Why are small bowel tumours rare? An experimental model

It is a well known clinical observation that small bowel tumours are rarer than tumours of the stomach or colon. The fluidity and relative sterility of small bowel contents, and rapid transit time and of the relative sterility of the small bowel itself have been suggested as possible factors contributing to this relatively low incidence. A further mechanism, that of a local immune response against malignant cells, has also been suggested and this investigation is the subject of this paper. A transplantable tumour (Gardner lymphosarcoma) was injected either into the stomach or small bowel of CBA mice, and the incidence of subsequent tumour growth was studied when the immune status of the host mouse was altered. Normal mice, mice ;deprived' by thymectomy and irradiation (T-cell-deficient mice), and ;reconstituted mice' (prepared as the ;deprived' animals but given a thymus graft) were used. The results showed that in normal and reconstituted mice more tumours arise in the stomach than in the small bowel but that in deprived mice the incidence of tumours was the same in both sites. This could be taken to suggest that local immune responses can suppress the development of tumours of the small bowel.