The transformation of a normal cell into a cancer cell is not a sudden but a slow continuous process which may take years. A systematic study of the morphological and structural changes that take place during this cellular transformation has only become possible since methods were developed to induce a high incidence of tumors in experimental animals. The growth of such tumors can be followed during all stages of their development. For several reasons, rat liver has proved to be particularly suitable. For more than thirty years it has been known that one can induce tumors in rat liver experimentally with many different substances. The azo dyes which were used originally (YOSHIDA, 1932; KINOSITA, 1937) have since been replaced by more potent carcinogens. The recently discovered nitrosamines, dimethylnitrosamine (MAGEE and BARNES, 1956), diethylnitrosamine (SCHMAHL et aI. , 1960) and N-nitrosomorpholine (DRUCKREY et aI. , 1961) should be mentioned in this context. Used in the proper dose, these carcinogens lead within a few months to the formation of multicentric hepatomas in practically 100 Ufo of the experimental animals, and are therefore a very useful tool for studies of the cytogenesis of cancer (see BUCHNER, 1961; GRUND MANN, 1961; GRUNDMANN and SIEBURG, 1962; OEHLERT and HARTJE, 1963; BAN NASCH and MULLER, 1964). The liver parenchyma as such offers a very big technical advantage because it consists of a rather homogenous cell population.

Early detection of cancer has been recognized as an important area of preventive medicine for quite some time. In a few sites, such as the cervix, the breast, and the skin, it has been shown beyond doubt that early detection and secondary prevention of cancer are realistic goals. However, the general success of this approach is limited and re- quires substantial additional effort in research and public health education. The present volume is based on an International Symposium on Cancer Diagnosis - Early Detection, which was organized by the Committee Coordinating Cancer Research in GroBforschungsein- richtungen of the Federal Republic of Germany. The meeting was held at the German Cancer Research Center in Heidelberg, June 3-15, 1991. I am grateful to all members of the Scientific Committee who gave valuable advice in preparing the program of the conference: Gunther Bastert (Heidelberg), John Collins (Braunschweig), Heinz Hofler (MunichINeuherberg), Ernst Jung (Mannheim), Gerhard van Kaick (Heidelberg), Stefan Meuer (Heidelberg), Michael Wannenmacher (Heidelberg), and Wolfgang Wilmanns (Munich/ Neuherberg). I am obliged to Volker Diehl (Cologne), Gunther Bastert (Heidelberg), Hans K. Miiller-Hermelink (Wiirzburg), Manfred Schwab (Heidelberg), and Georg Dhom (Homburg/Saar) for serving as chairmen of the sessions. I am also grateful to Horst Metzler and the administration of the German Cancer Research Center for all their efforts to guarantee a smooth running of the Conference.

In more than 20 contributions new trends in cancer therapy are reviewed and critically discussed by highly competent authors. Their contributions are based on an international symposium and cover all major fields of clinical oncology. Special emphasis is given to surgical oncology (including photodynamic laser therapy and radiosurgery), radiation oncology, hyperthermia, new antineoplastic agents, enzyme-pattern-targeted chemotherapy, drug resistance, biological response modifiers, bone marrow transplantation, antibody guided therapy, induction of cell differentiation in transformed cells, and the individualization of tumor therapy. The stimulating discussion at the end of each section covering these topics will particulary benefit the clinical oncologist and all researchers devoted to the improvement of cancer therapy.

A large body of information about possible causes of cancer has been ac cumulated by both clinical experience in man and observations in animal experiments. There is general agreement that in most cases cancer is due to exogenous factors, albeit the genetic disposition may also play an impor tant part. Three main categories of exogenous risk factors have been iden tified, namely chemicals, radiation, and viruses. In principle, it should be possible to prevent cancer by eliminating such risk factors. However, in spite of the rapidly growing number of well established risk factors, many shortcomings in our understanding of the causes of human cancer and in the introduction of effective preventive measures are evident. In addition to obvious gaps in our scientific knowledge, inadequacies in personal life style, irresponsible handling of environmental contaminants, and in some areas also deficiencies in political decisions seriously hamper the elimina tion of risk factors for cancer. It was this background which prompted the Committee coordinating Cancer Research in the GroBforschungseinrichtungen in the Federal Re public of Germany to organize a conference on Strategies for the Elimina tion of Cancer Risks. The conference was held in Heidelberg in June 1986. In areas which appeared to be especially pertinent to public health, author itative speakers presented the state of the art and prepared the basis for the discussion. Both the presentations of the speakers and the summaries of the discussions are contained in this book."