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The Dose Difference of Outside Region of Prescribed Dose in IMRT for Nasopharyngeal Carcinoma

CHEN Ju-hui, XU Lu-ying, LI Qi-xin, et al. (Fujian Province Cancer Hospital, Fuzhou 350014, China)  
[Purpose] To observe the dose variability to the tissue outside the isodose curve of the prescribed dose (the tumoricidal dose DT70Gy and the prophylaxis dose DT56Gy) in IMRT plan to nasopharyngeal carcinoma, to evaluate the dosimetric impact of inter-observer contouring variation. [Methods] Twenty cases with pathologically confirmed nasopharyngeal carcinoma underwent IMRT, target delineation and program design according to the procedure. The tissue volume enclose by the isodose curve of the prescribed dose DT70Gy and DT56Gy were defined as V70 and V56 respectively. 0.5cm and 1cm expansions of the V70 were performed followed by subtractions of the original V70 to generate axial 0.5cm and 1cm "rings of tissue" concentric around the V70 which were defined as V70+0.5 and V70+1. In the same way the V56+0.5 and V56+1 were defined, and DVHs were created for these rims. [Results] The D50 of the V70+0.5, V70+1, V56+0.5 and V56+1 were 65.94Gy, 62.28Gy, 51.51Gy and 47.03Gy respectively, and D95 of them were 58.86Gy, 49.66Gy, 38.87Gy and 29.03Gy respectively. [Conclusions] In an IMRT plan, there is a higher radiation dose at 0.5~1cm outside region of prescribed dose. "Missing doses" due to the difference on target delineation is not significant.
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