Biologic Potential, Grading, Staging, and Reporting of Sarcomas

Biologic Potential

Among the most important reasons for the accurate classification of soft tissue tumors is the communication of clinical behavior (i.e., assignment into a managerial category). The vast majority of soft tissue tumors can be classified as either benign or malignant. Some benign tumors may occasionally recur, but they typically do so in a nondestructive fashion; simple surgical excision with narrow margins is generally adequate therapy for such tumors. By definition, a benign tumor should not metastasize. However, it is now recognized that in exceptional cases some examples of benign tumors may in fact metastasize (e.g., cutaneous benign fibrous histiocytoma), although the incidence of such an event is likely much less than 1 in 10,000. In contrast, malignant mesenchymal neoplasms (i.e., sarcomas) have a significant potential for local recurrence (including destructive growth through normal tissues) as well as distant metastasis. The risk of metastasis varies widely among different types of sarcomas, sometimes determined by histologic grade (see later discussion).

There is a small group of soft tissue tumors that cannot easily be classified as either benign or malignant. Such tumors (with “intermediate” biologic potential) fall into two main categories: (1) those that exhibit locally aggressive behavior ( Box 2.1 ) and (2) those that may rarely metastasize ( Box 2.2 ). Rare tumors fulfill both of these criteria. The prototypical example of a locally aggressive mesenchymal neoplasm is desmoid fibromatosis (see Chapter 3, Chapter 4, Chapter 16 ). Although desmoid tumors do not metastasize, when they arise at particular anatomic sites (e.g., mesentery or neck), because of the proximity to vital structures, they may be associated with significant morbidity and may occasionally result in the patient's death. Several locally aggressive tumor types carry the name sarcoma despite the lack of significant metastatic potential. For example, in its conventional form, dermatofibrosarcoma protuberans (DFSP) does not metastasize, although local surgical control may occasionally be difficult. In contrast, the fibrosarcomatous variant of DFSP (representing a form of histologic progression) metastasizes in 10% to 15% of cases (see Chapter 15 ). Most of the tumors that fall into the “rarely metastasizing” category are very uncommon. Although drawing the line between this “intermediate” category and bona fide sarcomas may be somewhat arbitrary, a 2% metastatic risk has been used as a cutoff point. For the tumors in these unusual categories, good communication between the pathologist and the treating physicians is critical to convey the clinical significance of the diagnosis, particularly for rare tumor types that are unfamiliar to many clinicians. The remainder of this chapter is devoted to sarcomas.