Pathologic Findings Light Microscopy

Bence Jones cast nephropathy is characterized by prominent casts in renal tubules; the casts are usually large and "brittle," have fracture lines or are broken into many fragments often with geometric shapes, and are surrounded by tubular epithelium, neutrophils, and typically and diagnosti-cally by multinucleated giant cells of foreign-body type (Fig. 16.1). While they are more common in distal tubules, the casts may be formed in any segment of the nephron, including Bowman's space. The casts have reasonably typical tinctorial properties: periodic acid-Schiff (PAS) negative, brightly eosinophilic, fuchsinophilic with Masson's trichrome and, infrequently, Congo red positive. The staining is not always uniform within the same cast or among all casts in the same kidney, but the above colors are most typical. The casts may be lamellated, contain crystals of a variety of shapes, and, rarely at the periphery have a spicular appearance. There are reasonably constant abnormalities in tubular epithelium; proximal cells often contain numerous uniform cytoplasmic vacuoles. Cells of all tubular

Figure 16.1. Bence Jones (light chain)cast nephropathy with a tubule containing a pale-staining cast surrounded by a multinucleated giant cell. In contrast, casts composed primarily of Tamm-Horsfall protein are periodic acid-Schiff (PAS) positive (PAS stain).

segments may be necrotic and sloughed into lumina, where they may be adherent to the edges of the casts. Tubular basement membranes are discontinuous, thereby allowing free communication between the interstitium and the tubular lumina; it is through these gaps that monocytes and other inflammatory cells migrate from the interstitium. The adjacent interstitium is edematous and often infiltrated by monocytes and lymphocyte (1-4). This constellation of light microscopic abnormalities, especially the morphology and tinctorial properties of the casts and the surrounding giant cells, is sufficiently distinctive to be diagnostic of multiple myeloma. Indeed, it is not unusual for the renal biopsy to be the first test indicative of myeloma in a patient who presents with acute renal failure of seemingly unknown origin (5).

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