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SU‐FF‐T‐88: An Experimental Evaluation of the Impact of Setup Uncertainty On Dose Near the Surface for IMRT Plans Where the Skin Is Considered a Sensitive Structure and Is Excluded From the PTV
Author(s) -
Court L,
Tishler R
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2241013
Subject(s) - isocenter , imaging phantom , nuclear medicine , dosimeter , medicine , materials science , physics , optics , dosimetry
Purpose: (1) To investigate the impact of setup uncertainty on doses near the surface for IMRT plans where the skin is considered a sensitive structure and is excluded from the PTV. (2) To evaluate the impact of PTV design (how close to the skin the PTV is allowed) and number of gantry angles. Method and Materials: Effects of setup uncertainty were investigated experimentally using a semi‐cylindrical phantom with MOSFET dosimeters positioned at depths of 3, 6, 9 and 12mm. A CT image was taken of the phantom, and a node‐like CTV was drawn near the surface. Two PTVs were created by uniformly expanding the CTV by 5mm, and then pulling back 3 and 5mm from the body contour (PTV(3mm) and PTV(5mm), respectively). A 2mm skin structure was contoured. Seven and nine‐field IMRT plans were created using Eclipse for each PTV with the following guidelines: 99% of PTV volume to receive 90–93% of prescribed dose, maximum 105% hotspot, and minimize dose to skin structure. The phantom was then positioned at isocenter, the planned treatments delivered using a Varian 21Ex, and doses delivered to the build‐up region were measured. Setup uncertainty was simulated by shifting the phantom laterally in a range ±5mm, and the experiment repeated. Results: The number of beams did not significantly affect the results. For PTV(5mm), setup errors of 3 and 5mm reduced dose at 6mm depth by an average of 8% and 17%, respectively. For PTV(3mm), this was reduced to 4% and 12%, respectively. Corresponding numbers for 3mm depth were 8% and 12% for PTV(5mm), and 4% and 9% for PTV(3mm). Conclusion: To avoid daily dose to the surface nodes being reduced by more than 5%, the PTV should not be pulled back more than 3mm from the body contour, and setup uncertainty should be kept below 3mm.