Arc therapy

The accurate arc therapy requires three factors which should be correct:

1. The transmission ion chamber indication must be independent of gantry rotation – this is checked as in Section 12.4.
2. The gantry must start and stop at the required point.
3. The servo relationship between gantry rotation and dose must be correct. Among these the third is the most important.

It may be checked in a number of ways:
1. A film can be exposed in a cylindrical phantom.
2. An asymmetric phantom may be used and the expected dose calculated for a given arc. Then it can be checked that this dose is actually given when the arc is carried out.
3. If outputs of arc angle are available the delivered dose can be plotted directly against arc angle. (It should be noted that the dose rate is not necessary to be constant assumed that rotation speed is altered to compensate appropriately.) The accurate of the dose deivered in an asymmetric phantom should be within 2%. In many centres arc therapy is used only very occasionally or not at all and the frequency of testing can be set accordingly. If this possibility is not regularly tested teps must be taken to ensure that it is not used without the knowledge of the physics department. Arc therapy should not be skipped from acceptance testing, especially on a computer controlled accelerator where arcing may provide a useful test of the control system.