This chapter includes selected hematopoietic entities that are considered important in soft tissue pathology because of their more or less common presentation in extranodal soft tissues. Practical diagnosis is emphasized, based on histologic and immunohistochemical parameters (Table 35.1). Reactive histiocytic lesions and foreign body reactions are discussed in Chapter 34.

The concepts of soft tissue and cutaneous lymphoma overlap in the literature, because subcutaneous involvement is often included with cutaneous lymphoma. Selected cutaneous lymphoma types are therefore discussed in this chapter. Typically skin-based lymphomas, such as mycosis fungoides, related entities, and some rare T-cell lymphomas are excluded, however. Discussion of the other entities centers on extranodal, noncutaneous soft tissue manifestations, and genetics is included only in the context of established clinical tests. For more extensive discussion, the author refers readers to recent review articles and lymphoma classification handbooks. Comprehensive reviews on immunophenotyping of lymphomas and other hematopoietic neoplasms are recommended.

Soft tissue lymphomas outside the context of cutaneous lymphomas have been reported in older series that predate modern immunophenotypic classification. Morphologically, most examples have been diffuse large cell lymphomas that probably represented large B-cell lymphomas. Localized primary soft tissue lymphomas seem to be distinctly uncommon. In a Mayo Clinic series employing strict criteria for primary extranodal soft tissue lymphoma, there were only 8 cases among 7000 lymphomas (0.1%): 6 in the lower extremity and 2 in the arm. Most of these lymphomas formed large masses (≥5 cm), and systemic disease often developed during follow-up.