Using non-daily cone-beam computed tomography reduce dosimetric effect of set-up errors in intensity-modulated radiotherapy for nasopharyngeal cancer

Abstract

Objective To assess the impact of set-up correction on dosimetry using non-daily kilovolt cone-beam computed tomography (KVCBCT) for nasopharyngeal cancer patients treated with intensitymodulated radiotherapy (IMRT). Methods The mean shift values from 14 nasopharyngeal cancer patients received KVCBCT scans during the first 5 treatment fractions were calculated as prediction of systemic set-up errors and used for off-line correction at 1.5 mm threshold level. Presumed that the systemic errors can be corrected by moving couch without residual errors, the pre-correction set-up errors in the remaining fractions were the sum of actual set-up errors and predicted errors. The dosimetric effects of non-daily protocol were simulated in the planning system and analyzed with physical dose parameters in 14 IMRT plans. Results In 10 patients with predicted systemic errors ＞ 1.5 mm, target dose was reduced significantly. The mean reduction of GTV-D98 ( dose received by 98％ of the volume of GTV ), CTVnx-D95 ( dose received by 95％ of the volume of CTVnx ), CTV1 -D98 ( dose received by 98％ of the volume CTV1 ) were 3. 8 Gy ( Z =- 2. 81,P =0. 005 ) ,4. 8 Gy ( Z =- 1.96, P =0. 050 ), 1.0 Gy ( Z =- 2. 82, P =0. 005 ), respectively. The effect on dose to CTV2 was much less. After correction, mean 3D vector positioning errors was reduced from 3. 6mm to 2. 3 mm (t =2.00,P =0. 000). After correction, the dose led to increase in GTV-D98, CTVns-D95,CTV1-D95 was 3.8 Gy (t=-2. 70,P=0.007),5.0 Gy (t =-2. 15,P=0.030),0.9 Gy (Z=-2.80,P=0. 005 ) respectively, and reduced the dose deviation greater than 3％ or 5％ for organs at risk.Conclusion Non-daily KVCBCT correction reduced dosimetric effect of set-up errors in IMRT for nasopharyngeal cancer patients. Key words: Tomography, X-ray computed, cone-beam; Set-up errors; Nasopharyngeal neoplasms/intensity-modulated radiotherapy