Objective  To quantify dose variations in target area and organs at risk during fractionation therapy of patients with nasopharyngeal carcinoma undergoing intensity-modulated radiation therapy (IMRT).

Methods  A retrospective analysis was conducted on the clinical data of 27 patients with nasopharyngeal carcinoma who completed volumetric modulated arc therapy (VMAT) plans in the Department of Oncology of the First Affiliated Hospital of Chongqing Medical University from February to April 2022. The group consisted of 18 males and 9 females, with an average age of (57.8±9.0) years, ranging from 40 to 72 years old. For all patients, the gross tumor volume of the primary lesion (GTV_nx), the clinical target volume of the area invaded by the primary lesion (CTV1), the planning target volume of the nasopharyngeal carcinoma primary lesion area generated by expanding 3 mm outward in all directions from CTV1 (PTV1), and the organs at risk such as the brainstem, spinal cord, left parotid gland, and right parotid gland were delineated. The three dimensional dose verification system Delta4 was used to perform 33 dose verifications based on the initial plans of the patients and to collect data from each dose verification. Compare the differences in measured dose and percentage dose differences (%DD) between the completed 33 dose verifications and the first dose verification. Intergroup comparisons of quantitative data were conducted using paired t-tests or Wilcoxon tests.

Results  After completing 33 dose verifications, the mean dose (D_mean) of GTV_nx (2.159(4.357) Gy vs. 2.173(4.375) Gy), the 95% target volume dose (D_95%), the 5% target volume dose (D_5%), the D_mean of CTV1 ((50.859±1.753) Gy vs. (51.305±1.756) Gy, 64.261(2.979) Gy vs. 64.395(2.984) Gy, 0.135(0.064) Gy vs. 0.136(0.065) Gy), the D_95%, D_5% and D_mean of PTV1 ((49.364±1.440) Gy vs. (49.827±1.459) Gy, 64.105(3.201) Gy vs. 64.149(3.273) Gy, 0.089(0.032) Gy vs. 0.090(0.033) Gy), and the D_mean of the left parotid gland and right parotid gland (1.185(0.612) Gy vs. 1.188(0.686) Gy, 1.227(0.640) Gy vs. 1.252(0.619) Gy) showed a decrease in measured doses compared with the first dose verification, with all differences being statistically significant (Z=−4.397, t=−6.060, Z=−3.339, Z=−4.541, t=−6.870, Z=−3.363, Z=−4.541, Z=−2.667, Z=−3.460; all P<0.05). The dose received by the 1 cm³ target volume (D_1cc) of the brainstem ((40.770±3.670) Gy vs. (40.228±3.555) Gy) increased compared with the first dose verification, with a statistically significant difference (t=5.903, P <0.001). After completing 33 dose verification, the 2% target volume dose (D_2%) of GTV_nx (0.090(1.993)% vs. 0.209(1.696)%), the D_95%, D_5% and D_mean of CTV1 ((0.153±1.575)% vs. (0.905±1.626)%, −0.203(1.737)% vs. 0.050(1.572)%, (0.145±0.903)% vs. (0.475±0.956)%), the D_95%, D_5% and D_mean of PTV1 ((−1.017±1.237)% vs. (−0.213±1.303)%, −0.452(1.583)% vs. 0.044(1.430)%, −0.003(1.130)% vs. 0.385(0.960)%), and the D_mean of the left and right parotid glands (9.778(5.093)% vs. 10.018(5.795)%, 4.101(4.975)% vs. 7.050(5.177)%) decreased in %DD compared with the first dose verification, with all differences were statistically significant (Z=−3.195, t=−7.594, Z=−2.763, t=−3.254, t=−8.709, Z=−2.667, Z=−3.099, Z=−2.258, Z=−3.243; all P <0.05). The %DD of D_1cc of the brainstem and the spinal cord ((3.895±3.135)% vs. (2.346±2.574)%, 2.935(2.929)% vs. 2.032(2.897)%) increased compared with the first dose verification, and both differences were statistically significant (t=7.469, Z=−4.469, both P <0.001). After completing 12 dose verifications, the %DD of D_1cc of the brainstem ((3.005±2.841)% vs. (2.346±2.574)%) increased compared with the first dose verification and the difference was statistically significant (t=4.398, P <0.001) and %DD>3%.

Conclusions  During fractionation therapy of nasopharyngeal carcinoma with IMRT, significant changes occur in the doses to the target area and organs at risk. Only the dose variation in the brainstem exceeds the clinically acceptable range. This finding suggests that attention should be paid to the dose variations in the brainstem during the implementation of radiotherapy for nasopharyngeal carcinoma to maximize the therapeutic benefit.