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Dr Grange and colleagues conclude that recent advances in the understanding of the nature of the immune response will enable a move ‘from the empirical to the rational approach’ in devising immunotherapy for cancer (June 2002 JRSM1). They quote latent periods in the progress of cancer and of tuberculosis as evidence that an immune response is involved in both cases. This need not be so.
In both diseases there is cell death in the centre of masses (whether tubercles or cancers) that have outstripped their circulation2. In cancers the dead tissue may become organized and potentially proliferative cells sequestered, remote from blood-borne treatment and resistant to radiation through hypoxia. Successful treatment of the outer well-vascularized tumour leaves a core that may be slowly resorbed until the sequestered cells are exposed to a fresh blood supply; regrowth then occurs.
In the interpretation of so-called latent periods in cancer the most influential factor is a huge range of growth rates. In our series of unselected breast cancers3 the 95% confidence intervals of the distribution of inferred volume doubling times were 6 and 121 days. Some 30 doublings of volume from a single cell are required to produce a clinically detectable tumour of about 1 cm in diameter. Thus growth from a single cell might take more than 10 years to achieve a diagnosable size. The published work abounds with much more slowly growing examples4. After treatments, any residual subclinical disease is unmeasurable over a range of sizes that represent three-quarters of the life-span of a tumour.
One may not assume that apparent delays in growth result from an immune response.