Avoidance sectors are regions along the arc rotation that the dosimetrist has specified that no dose be deposited. During treatment, the gantry will rotate as normal through an avoidance sector, but the console screen will show a dose rate of zero. The dose rate will return once the gantry exits the avoidance sector.
For the example shown above, using a partial arc from ‘181° to 345°’ may suffice, but having the arc also rotate from ‘125° to 179°’ will result in a tighter dose carve to spare the brainstem and improved midline coverage of the PTV. Instead of using two separate partial arcs, you can use a full arc with an avoidance sector from ‘345° to 125°’. This can be achieved in the optimizer by selecting plan information and inputting the arc rotation length. Avoidance sectors can also be used in areas of high artifact such as a prosthesis in a prostate plan even after manual density corrections.
Using a full arc with an avoidance sector ‘345° to 125°’ will result in a shorter treatment time compared to using two separate partial arcs of ‘181° to 345°’ and ‘125° to 179°’ due to the downtime between moding fields. Additionally, using two partial arcs is technically equivalent to one rotation around the patient, but plans will commonly necessitate at least two rotations around the patient, e.g., ‘CW 181° to 179°’ and ‘CCW 179°-181°’. Thus, the number of partial arcs required will double the number of desired rotations. Lastly on a minor note, using avoidance sectors will save time for the department as a whole as it means less fields for IMRT QA and chart checking, and also reducing unnecessary cost (patient billed per dose calculation) onto the healthcare industry.
Avoidance sectors should be used with caution as its major concern is associated with clearance. If the isocenter, when aligned to the PTV, is set off midline with a high enough table top, the gantry can collide with the couch as it rotates around the contralateral side. Shifting the isocenter more midline and posterior can reduce clearance issues, but be aware that large shifts away from the center of the PTV can result in dosimetric consequences as well. In the case that isocenter shifts cannot be made and full arcs with avoidance sectors cannot clear, using partial arcs can be the method of choice. Using the example from above, the therapists will set up the patient up as normal and treat the partial arc from ‘181° to 345°’. The therapists will re-enter the room and shift the couch laterally away from region of collision and then manually move the gantry to 125° to clear the couch. They will then shift the couch back to its initial position, and resume treatment to treat the partial arc from ‘125° to 179°’.