What should the collimator angle be when planning an opposed portal with a gantry angle of 270° and a left lateral field?

In radiation therapy, the collimator angle is crucial when setting up fields for treatment, especially for an opposed portal. When the gantry angle is set at 270° for a left lateral treatment position, the collimator angle must be determined to ensure proper alignment of the radiation beam with the target area while minimizing exposure to surrounding healthy tissues.

In this instance, a collimator angle of 335° is appropriate because it accounts for the rotation from the gantry's position. The gantry's 270° represents the lateral aspect, and when planning for an opposed beam, the collimator is typically adjusted to achieve optimal alignment with the treatment volume and to respect the geometry of the patient’s anatomy.

To visualize it, if you think of a compass, with 0° (or 360°) pointing to the North, 270° points directly to the West. When adjusting the collimator angle, moving counterclockwise from 270° to 335° maintains the left lateral treatment orientation while ensuring that the radiation beams from the opposed portals intersect correctly at the target. This angle effectively integrates the dosimetry considerations for margin and scatter, making it the correct choice for achieving a well-planned treatment delivery.