[Preliminary application of kilo-volt cone-beam computed tomography to intensity-modulated radiotherapy of nasopharyngeal carcinoma].

Background & Objective: The intensity-modulated radiotherapy (IMRT) with its highly conformed dose distribution to nasopharyngeal cancer (NPC) and the surrounding critical organs is being accepted increasingly in clinical practice. Due to the steep dose fall-offs of IMRT at the target margin, precise patient positioning and verification are required. This study was to evaluate the role of kilo-volt cone-beam computed tomography (kV-CBCT) in guiding the accurate positioning of IMRT for NPC.

Methods: kV-CBCT was performed on 22 NPC patients before radiotherapy. The acquired CBCT were co-registered with the planning CT for online set-up correction and offline planning target volume (PTV) analysis.

Results: The 22 patients received a total of 754 kV-CBCT scans. Among the 505 scans before couch correction, the detection rates of deviation of < or =2 mm were 76.4% in left-to-right (X) direction, 76.0% in superior-to-inferior (Y) direction, and 85.7% in anterior-to-posterior (Z) directionû among the 106 scans after correction, the detection rates were 97.2%, 97.2%, and 100% in X, Y and Z directions, respectivelyû among the 143 scans after treatment, the detection rates were 87.4%, 87.6%, and 90.0%, respectively. The overall setup errors in X, Y and Z directions were (-0.7+/-1.6) mm, (-0.7+/-1.8) mm and (-0.3+/-1.7) mm, respectively, before correctionû (-0.4+/-0.8) mm, (0.3+/-0.8) mm and (0.0+/-0.7) mm, respectively, after correctionû (0.2+/-1.2) mm, (0.3+/-1.3) mm and (0.1+/-1.1) mm, respectively, after treatment. The maximal PTV margin was 4.0 mm before correction and 2.1 mm after correction.

Conclusion: kV-CBCT image-guided radiotherapy may improve the set-up precision of IMRT for NPC.