Foetus in Foetu

Foetus in foetu, a descriptive term attributed to Meckel (circa 1800), should be applied only to those rare cases where a parasitic twin is found included within the abdomen of its partner. Lord (1954; 1956) reported two definite cases and reviewed other possible cases. In contrast, the variety of teratoma which occurs in the retroperitoneal space is much less defined. Since the description of the first recognizable case by Dickinson (1871), an increasingly complex volume of literature has accumulated. Palumbo, Cross, Smith and Baronas (1949) documented many of the earliest cases of retroperitoneal teratoma and their work drew together some of the facts concerning a condition which they rightly say is seldom discussed in the medical reference volumes. Charles (1953) traced 82 case reports in the literature and since this time at least five further reports have appeared including some from Russia and Bulgaria. Many theories of origin have been suggested, with some authors contending that the teratoma represents a form of included twin. Although this is now unacceptable, it has resulted in the description of many retroperitoneal teratomas in confusing terms. Willis (1958) pointed out the separate nature of the retroperitoneal teratoma and foetus in foetu, emphasizing that the former is a true tumour while the latter is not. However, as both conditions occur in the upper part of the retroperitoneal space, it is not surprising that terms such as 'foetus in foetu', 'parasitic twin' and 'suppressed twin' have been misapplied to teratomas having a marked degree of differentiation and some foetiform characteristic. Indeed, Willis concedes that final classification may be difficult, though he does not admit that the overlap of certain features implies that the two conditions are the same. The purpose of this report is (1) to describe a further case where borderline pathological features are present; (2) to discuss and compare briefly the