RESUMEN
Objective To analyze setup errors in cone beam computed tomography(CBCT)-guided radiotherapy for nasopharyngeal cancer(NPC)and evaluate the use frequency of CBCT. Methods Twenty-six patients newly diagnosed with NPC who received CBCT scan no less than 3 times per week during the whole course(7 weeks)of intensity-modulated radiotherapy(IMRT)were enrolled as subjects. Two setup errors were recorded each week:the setup error in the first CBCT analysis(Ef)and the setup error in the subsequent CBCT analysis(Ec). Comparisons of Efbetween two different weeks, Ecbetween two different weeks,and between Efand Ecin the same week were made by analysis of variance. Results During the 7 weeks,there were no significant differences in Efin the medial-lateral(ML), superior-inferior(SI), or anterior-posterior(AP)direction between any two weeks(all P>0.05);there were also no significant differences in Ecin the ML or SI direction between any two weeks(all P>0.05);there was a significant difference in Ecin the AP direction between two different weeks(P<0.05);there were no significant differences between Efand Ecin the ML or SI direction in any week(P>0.05);there were also no significant differences between Efand Ecin the AP direction in any week(P>0.05)except for the first week (P<0.05). Conclusions In IMRT for NPC,the setup errors in the ML and SI directions are stable in the whole course of radiotherapy,while the setup errors in the AP direction are different between the first week and subsequent weeks. Daily CBCT scan in the first week and weekly CBCT scan in the subsequent weeks are highly recommended for most patients to reduce the use frequency of CBCT.
RESUMEN
Some patients who have neck tumor but cannot tolerate the thermoplastic immobilization may be supported by simple cushions, and are marked on the neck skin during CT simulation. We therefore set 5 mm as the spinal cord-planning risk volume margin in the intensity-modulated radiotherapy plans in our Centre. Cone beam CT (CBCT) scans were acquired for three times, and matched with the simulation CT images in each radiotherapy. The mean and the standard deviation of the individual, the root mean-square and the standard deviation of the individual were calculated. The matched results of the third CBCT were used to calculate the spinal cord- planning risk volume margin. The results showed that the interfraction error was significantly reduced and the intrafraction error was stable by CBCT guiding. CBCT and 5 mm spinal cord-planning organ is feasible and safe without threatening volume margin to high dose radiotherapy for the patients with neck tumor and not able to tolerate thermoplastic immobilization.