Targeting cancer cells and missing all of the normal tissue is the Holy Grail of cancer therapy. It is the cancer equivalent to the perfect diet: eat everything you want, never exercise and stay perfectly skinny and fit. Doesn’t happen in metabolics and doesn’t happen in cancer therapy. Yet, to hear radiation oncologists or medical oncologists talk, you would think that all of our therapies are super precise. Let’s consider each in turn, but being a radiation oncologist I will spend more time on that.

Therapeutic radiation beams given from outside the body (as opposed to brachytherapy –otherwise known as implanted radiation or “seed therapy” in prostate cancer, which will not be considered here at all) are like visible light waves, but much higher energy. Like visible light, the physics of therapeutic radiation beams has the same wave/particle duality that make theoretical physicists lay awake at night. From a therapeutic perspective, though, the fact that the beam “travels in a straight line” is useful for it allows us to point the radiation beam at the tumor and miss the adjacent normal tissue (like using a knife to cut a bruised part out of a piece of fruit). However, like that bruised piece of fruit, the knife has to start on the outside of the fruit and cut deep enough to get out the bruise—if the bruise is deep, all of the fruit from the skin down to the bruise will also get cut away. In cancer therapy, this means that all the tissue from the skin down to the tumor “sees” the radiation beam.

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