Which method can significantly reduce field placement errors?

Using thermoplastic masks for immobilization significantly reduces field placement errors because these masks are specifically designed to fit the contours of a patient's anatomy, ensuring that they remain in the same position throughout the treatment process. Proper immobilization is critical in radiation therapy, as even small movements can lead to misalignment of the treatment beams, potentially affecting the accuracy of radiation delivery and impacting treatment efficacy.

Thermoplastic masks provide a customized support that restricts patient movement and helps in consistently reproducing the treatment setup for each session. This reduces variability and enhances the precision of targeting the treatment area while minimizing exposure to surrounding healthy tissues.

In contrast, other methods listed do not address the core issue of patient positioning to the same effect, leading to potential inaccuracies in treatment delivery. This is why using thermoplastic masks is a recognized best practice in the field of medical dosimetry for enhancing treatment precision and patient safety.