Which device indicates the isocenter of a linac?

The optical distance indicator (ODI) is a device used in linear accelerators (linacs) to help visualize and indicate the isocenter, which is a crucial point where radiation beams intersect for treatment. The isocenter is pivotal in ensuring that radiation is precisely delivered to the target area while minimizing exposure to surrounding healthy tissues. The ODI projects a laser beam that highlights this point, making it easier for the medical staff to align both the patient and the treatment machinery accurately.

The multileaf collimator, while essential for shaping and modulating the radiation beam during treatment, does not directly indicate the isocenter. Its primary function is to conform the radiation dose to the shape of the tumor.

The vertical laser array serves primarily as a reference for patient positioning in conjunction with the ODI. It helps align the patient in relation to the machine but does not specifically identify the isocenter.

The electronic distance meter measures distances within the treatment room and can be a valuable tool for various calculations but is not specifically designed for indicating the isocenter itself.

Thus, the ODI stands out as the correct answer because of its specific role in directly marking the isocenter, aiding in precise treatment delivery.