Optimum Elevation between the Skin Surface and the Skin Markers for Proton Therapy CT Simulation
Abstract
Purpose
Proton dose calculation is sensitive to the metal wires, which are placed on the patient's skin during CT simulation to precisely outline the location of surgical scars and the intended radiation treatment field borders. In order to avoid the dose uncertainties, dosimetrists must contour the skin markers and manually assign the density of air, as the wires will not be present during treatment. This is a time-consuming process, which requires significant time for the planner. Hence, markers, elevated from the skin, are needed, which can mitigate these issues and strike a balance between acceptable contouring time and geometric accuracy.
Methods
A commercially available marker was modified so that the markers are elevated from the skin with various thicknesses of radiolucent foam backing. An anthropomorphic phantom with skin markers was placed by 4 professions (physicist, physician, dosimetrist, and therapist). The phantom was CT-scanned, and the marker and the body contours were drawn by 3 same-level dosimetrists. The markers were tested for contouring time, positional, and dosimetric accuracy to optimize the skin elevation.
Results
The contouring time has been dramatically reduced by 33% with the 1.5mm elevated markers compared to the traditional markers without elevation. Beyond the 1.5mm thickness, the contouring time reduces marginally. Geometric errors, however, increased from 0.9 to 1.3 mm when the elevation increased from 1.5 mm to 4.5 mm. Based on these findings, foam thicknesses of 1.5 mm - 3.0 mm offer a favorable balance between minimizing contouring time and maintaining good positional accuracy, making them optimal choices for practical implementation. The dosimetric accuracy was maintained for all the markers.
Conclusion
After successfully evaluating all the markers for clinical use for the proton therapy planning, the 1.5 mm (1-layer) foam back-up marker has been used in our clinic for breast and head-and-neck patients.