Which of the following is not a beam-modifying device used by radiation therapists?

A thermoplastic mask is primarily used for patient immobilization and does not modify the radiation beam itself. In radiation therapy, beam-modifying devices are those that alter the characteristics of the radiation beam, such as intensity or distribution, to achieve a desired treatment effect.

Bolus, for instance, is a beam-modifying device that helps to bring the dose of radiation closer to the surface of the skin, compensating for irregularities in patient anatomy. Wedges are utilized to modify the shape of the radiation beam to ensure uniform dose distribution across the treatment area. Dynamic collimation involves adjusting the beam's shape during treatment to match the contour of the tumor, also qualifying as a beam-modifying technique.

In contrast, while a thermoplastic mask is essential for accurately aligning the patient during treatment, it does not alter the beam parameters directly to modify how the radiation interacts with the tissue. This distinction makes the thermoplastic mask the appropriate choice for this question.