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One thing has not changed: the radiologist in the tropics must have sound knowledge of local disease patterns amongst his or her patients if a reputation for accurate diagnosis is to be maintained (Fig. 43.1). Reference to standard textbooks from America or Europe will be of little help, and this is particularly true for those which are based on statistical aspects of probability for their own populations. Indeed, this applies not only to malignant disease but also to almost all illnesses (see chapter on Non-Malignant Neoplastic Diseases).

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Fig. 43.1 A-D. Metastases may occur from primary tumors which have quite a different incidence in tropical countries compared with nontropical regions, or even between various parts of the tropics. (A, B) The upper cervical metastasis shown in A came from a primary hepatoma, as did the metastasis in the thumb shown in B. Both were the presenting lesions in these two patients. Primary hepatoma is very com mon in many parts of Africa and Asia. (C, D) In the tropics, pulmonary metastases often originate from unusual primary tumors: (C) pulmonary metastases from a carcinoma of the esophagus, which also caused widening of the upper mediastinum; (D) pulmonary metastases from a carcinoma of the penis. It is not possible to locate the primary tumor by looking at the chest radiograph..

This chapter therefore gives a brief summary to emphasize certain problems in diagnostic imaging, which may be of help to physicians in a temperate climate caring for a patient whose home is in the tropics. It may also be of assistance to radiologists or physicians practicing for the first time in a tropical country. None of the sections are really comprehensive, and, as in the Chapter on Non Malignant Neoplastic Diseases, almost anyone working in the tropics will be able to claim differences in their own experience. Only broad outlines have been given, recognizing the numerous local variations which will occur with each geographic area. No text can be a substitute for local knowledge and experience, but it is important to accept the principle that malignant disease varies, not only in its incidence, but in its relative malignancy and natural history. A cancer with a high rate of cure in one area may be histologically identical and yet highly malignant and almost untreatable in a different population. The pathology may be the same, but the patients and the course of their illness are different.

It is impossible in a chapter such as this to give absolute figures, and it would be misleading to do so when comparing such widely different populations. This chapter refers to any malignancy in the tropics as being common, or very common, or, alternatively, uncommon or rare, using its general frequency in Europe or North America as a standard. "Very common" would suggest that the radiologist at a large general hospital is likely to see a case every week, whereas "rare" would indicate that a case will be seen only once or twice a year.

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