mmWave Sensing for Detecting Movement Through Thermoplastic Masks During Radiation Therapy Treatment

TL;DR

The paper tackles intrafraction motion during radiotherapy and the limitations of optical and ionizing imaging for motion tracking. It proposes millimeter-wave sensing in the band $[28,38]$ GHz to detect motion through thermoplastic masks, validating mask transmission and motion-induced signal changes with choreographed ground-truth events. Key contributions include characterizing the mask’s RF transparency, demonstrating detectable amplitude and phase changes behind the mask, and outlining pathways to scalable antenna arrays and motion-type classification. The work offers a non-ionizing, real-time approach to through-mask motion monitoring that could enable gating or automatic treatment adaptation, improving precision and patient safety in radiotherapy.

Abstract

Precision in radiation therapy relies on immobilization systems that limit patient motion. Thermoplastic masks are commonly used for this purpose, but subtle voluntary and involuntary movements such as jaw shifts, deep breathing, or eye squinting may still compromise treatment accuracy. Existing motion tracking methods are limited: optical systems require a clear line of sight and only detect surface motion, while X-ray-based tracking introduces additional ionizing radiation. This study explores the use of low-power, non-ionizing millimeter-wave (mmWave) sensing for through-mask motion detection. We characterize the RF properties of thermoplastic mask material in the 28-38 GHz range and perform motion detection using a 1 GHz bandwidth centered at 28 GHz. We use a frequency-domain system with horn antennas in a custom-built anechoic chamber to capture changes in the amplitude and phase of transmitted RF waves in response to subtle head and facial movements. These findings lay groundwork for future real-time through-mask motion tracking and future integration with multi-antenna systems and machine learning for error correction during radiotherapy.

Figures (6)

• Figure 1: Envisioned clinical setup where a mmWave antenna array is integrated into the radiotherapy system to sense patient motion through the thermoplastic radiotherapy mask, with a processing unit extracting motion information and feeding real-time feedback to the treatment control system.
• Figure 2: Experimental scenario for mannequin and human subject measurements through the thermoplastic mask, and for material characterization (highlighted in green).
• Figure 3: Results for the thermoplastic material characterization: Penetration gain over 28-38 GHz band.
• Figure 4: Unwrapped phase and amplitude variation over time when plastic mannequin is in subject's place.
• Figure 5: Amplitude and phase change at 28--GHz for each facial movement type, indexed as (1) Eye squinting, (2) Smiling, (3) Yawning, (4) Moving right, (5) Moving left.