The collimator head rotates slowly around the patient. Definitely not what I would call spinning!
You can also hear the “hum” from the high voltage supply making the high energy x-ray photons which blast the cancer.
I used to wait for the treatment team finishing any adjustments and knew when the high voltage sound kicked in those little sods were being zapped!
Below is a good description from the cancer research website.
“The LINAC has a device called a multileaf collimator. The multileaf collimator is made up of thin leaves of lead which can move independently.
They can form shapes that fit precisely around the treatment area. The lead leaves can move while the machine moves around the patient. This shapes the beam of radiation to the tumour as the machine rotates.
This means that the tumour receives a very high dose and normal healthy cells nearby receive a much lower dose.
Each radiotherapy beam is divided into many small beamlets that can vary their intensity. This allows different doses of radiation to be given across the tumour.
IMRT can also create a U shaped (concave) area at the edge of the radiotherapy field. This avoids high radiation doses to structures that would otherwise be damaged by the radiotherapy. “
I am a Physics geek so I like to know the detail!