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In page Metastasis:

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The use of immunohistochemistry has permitted pathologists to give an identity to many of these metastases. However, imaging of the indicated area only occasionally reveals a primary. In rare cases (e.g., of melanoma), no primary tumor is found, even on autopsy. It is therefore thought that some primary tumors can regress completely, but leave their metastases behind. In other cases, the tumor might just be too small and/or in an unusual location to be diagnosed.[citation needed]