Objective:To assess the application value of low-dose contrast agent combined with bolus tracking technique in radiotherapy localization for nasopharyngeal carcinoma (NPC).Methods:One hundred patients with NPC for radiotherapy localization were enrolled prospectively and randomly divided into observation ( n = 50) and control ( n = 50) group using random number table. All patients in the control group underwent routine computerized tomography (CT) for radiotherapy localization with an intravenous bolus of 80 ml iodinated contrast media. Images were obtained at a fixed delay of 50 s from the beginning of contrast injection. Patients in the observation group underwent CT scan using a bolus tracking technique with an intravenous bolus of 50-60 ml iodinated contrast media. By observing the time-CT value monitoring curve, the scanning was manually triggered when the peak fell. The score of image quality, enhanced CT value, iodine contrast dose, and dose calculation variance of planning target volume were compared between the two different dose groups. The differences of dose were compared using independent sample t-tests and the differences in graph scores were compared using Wilcoxon rank sum tests. Results:The dosage of iodine contrast agent in the observation group was 28.75% less than that in the control group, and the difference achieved statistical significance ( t=-42.11, P<0.001). The enhanced CT value of carotid artery and jugular vein in the control group increased 32.58% and 21.02% compared with patients in the observation group, respectively ( t=-8.11 and -4.82, P<0.001 for both). No statistically significant differences in the enhanced CT value were found for the gross tumor volume (GTV) of nasopharynx or sternocleidomastoid between the two groups ( P>0.05). The images in both groups can met the requirements of target volume delineation, and the difference of subjective score was not statistically significant ( Z=-1.36, P=0.175). There were no significant differences in the signal-to-noise ratio (SNR) or contrast-to-noise ratio (CNR) of nasopharynx between the two groups ( P>0.05). The results of subjective score and GTV target area showed high consistency between the two doctors with a κ value of 0.77. The differences in dose calculation variance of planning target volume between the two different dose groups was also not statistically significant ( P>0.05). Conclusions:In the radiotherapy localization for nasopharyngeal carcinoma, the application of low-dose contrast agent combined with bolus tracking technique could obtain good images for target delineation and effectively reduce the intake of iodine contrast agent and the risk of adverse reactions. Based on these results, low-dose contrast agent combined with bolus tracking technique has wide range of application.

Objective: To evaluate the dosimetric effects of set-up errors on nasal NK/T cell lymphoma by introducing set-up errors into the radiotherapy planning system for dose reconstruction. Methods: Ten patients with nasal NK/T cell lymphoma were recruited. A non-coplanar volumetric modulated arc therapy plan was designed for CT image and clinical target area of each patient. After the completion of the plan, the set-up errors were introduced into the radiotherapy plan by changing the ISO of the treatment, and dose calculation was performed to reconstruct the dose distribution. Results: With the increase of system set-up errors, the dose of target was decreased and the order affected by set-up errors in different directions was: left-right direction> head-foot direction> front-rear direction. When the translational set-up errors in each direction were -3 mm to 3 mm and the rotating set-up errors were -3° to 3°, the range of dose change in all targets was less than ±3%. When the set-up errors in all directions were ≤ 3 mm, the dose of organ at risk was less than or similar to the prescribed dose. When the set-up errors were> 3 mm, the doses of lens, spinal cord, parotid gland and optic nerve gradually exceeded the prescribed dose. Only when the rotating set-up errors were ≥ 3°, the dose of lens exceeded the prescribed dose. Special attention should be paid to the influence of the greater set-up errors in the left and right direction on lens, spinal cord and parotid gland, as well as on the spinal cord due to the larger set-up errors in the front and rear direction. After the actual set-up errors were introduced from our department, it exerted slight effect on the irradiation dose of GTV and CTV, which was less than ±2%. In a few cases, the dose of organ at risk potentially exceeded the prescribed dose limit, and special attention should be diverted to overdose of the lens and optic nerve. Conclusions The set-up errors will result in target dose deficiency and overdose of organ at risk in nasal NK/T cell lymphoma, especially upon the set-up errors in the left and right direction. The effect of 3 mm and 3° set-up errors on target and organ at risk is limited. Therefore, it is recommended to maintain the single direction set-up errors within 3 mm and 3°. The actual set-up errors introduced from our department exert little effect on the target dose, but a small number of organs are at risk of exceeding the prescribed dose limit. It is necessary to increase the evaluation of the extension region of organ at risk.