Mike

There have been amazing advances in radiation therapy. That's why I used those quotation marks with maximum. My understanding from my RO is that generally for head and neck cancer, 77 GY used to be considered the "maximum". Beyond that dosage, too many bad side effects. Many oncologists still mistakenly believe that once you have had the "maximum" then you cannot be radiated again in the same area based on old outdated studies. But with the alternative radiation delivery systems to IMRT, including CyberKnife (sterotatic radiation), Proton Therapy, especially IMPT, etc , people like me who get a recurrence can now be treated again instead of dying. Sad to say, we still get posters here whose ROs are living in the past.

Different cancers in different areas may be different, I never researched that.

Since you mentioned scanners, they and Xrays CTs etc use a different measurement. In one sense, one Gy (Gray) equals one Sv (sievert) which is how they measure scanners (actually mili sieverts) but there are other factors than dosage Wikipedia explains it the simplest
[quote]The sievert (symbol: Sv) is the International System of Units (SI) derived unit of equivalent radiation dose, effective dose, and committed dose. Quantities that are measured in sieverts are designed to represent the stochastic biological effects of ionizing radiation. The sievert should not be used to express the unmodified absorbed dose of radiation energy, which is a clear physical quantity measured in grays. To enable consideration of biological effects, further calculations must be performed to convert absorbed dose into effective dose, the details of which depend on the biological context. This can be far more complicated than just multiplying by a weighting factor..[/quote] Best guess is that we've had hundreds of thousands scanner doses