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This issue is not new in radiology imaging.

CT and MRI are imaging modalities that predominantly image the size of a lesion. A CT depiction is merely a graphical interpretation of physical density. An MRI examination in some body parts may also demonstrate the same anatomy but may further infer some chemical composition of the material by means of its different signal characteristics. The activity of some tumors has been has been associated with these signal characteristics. Either or a CT or MRI examination may be performed with intravenous contrast which may help evaluate the vascularity/perfusion of a lesion which again may infer biological activity. A PET (positron emission tomography) examination is an even better indicator of biological activity because it depicts glucose utilization anatomically. There are size and location limitations to the lesions evaluable by PET.

Here is the problem. For a long time it has been known that tumors may regress minimally in size but significantly in biological activity. For lymphoma treated with conventional chemo this was known as the residual mass or sterile mass.The problem predates immunotherapy.

However, the WHO and later RECIST criteria have remained steadfast in their utilization of a simple size measurement to determine tumor treatment response. The absence of significant radiology input in the development of these criteria was likely in part to blame. The RECIST criteria in particular are a shambolic embarassement to oncology, a step backwards from the WHO criteria.

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