Chronic administration of testosterone induces a low to moderate (5-56%) incidence of prostate cancer in several rat strains [190,194-199], but not in all strains [200]. The induced tumors were adenocarcinomas in all studies but one, in which also some squamous cell carcinomas were observed [197], and these carcinomas appeared to develop from the dorsolateral prostate and/or coagulating gland, but not the ventral prostate lobe [190,194-198]. The prostate carcinoma incidence in most of these studies was low (5-20%) [190,194,197,198]. Only the studies reported by Pollard and co-workers using the Lobund Wistar strain sometimes had higher carcinoma incidences, but the incidences varied considerably (0-60%) [195, 196, 201-204]. In the only other study with the Lobund Wistar strain, a 7% incidence was found [198]. The actual dose of testosterone considerably fluctuated over time in many of these studies from five to ten times control values down to control values [198,200], but even when the level of circulating testosterone was kept steadily elevated by two- to threefold, prostate carcinomas were induced [199]. These data indicate that testosterone acts as a complete carcinogen for the rat prostate.

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