Putting things in the way of the beam to measure dose is actually quite common.<p>You can do real time dose monitoring using diodes (see [1]). The diodes are quite small and don&#x27;t generate huge amounts of scatter.<p>It is quite common to use TLD&#x27;s[2] to monitor patient dose which is then analyzed offline to give dose estimates after the fact.<p>edit: that said, I don&#x27;t think anybody was doing real-time in-vivo measurements during the era of the Therac incident.<p>[1] <a href="https:&#x2F;&#x2F;www.aapm.org&#x2F;pubs&#x2F;reports&#x2F;RPT_87.pdf" rel="nofollow">https:&#x2F;&#x2F;www.aapm.org&#x2F;pubs&#x2F;reports&#x2F;RPT_87.pdf</a><p>[2] TLD=thermoluminescent dosimeter, little hunks of metal whose electrons get stuck in an excited state when exposed to radiation, you then heat them up and measure the light they give off as they relax back to ground state.  The amount of light they give off can then be correlated with the dose the patient received.