Anatomical changes and dose distribution deviations of locally advanced nasopharyngeal carcinoma patients during IMRT

Objective To investigate the anatomical changes and dose distribution deviations of locally advanced nasopharyngeal carcinoma (NPC) patients during intensity-modulated radiation therapy (IMRT). Methods Weekly cone beam CT (CBCT) was applied to investigate the anatomical changes of enrolled 18 patients with locally advanced NPC. Dosimetric deviations were investigated with new IMRT plans after 20 fractions. Results The volumes of gross tumor volume (GTVnx) and GTVnd were decreased by 3.15% and 5.67% weekly, and decreased by a total of 22.03% and 39.68%, respectiely. The volumes of left and right parotids were decreased by 4.93% and 5.26% weekly, and decreased by a total of 29.60% and 31.56%, respectively. The volumes of GTVnd and parotids showed significant retraction in the first four weeks of radiation, then the retraction slowed down. The D95 of PGTVnd in Plan 2 decreased by 2.20% (t=2.382, P<0.05) compared that in the initial Plan 1, while there was no significant difference on the D95 of PGTVnx, PTV1 and PTV2.The Dmean, D50 and V30 of the left and right parotids in Plan 2 increased by 7.34%, 12.68%, 10.90% (t=-3.376, -3.738, -3.679, P<0.05), and 6.13%, 11.17%, 9.72% (t=-2.550, -2.446, -2.673, P<0.05), respectively, compared with Plan 1. The Dmean of larynx increased by 8.69% in Plan 2 compared with Plan 1 (t=-3.099, P<0.05). The D95 of PGTVnd in Plan(1+ 2) increased by 1.37% (t=-3.555, P<0.05) compared with Plan 2.The Dmean, D50 and V30 of the left and right parotids in Plan(1+ 2) were decreased by 2.90%, 2.73%, 4.62% (t=3.089, 2.718, 2.705, P<0.05), and decreased by 3.49%, 3.44%, 3.80% (t=2.781, 2.958, 4.275, P<0.05), respectively, compared with Plan 1. The Dmean of larynx decreased by 3.29% (t=2.747, P<0.05) in Plan(1+ 2) compared with Plan 1. Conclusions The volumes of GTV and parotids of locally advanced NPC patients showed significant shrinks during IMRT. Replanning is necessary during the middle of IMRT to improve the target coverage and to spare the organ at risks (OARs). Key words: Nasopharyngeal carcinoma; Intensity modulated radiation therapy; Cone beam CT(CBCT); Replanning