Metastatic potential of malignant tumours.

The term 'metastatic potential' is examined, together with some of the factors that may contribute to it. The evidence is drawn from clinical and experimental sources. Morphological factors include the site of the tumour, its size, histological appearances and pattern of local spread. The scope and limitations of each are discussed and the potential application of newer morphological techniques is noted. Certain non-morphological factors are considered: these are (1) the functional heterogeneity of tumour cell populations with resepct to the metastatic capacity; (2) the elaboration of certain tumour-associated products which may potentiate tumour spread; and (3) immunological factors in the form of local host cell infiltrates in and around primary tumours. The tumour-associated products include collagenases, prostaglandins and angiogenesis factor; their likely origin from tumour and host cells is stressed, together with their implications in physiological and pathological processes other than neoplasia. It is concluded that the metastatic potential of tumours cannot be assessed in the terms of conventional tumour pathology alone. A more functional approach is needed before the term 'metastatic potential' acquires solid conceptual or practical value.