I found the information that in order to see cancer cells we need a volume of few mm^3. I wonder how this value may be supported by actual medical data.
Modern clinical PET scanners have a resolution limit of 4 mm, corresponding to the detection of tumors with a volume of 0.2 ml (7 mm diameter) in 5:1 T/B ratio.
c.f . Article Limits of Tumor Detectability in Nuclear Medicine and PET