Room: Exhibit Hall
Purpose: To quantify the dosimetric impact of positionning and immobilization devices on patientâ€™s skin for IMRT treatments.
Methods: For this study two widely used systems, Curve Fibreplast mask (Qfix) for head and neck and Vac-Lok cushion (CIVCO) were selected. These systems were mounted on two different table tops, Exact Couch top (carbon fiber grid) and IGRT Couch table top (carbon fiber homogeneous). A clinical sliding window IMRT plan was delivered by a clinac 2100 CD (Varian) on a QUASAR phantom (Modus Medical). 6 MV X-ray beams were used. Skin dose was measured using EBT3 radiochromic films attached to its surface. Measurements were repeated for each of the combinations of immobilization systems and table tops. EBT3 films were scanned using EPSON EXPRESSION 10000XL scanner and Film QA Pro2014 Software (Ashland Advanced Materials) was used for dose analysis. Cross plane dose profiles through the beam axis were extracted from 2D dose distributions.
Results: An increase in skin dose from 0.35 to 0.65(Gy/fx) was observed when mounting the vacuum mattress on the Exact board, which also softened the effect of the grid. The increase of skin dose when combined with the IGRT couch was much lower from 0.10 to 0.12(Gy/fx) . When using Curve fibreplast masks the increase in skin dose was between from 0.63 to 0.99(Gy/fx) and from 0.71 to 0.95(Gy/fx) depending on mask stretching.
Conclusion: The effect of immobilizers and positioning devices should be taken into account when high doses are expected near the skin as the skin dose can significantly increase. As TPS do not calculate skin dose accurately, in vivo dosimetry with EBT3 films using the same methodology as for the phantom measurements should be implemented to assess skin dose.This work was partially financed by the grant Singulars Projects 2015 of the Spanish Association Against Cancer (AECC).