scans instead of one. Some centers also do spectroscopy, a fourth modality. Spectroscopy is used to determine if lesions detected on standard MRI are malignant. Abnormal spectroscopy suggests a higher likelihood of malignancy when it detects abnormal concentrations of choline which is characteristically elevated in malignant tissue. Endorectal MRI scanning is quite useful when the study is performed at a center using state-of-the-art technology and read by physicians with special experience and training.
Body Scans
Prostate cancer, if it spreads to other parts of the body, has a strong predilection for lymph nodes and bones. Therefore, scans performed to detect prostate cancer should focus on these areas. The best scans for lymph nodes are 1) Computerized Tomography (CT) scan of the abdomen and pelvis with oral and IV contrast, 2) Magnetic Resonance Imaging (MRI) of the abdomen and pelvis, 3) FDG PET scans. Pet scans for prostate cancer are less accurate in prostate cancer, especially in its early stages because prostate cancer tends to have a slow metabolism compared to other cancers. When prostate cancer is more advanced FDG PET scans can provide a quicker indication of disease response or progression than either MRI or CT scans.
Bone Scans
A standard bone scan is performed by injecting radioactive technetium, a calcium analogue that accumulates in areas where the bone is "irritated." A sophisticated Geiger counter is passed over the body creating an image of the bones. Areas of accumulated radioactivity show up as "hot spots." These areas of abnormal accumulation can result from cancer or trauma. Bone scan technology has recently been upgraded. Fluoride PET scan of the bone is decidedly more accurate than standard technetium bone scans that have been in use for several decades.
MRI of the bones with gadolinium contrast is even more accurate. However, general use of MRI for scanning the bones is impractical. First, MRI of the ribs is impossible due to respiratory motion. Second, MRI scanning of large areas of the body is very time consuming. MRI of the bones, therefore, is usually reserved for confirming suspicious but unconfirmed abnormalities detected with a bone scan. MRI of the spine and pelvis is sometime used as a screening tool to obtain the greatest assurance about the absence of bone metastasis.
Bone Density Scans
Bone density scans are used to detect weakening of the bones through loss of calcium, otherwise known as osteoporosis. Osteoporosis needs to be diagnosed before a fracture occurs. Unfortunately, the most common scanning technique, the DEXA scan, grossly underestimates calcium loss from the spine in men. This is because degenerative arthritis with calcium deposition in the ligaments surrounding the spine is mistakenly read as bone.
Fortunately, another more accurate technique called quantitative CAT scan or QCT is available. This scan measures the calcium density in the center of the vertebral column, avoiding the problem of surrounding calcium in the ligaments. DEXA limitations have been well documented in a study from Massachusetts General Hospital, yet most health care providers are unaware of these limitations. For example, in a study that compared DEXA and QCT in 41 men with prostate cancer, QCT detected osteoporosis in 26 but DEXA only found osteoporosis in two.
