Generally, there are two types of spots on a bone scan. Hot spots on a bone scan form when the radioactive material, or tracer, accumulates in areas of the bone. Cold spots form in areas where the tracer has not accumulated. Bone loss may cause cold spots on a bone scan while arthritis, infections or broken bones can cause hot spots. Tumors and cancer can cause either hot or cold spots.

Some cancers, such as prostate cancer, cause hot spots on a bone scan when the cancer has metastasized, or spread to the bone. Other cancers that can metastasize and show up as hot spots on a bone scan include lung and breast cancer. Bone cancers will also appear as hot spots on a scan. The hot spots are dark areas on the scan due to the growth of excess tissue and the accumulation of the tracer.

Hot spots also appear on bone scans due to infections or diseases in the bone. One disease that causes hot spots is Paget's disease of bone, which leads to fragile bones and pain in the bones. A person with Paget's does not metabolize bone properly. Old bones may break down faster than new bones are formed. As the disease progresses, new bones are formed too quickly to compensate for the deterioration of the old bones. The new bones are not formed fully and may be soft and weak.

Osteomyelitis, a type of bone infection, will also cause hot spots on a bone scan. The infection is typically caused by staphylococcus bacteria. It can infect the bones as well as the joints. A person can get a bone infection when she suffers a puncture wound or broken bone. In some cases, the bacteria can enter the body when the broken bone is being treated.

Cold spots on a bone scan indicate that the tracer has not accumulated in that section of the bone. A loss of blood flow to the bone, or a bone infarction, can cause a cold spot. When a person suffers a bone infarction, the part of the bone that no longer receives blood can die, leading to a collapse of the bone.

Another cause of cold spots on a scan is multiple myeloma, a type of cancer. Multiple myeloma causes the body to produce excess plasma or white blood cells. These cells then take over the bone marrow, reducing the number of red blood cells. The plasma cells can also form small tumors on the bone and damage the bone.