Clinical study of the time of repeated computed tomography and replanning for patients with nasopharyngeal carcinoma

Purpose: To study the necessity of repeat computed tomography (CT) scan and replanning and know a more accurate time using weekly kilovoltage cone beam computed tomography (kV-CBCT) scans for patients with nasopharyngeal carcinoma (NPC) during radiotherapy. Methods and Meterials: Thirteen NPC patients treated with IMRT were enrolled into this prospective study. Weekly pretreatment kV-CBCT scans were performed on the 1st, 6th, 11st, 16th, 21st and 26th radiation time, respectively. Target delineations were contoured on all fractionated CBCT images, including the gross tumor volume of the primary nasopharyngeal tumor (GTVnx) and parotid glands. The volumes of GTVnx and parotid glands were calculated automatically using the Pinnacle(3) 8.0 system. Compared to the original GTVnx, the percentage of shrinking volume (Delta P) >= 50% was considered significantly. Results: As the radiation proceeding, the GTVnx had a trend of shrinkage. Of all 13 patients, 11 cases (84.6%) had the volume shrinking >= 50% before the 21st radiation and 12 cases (92.3%) before the 26th radiation. And the parotid volume decreased significantly in the first four-week radiation, 6.45 +/- 3.16cm(3) (range, 3.06-13.9cm(3)) for the left parotid gland and 5.78 +/- 2.39cm(3) (range, 2.70-11.2cm(3)) for the right. Furthermore, only a little displacement occured to bilateral parotid glands. Conclusion: The replanning for NPC patients with IMRT is necessary, and the time between the 21st to 25th radiation is appropriate.