KEY POINTS
- Radiopaque skin wires used to identify scars and field borders can perturb proton dose calculations and are therefore commonly manually contoured and overridden to air. The study tested whether physically separating the marker from the skin could simplify this step.
- An anthropomorphic phantom was scanned with conventional CT-SPOT markers placed directly on the surface and foam-backed RT-SPOT markers elevated by 1.5, 3.0, or 4.5 mm. Three experienced dosimetrists independently measured body- and marker-contouring time.
- Contouring time fell from 6.7 minutes with standard skin markers to 4.7, 4.2 and 4.0 minutes with 1.5-, 3.0- and 4.5-mm elevation, respectively—reductions of approximately 30%, 37% and 40%.
- The workflow gain plateaued quickly. Moving from 1.5 to 4.5 mm saved only another 0.7 minutes, while geometric error progressively increased; mean positional error was 0.97, 1.16 and 1.31 mm at 1.5, 3.0 and 4.5 mm, respectively.
- At 70 MeV, a conventional skin marker caused a 2.6-mm Bragg-peak shift and 2.2% dose change when not density-overridden. Elevated markers eliminated the measurable Bragg-peak shift, with only approximately 2% dose change.
- At 160 MeV, the conventional marker still produced a 1.9-mm Bragg-peak shift, although no meaningful dose change was observed. At 200 MeV with a 5-cm range shifter, none of the marker configurations produced measurable dose or range changes.
- The investigators concluded that 1.5–3.0 mm elevation provided the best compromise between contouring efficiency and geometric accuracy. Their centre subsequently adopted the 1.5-mm RT-SPOT clinically for breast and head-and-neck proton patients.
CLINICAL TAKEAWAY
A few millimeters of physical separation between a radiopaque skin marker and the patient surface can materially simplify proton simulation and reduce dependence on manual contouring overrides. The finding is practical but remains a phantom-based workflow study, and its dosimetric importance is greatest for lower-energy, superficial proton beams.