Enchondromas are the most common type of tumor that is found in the hand.
Whenever people hear tumor, they think cancer, they think chemotherapy and they think life-threatening illness. Yet tumor does not always mean cancer. A tumor is an abnormal growth of tissue (like a weed in a garden, it pops up despite your best efforts to patrol the garden to control growth of flowers only). Cancer is the spread of this poorly controlled tissue to other parts of the body (like if you blow on a dandilion and all its little seeds spread out over your garden and lawn and now you have weeds growing everywhere). But some weeds don't spread, they are happy to stay put. Therefore some tumors are benign, meaning they are not spreading. Other tumors are malignant ("cancer") meaning, they want to spread out all over.
The good news is that enchondromas are benign meaning they will not spread. Enchondroma is an abnormal growth of cartilage (cartilage cells are called "chondrocytes"...chondroma = abnormal growth of cartilage cells). It is actually believed that some left over cells from your growth plate as a child (your growth plate is actually cartilage cells that get covered in bone...read more here) get "turned-on" and start replicating abnormally. Its usually an isolated tumor (although some genetic conditions like Ollier's disease, or Maffucci syndrome cause enchondromas to appear in many bones of the body).
The bad news is that enchondromas can occupy space that should be held by bone cells, and therefore weaken bones, and cause them to break more easily. Many enchondromas are found after someone breaks their hand or arm doing some simple task, and it occurs because the bone is abnormally weak.
Another problem with enchondromas is that they look very similar to Chondrosarcoma, which is the evil twin and a true "cancer" that invades other parts of the body. There are ways to tell the difference. On x-ray, a chondrosarcoma looks more aggressive: its invading the near by soft tissue, its causes surrounding inflammation (periosteal reaction), completely eats away the nearby bone (cortical thinning). A enchondroma looks like a puffy cloud within the bone (called "stippled calcification") and looks pretty harmless. Sometimes a simple cyst within the bone will be mistaken for an enchodnroma. It can also be mistaken for Giant Cell Tumors (which can be cancer), but remember enchondromas are unique in having calcifications. Very rarely a metastasis from another type of cancer (like breast cancer, or lung cancer) can travel to the finger tip (<0.1% of the time), and may look like an enchondroma.
Usually an x-ray is all that is needed. However, if someone has a past history of cancer, or something else that seems unusual, then an MRI can be ordered to better see the tumor. Treatment of an enchondroma is usually just watching it to make sure it doesnt cause pain or swelling. Most enchondromas, even if they cause a broken bone, can be treated without surgery.
However, surgery is recommended if it is causing a lot of pain. Surgery involves scooping out the lesion (curettage). The gap in the bone is then filled with cement or alcohol is poured into the hole to kill any last remaining tumor cells and then normal bone is able to grow back.