SU‐FF‐T‐360: Planning Strategies to Reduce Unnecessary Skin Dose in Head and Neck IMRT, Including Experimental Verification

Purpose: To investigate IMRT planning techniques that reduce skin dose without adversely impacting important doses at depth and to experimentally confirm these findings. Method and Materials: A semi‐cylindrical phantom was constructed with catheters at 3, 6, 9 and 12mm depths, allowing micro‐MOSFET dosimeters to be inserted to measure dose at multiple points on the surface and in the build‐up region. Critical structures, a node‐like CTV, and 2mm surface structure (‘skin’) were contoured on a CT image of the phantom with and without an immobilization mask. A PTV was generated by expanding the CTV 5mm in all directions up to the body contour; three modifications were then implemented, pulling back the PTV 0, 3 or 5mm from the body contour. Seven‐field IMRT plans were created using Eclipse to maximize PTV coverage, with one of the following strategies: (1) aim for maximum 110% hotspot, 115% allowed, (2) maximum 105% hotspot, (3) maximum 105% hotspot and 50% of skin to get maximum 70% of prescribed dose, (4) 99% of PTV volume to receive 90–93% of prescribed dose, maximum 105% hotspot, and dose to skin structure minimized. All twelve PTV and planning strategy combinations were investigated, with and without immobilization mask. The plans were then delivered using a Varian 21Ex, and dose to skin and build‐up region was measured. Results: From highest to lowest skin dose, the planning strategies were (3), (4), (2), (1), with little dependence on the PTV expansion approach. Technique (3), however, showed a tendency to underdose tissues at depth. Conclusion: The best strategy to reduce unnecessary skin dose, while maintaining target dose, is to pull the PTV back 3–5mm from the skin surface, and plan such that PTV coverage is maximized, with 99% of PTV volume receiving 90–93% of prescribed dose, maximum 105% hotspot, and the skin structure dose minimized.