During a review of a new treatment vault, the architect points out three areas: an east wall that can be struck by the useful beam at gantry 90°, a north wall several meters from the couch that never crosses the beam path, and a maze wall seen only around a corner from the target. Which classification correctly identifies the shielding basis for each area?

The east wall can intercept the useful beam at gantry 90°, so it is a primary barrier designed for direct-beam exposure. The north wall is outside the beam path, and the maze wall is around a corner from the target; both are secondary barriers designed for the applicable combination of patient or room scatter and treatment-head leakage. Describing the east wall as primary but saying leakage is not checked there is incomplete: its shielding must address the direct beam, and leakage must also be assessed to determine whether it affects the required thickness. Treating the north wall as leakage-only is also incorrect because being outside the beam does not eliminate scattered radiation. Likewise, the maze corner prevents direct-beam exposure but does not eliminate scatter, so a leakage-only basis is insufficient. Exam caveat: primary and secondary identify the radiation basis for shielding, not a guarantee that only one radiation component contributes to final barrier thickness. Practical check: verify the approved workload, use factors, beam orientations, occupancy, and maze geometry in the site-specific shielding calculation, including scatter and leakage where applicable.