Pathology of Renal Tumors

THIS paper is limited to a consideration of tumors of the kidney and perirenal region, particular attention being given to the gross pathological features which may have an important bearing upon the interpretation of roentgenograms. The more frequent types of intra- and extra-renal neoplasms and lesions simulating neoplasms are discussed briefly, and points of interest in each group are noted. General Considerations Embryological.—By reason of the embryological derivation of the kidney and the intimate anatomical relationship of the original anlage to the anlage of other important organs and structures in the immediate vicinity, it is not difficult to conceive of misplacements and inclusions of tissues which are not ordinarily destined to be integral parts of the fully differentiated kidney. The anatomical possibility of such inclusions affords one explanation for the occurrence of intra-renal neoplasms, the structure of which has little or no resemblance to the kidney substance. An alternative explanation for the occurrence of heterotopic neoplasms in the kidney, which is also derived from embryological investigations, is based upon the fact that a mass of undifferentiated cells of common origin have the capacity of differentiating along diverse lines and of resulting in the establishment of organs and tissues so obviously different in structure and function that their primordial common ancestry is easily forgotten. Deviations from the line of normal differentiation or incomplete differentiation along the normal line may eventuate in the presence of two structurally different tissues of identical origin in the same organ. Granting this theoretical possibility, it should not seem strange that the apparently ectopic tissue may become neoplastic. Each of the foregoing conceptions of the genesis of neoplasms has the sanction of authority. Neither is accepted to the exclusion of the other. De-differentiation, Anaplasia, Metaplasia.—In the genesis of neoplasms increasing importance is attached to the possibility that adult tissue, under circumstances not too well understood, may revert to a more or less embryonal or undifferentiated state. The multiformity of renal neoplasms conceivably originating in such reverted tissue is limited primarily by the degree to which we are willing to admit that de-differentiation may proceed. If it be conceivable that a renal epithelial cell may revert to a cell having the potentialities of its ultimate ancestor, it would not be inconceivable that by a process of growth and re-differentiation it might develop the characteristics of an adrenal epithelial cell. Thus we have a third possible explanation of heterotopic neoplasms. In a restricted sense, metaplasia implies the transition of one type of tissue into another type.