Session Item

The novel Exactrac Dynamic (Brainlab AG, Germany) provides real-time 3D surface imaging by combining structured light with thermal information, and is complemented by an in-room kV X-ray imaging system. The aim of the study was to compare the positioning accuracy of the combined surface/thermal imaging system and the gold-standard, stereoscopic X-rays, for stereotactic radiosurgery (SRS) treatments. In order to provide a more realistic scenario, a head phantom prototype with a specific heat signature profile was used. Phantom specifications regarding surface temperature stability were also investigated.

An anthropomorphic 3D-printed head phantom (Prime, RTsafe, Greece) with bone equivalent material, 3 embedded ball bearings and can be filled with water up to 45°C was used. It was fixed to the table using a 4Pi open face mask (Brainlab AG, Germany). To investigate the phantom’s surface temperature stability, the phantom was filled with warm water (41°C) and surface temperature was measured with an infrared thermometer at 7 locations within the area of the face opening, app. every 10 min, for 65 min. The surface/thermal imaging positioning (D_S+T) was measured and compared to X-ray based positioning (D_X-RAY) at an Elekta Versa HD linac. The couch (at 0°) was displaced in all 3 directions (lateral, longitudinal and vertical) from the planned position (10 random translations, max. translation 2.5 mm), using the HexaPOD^TM evo RT interface (Elekta AB, Sweden). An empty (cold) phantom at room temperature and a warm water filled phantom were used. The difference between both positioning methods (ΔD_POSITION=ǀD_X-RAY-D_S+Tǀ) was calculated and an independent t-test was done to investigate the significance of the differences for a cold and a warm phantom.

Fig. 1 shows the temperature curves, where a stabilisation is seen 10 min after filling the phantom and during the next 55 min. In 6 out of 7 locations values measured within nearly 1h were between 36°C and 32°C. The 4°C difference is not expected to impact the measurements as the surface/thermal imaging reference is updated and zeroed after X-rays are acquired.

Tab. 1 shows the couch translations and the difference between both positioning methods. No statistical significance between the positioning of the cold and warm phantom was found (p ˃ 0.05).

Considering the surface temperature stability this phantom demonstrated to be suitable for measurements with ExacTrac Dynamic for at least 1h. The measurements with a warm phantom showed good agreement between surface/temperature and X-ray imaging. No deviations larger than 0.1 mm were found, leaving open the possibility of monitoring the patient using more surface guidance and less ionising radiation for cranial treatments. Measurements with a cold phantom showed slightly higher deviations. However, as the differences were not statistically significant, a larger set of measurements is planned to determine whether a heated phantom is essential for SRS QA.