Optimum elevation between the skin surface and skin markers for proton therapy CT simulation

Radiopaque wires are commonly placed on the patient's skin during CT simulation to identify surgical scars and treatment field borders. Because proton dose calculations are sensitive to these markers, planners must manually contour them and assign an air density override, a time-consuming process. To address this challenge, commercially available RT-SPOT markers (Beekley Medical, Bristol, CT) were modified with radiolucent foam backing of varying thicknesses to elevate the marker from the skin surface. Elevated markers significantly improved workflow, reducing contouring time by up to 40%, with greater elevations providing shorter contouring times. However, geometric error increased with increasing elevation. An elevation of 1.5-3.0 mm provided the optimal balance, reducing contouring time by approximately 37% while maintaining geometric errors below 1 mm.