Abstract
Purpose: The aim of replanning at the midcourse and the latter course was to illustrate the superior timing of replanning during intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC) patients. Methods: Twenty-three NPC patients received IMRT in 33 fractions were enrolled. Each patient with two repeated CT scans before the 16 fraction and before the 25 fraction, respectively. The anatomic changes were determined in the three CT scans. The differences of the dose distribution were compared in inital plan vs. phantom plan 1 and replanning 1vs. phantom plan 2. Results: The mean decrements of the transverse diameter of nasopharyngeal level (d1) and the transverse diameter of the neck level (d2) were 4.66mm, 2.63mm and 7.94mm, 5.78mm before the 16th and 25th fraction. The target volume reduced mainly in the first half. The mean decrements of volume GTVnx and GTVnd were 12.20cc, 8.91cc and 18.25cc, 13.71cc before the 16th and 25th fraction. While the volume of the parotid glands reduced mainly in the second half. The mean decrements of the left and right parotid glands volume were 1.40cc, 1.98cc and 6.31cc, 6.08cc before the 16th and 25th fraction. But the dose changes of bilateral parotids were more prominent in the first half than in the second half. There was a statistically significant increase in the mean dose and V30 to the bilateral parotid only in the first half (P < 0.05). Based on dose constraint criterion in the RTOG0225 protocol, the dose of the normal critical structures for 39.13% (9/23) were out of limit comparing to in the first half than 17.39(4/23) in the second half. Conclusions: Replanning at the midcourse may reach more benefits both in the coverage of the targets and sparing of normal structures for NPC patients. This study was supported by Zhejiang Provincial Medical and Health science Foundation of China (2008B198).
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