Optimising a Radiotherapy Optical Surface Monitoring System to Account for the Effects of Patient Skin Contour and Skin Colour

Optical surface monitoring systems (OSMSs) are designed to assist patient setup and patient motion management during radiotherapy treatments. Systems use projected and reflected patterns of coloured light on the patients surface and therefore depend on the skin’s optical absorbance and reflectance properties, which can vary with surface shape and colour. This study aimed to identify optimal operating parameters for the Catalyst HD OSMS (C-rad, Uppsala, Sweden) when used to monitor the surfaces of 3D-printed objects with various convex and concave surfaces, one of which was painted in six different colours with various levels of red and black saturation (from light pink to dark grey). The degree of surface detection was assessed via the Catalyst HD interface, with different levels of gain (100–600%) and signal integration time (1–7 ms). The OSMS was able to detect horizontal and convex shapes more consistently than vertical or steeply angled surfaces. The OSMS was not able to detect the darkest surface at all, even with the highest gain and the longest integration times. Mid-grey surfaces were detectable only when the integration time was increased to 2 s. All pink surfaces were easily detectable at the shortest integration time, with the OSMS performing best when red saturation was highest. Further work is recommended, as the red undertone of all human skin may lead to improved results for real patients. However, these preliminary results indicate that careful commissioning and optimisation of OSMS systems may be required before they can be used in radiotherapy treatments for a broad patient cohort.