Which measurement is critical to ensure proper linac functioning during QA checks?

Ensuring the proper functioning of a linear accelerator (linac) during quality assurance (QA) checks is essential for delivering accurate and safe radiation treatment to patients. Each of the measurements listed contributes to the overall performance and safety of the device, making them critical.

Monitoring energy output is vital because it confirms that the linac is delivering the correct dose of radiation at the specified energy level. Any deviation from the expected energy output can lead to underdosing or overdosing patients, which could seriously impact treatment efficacy and patient safety.

Field size accuracy is another important parameter. It ensures that the treatment area is appropriately defined according to the treatment plan. Accurate field size helps to ensure that only the intended area receives radiation, limiting exposure to surrounding healthy tissues and minimizing side effects.

Spatial alignment of beams is also crucial for effective radiation delivery. Proper alignment ensures that the beams converge accurately on the target tumor while avoiding critical structures. Misalignment can result in incorrect treatments, potentially leading to harm rather than benefit.

Since each of these aspects is essential for the safe and effective functioning of the linac, the answer encompasses the necessity to assess all three during quality assurance checks, confirming that the system operates as intended across multiple critical parameters.