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Spontaneous remission, also called spontaneous healing or spontaneous regression, is an unexpected improvement or cure from a disease that appears to be progressing in its severity. These terms are commonly used for unexpected transient or final improvements in cancer. Spontaneous remissions concern cancers of the haematopoietic system (blood cancer, e.g. leukemia), while spontaneous regressions concern palpable tumors; however, both notions are often used interchangeably.
The spontaneous regression and remission from cancer was defined by Everson and Cole in their 1966 book as "the partial or complete disappearance of a malignant tumour in the absence of all treatment, or in the presence of therapy which is considered inadequate to exert significant influence on neoplastic disease."
It has long been assumed that spontaneous regressions, let alone cures, from cancer are rare phenomena, and that some forms of cancer are more prone to unexpected courses (melanoma, neuroblastoma, lymphoma) than others (carcinoma). Frequency was estimated to be about 1 in 100,000 cancers; however, in reality this ratio might be largely under- or overestimated. For one, not all cases of spontaneous regression can be apprehended, either because the case was not well documented or the physician was not willing or literate enough to publish, or simply because the patient did not show up in a clinic any more. On the other hand, for the past 100 years almost all cancer patients have been treated in one way or the other, such that the influence of treatment cannot always be excluded.
At least for small tumors the frequency of spontaneous regression most likely was drastically underrated. In a carefully designed study on mammography it was found that 22% of all breast cancer cases underwent spontaneous regression.
Everson and Cole offered as explanation for spontaneous regression from cancer:
In many of the collected cases ... it must be acknowledged that the factors or mechanisms responsible for spontaneous regression are obscure or unknown in the light of present knowledge. However, in some of the cases, available knowledge permits one to infer that hormonal influences probably were important. ... In other cases, the protocols strongly suggest that an immune mechanism was responsible.
Challis and Stam, even more at a loss, concluded in 1989, "In summary, we are left to conclude that, although a great number of interesting and unusual cases continue to be published annually, there is still little conclusive data that explains the occurrence of spontaneous regression."
In medical circles sometimes apoptosis (programmed cell death) or angiogenesis (growth of new blood vessels) is discussed as "cause" of spontaneous regression. But both mechanisms need appropriate biochemical triggers and can not be caused on their own. To the contrary, in many cancer cells apoptosis is defective, angiogenesis is activated, both caused by mutations in cancer cells; cancer exists because both mechanisms are malfunctioning.
A great number of spontaneous regressions from cancer occurred after a feverish infection. If this coincidence in time would be a causal connection, it should as well precipitate as prophylactic effect, i.e. feverish infections should lower the risk to develop cancer later. This could be confirmed recently by collecting epidemiological studies.