The aim is to reduce the dose delivered without effecting the X-ray beam profile. Changing E-gun voltage will reduce the current and hence the dose delivered is what I understand. Same is the case with width reduction or by delaying E-gun pulse with respect to RF pulse. But does this have any unwanted effects ? Does presence of only electron beam without RF in the LINAC tube effect the system in someway? Is it a common practice to change E-gun parameters like voltage, pulse width and pulse delay wrt RF input for dose variation in VMAT ?
Ahhhhh...... this is not so easy! With a linac it is the manufacturer whom chooses the gun filament voltage to optimise its function/life. Tinkering with that voltage is not wise and will most likely result in the gun falling out of the space charged region into the temperature limited region and everything will go funky. DO NOT tinker with the filament voltage! We also have a gun grid & gun cathode voltage. The gun grid voltage determines the gun current whilst the cathode voltage controls the injecting electron potential (which is a fixed voltage for each energy). Changing of the gun current will change the 'dose rate', but with changing gun current the RF will also need adjustment otherwise you will fall off the load line and the beam won't make it out the 3% energy slit, seen by the dose rate dropping. As an example of one linac manufacturer the dose rate is adjusted by changing the coincidence of the gun pulses and modulator pulses. The pulse width is 'fixed' (fixed for the dose rate, more on that later...) optimised with for the RF (modulator) pulse, the delay is also optimised so the electron pulse is timed correctly with the forward power, when we have too much delay the width of electron pulse can fall outside of the RF pulse and a number of electrons won't be accelerated. In VMAT, well it depends on the era of the linac, and manufacturer, the world I know; 'once upon a time' maximum dose rate was set but dose pulses were dropped to effectively alter the dose rate, however that has changed slightly with later linacs, see this to readArticle Treatment planning and dosimetric comparison study on two di...
Not sure whether I have answered your question or not
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