Basically, any localized mass can be regarded as a tumor. In the broadest sense, this also includes swellings and conditions that result from a hematoma or inflammation. Accordingly, the term „tumor“ does not allow any statement about the nature and biological behavior of the mass. In a narrower sense, however, a tumor is regarded as a benign or malignant neoplasia. Neoplasia is defined as a new formation of body tissue resulting from a misregulation of cell growth (autonomous, pathological tissue proliferation). A musculoskeletal tumor can originate from bone or soft tissue and is classified by the WHO according to the underlying cell type (e.g. neurogenic tumors, chondromatous tumors [Fig.1]).

Tumor-like lesions or pseudotumors have to be distinguished from true neoplasia. A strict differentiation between the two terms tumor-like lesions or pseudotumor is not possible, generally valid definitions for these terms do not exist. In general, tumor-like lesion and pseudotumors provide the clinical and, in some cases, radiological aspect of a true neoplasm, but not the biological behavior. Amongst others, these include ganglia (intraosseous and soft tissue), foreign body granulomas (soft tissue), but also accessory muscles (e.g., soleus accessorius muscle) and abscess formations. Other types of pseudostumors, especially localized on the foot, include warts and hyperkeratoses. Very rare examples of pseudotumors localized on the foot include tumorous calcinosis (Fig.2) and melorheostosis (Fig.3).

The WHO classification of musculo-skeletal tumors also distinguishes „tumors of undefined neoplastic nature“, i.e., bone lesions that are generally considered non-neoplastic but require consideration in the differential diagnosis of true bone tumors. Some possess morphologic and cytogenetic features of true neoplasms, although clinical behavior tends to support a non-neoplastic nature. Common examples include simple bone cyst and fibrous dysplasia.
A true tumor, on the other hand, is characterized by disinhibited, autonomous, and usually irreversible excess growth of endogenous tissue associated with varying degrees of loss of specific cellular and tissue function.

It is important to emphasize that the diagnosis „tumor“ does not allow a definite statement on the dignity (benign vs. malignant) of the mass and is only in the rarest cases equivalent to cancer.