TheAero1221 t1_ivo99i1 wrote on November 9, 2022 at 12:15 PM

Reply to comment by Xist3nce in Experimental “FLASH” cancer treatment aces first human trial by tonymmorley

REFER TO GUY BELOW

Not a researcher, but I know that traditionally there has been a concerted effort to deliver radiation to the cancer site via small angles of attack that vary over time. They do this because even though the radiation is not at its maximum concentration when traveling through the noncancerous tissue, there is still some chance of causing undesired damage if those areas are repeatedly exposed. Subsequent treatments change the angle of attack so that they can minimize the radiation exposure to any given area of tissue that they do not wish to damage. Attacking from a wider angle may mean less total safe treatments are possible since fewer viable attack angles will remain after each treatment. Idk. Again not a researcher, just guessing. Could be totally wrong.

Mounta1nK1ng t1_ivpj6t8 wrote on November 9, 2022 at 5:50 PM

You are correct that you're totally wrong. We don't vary the angles over time, we just select a large number of angles. Basic idea, if you shot from just one angle, all the tissue on the entrance path would be getting more dose than the tumor. If you shot through 10 different angles for a single treatment, than each entrance path through normal tissue would be getting only 10% of the dose, but where they all overlap at the tumor, it's getting 100% of the dose. This treatment with the 10 angles (usually actually only 5, 7, or 9 for static IMRT) would be given every day for 5-8 weeks in conventional radiotherapy using those same angles for each daily treatment.

The evolution of this is having the gantry rotate constantly through the treatment, varying the dose rate and shape of the beam during the rotation to avoid dose to especially sensitive tissues, while making sure you completely cover the target. So instead of 10 (or 5 or 7) discrete angles you have the maximal spread of the entrance dose. It's called VMAT, volumetric modulated arc therapy.