In the strict sense of the word, FOP lesions are “tumors” or “neoplasms” – new growths that should not be where they are. FOP lesions may appear suddenly and lead to severe swelling within hours. This is often a source of much consternation for cancer doctors who are often asked to see FOP patients. They are often baffled as to the sudden expansion and growth of such a tumor, and are at a loss to understand what type it might be. This often leads to the suggestion for a diagnostic biopsy. A biopsy of an early FOP lesion can be mistaken for several types of cancer depending upon the state of maturation of the lesion. 

The reason for this is that when a new bone forms through an endochondral process (as FOP bone does), it goes through various stages of massive cell proliferation including cells involved in connective tissue, cartilage, and bone. There are often many dividing cells which is a worrisome sign for cancer, but of course when a bone heals such as in a fracture, much new tissue has to be formed as well.

In a sense, new FOP bone formation (whether it occurs in the muscle, tendon, ligament, or connective tissue) is essentially almost identical to the type of process seen in a healing facture. When someone breaks a bone, there is no question what the diagnosis is, and we rarely biopsy a fracture. But, if we did, and we looked under the microscope, we would see a process that could confuse us with cancer if we didn't know what we were looking at.

The same is the case of FOP. Thus, it is the tumor-like lesions in FOP and the microscopic appearance of the biopsy specimens that often lead doctors to the erroneous and dangerous misdiagnosis of cancer rather than the correct diagnosis of FOP. It is easy to look back in retrospect and say if one only suspected FOP and looked at the toes, the correct diagnosis could have been made without a biopsy. Also, a careful history would have clearly revealed that the FOP lesions grew much more rapidly then any cancer could ever possibly grow.