Using Radiopaque Fiducial Markers Embeded In Occlusal Splint To Measure Setup Error In Conformal Radiotherapy For Nasopharyngeal Carcinoma

OBJECTIVE: Using radiopaque fiducial markers embeded in occlusal splint('occlusal splint method') to measure setup error in conformal radiotherapy for nasopharyngeal carcinoma and examining its feasibility and reliability. METHODS: Equipment and techniques for the 'occlusal splint method' were developed and used in 22 nasopharyngeal carcinoma patients underwent conformal radiotherapy. We calculated the Setup errors through comparing the coordinates of three fiducial markers read from portal images with those read from computed tomography (CT) data. The reproducibility of the occlusal splint positions was assessed in 6 patients through CT 'image fusion method', while the distortion of the occlusal splint was evaluated in 22 patients through calculating the differences of distances between three fiducial markers in portal images and CT data. According to formulation Mpvt = 2.5 + 0.7 we can calculate the size of MPTV with and withoutconsideration of organ motion.RESULTS: Six patients wore occlusal splint and underwent CT scanning before and in the midst of conformal radiotherapy. According to 'image fusion method, the discrepancies were 0.17mm(range -0.4mm ~ 0.5mm), -0.07mm(range -0.7 0.4mm),and 0.86mm(range -0.7mm~2.lmm) in X(medio-lateral), Y(antero-posterior) and Z(cranio-caudal) directions, respectively. The differences of distances between three fiducial markers in portal images and CT data were within 1mm. The standard deviations of the systematic errors were 1.13mm, 1.47mm, and 1.31mm in X(medio-lateral), Y(antero-posterior) and Z(cranio-caudal) directions, respectively.While the standard deviations of the random errors were 0.81mm, 0.45mm, and0.80mm in X, Y and Z directions, respectively. Without considering the organ motion,the size of MPTV was 3.40mm, 3.98mm, and 3.83mm in X, Y and Z directions,respectively. If the organ motion was considered, the size of MPTV was 3.75mm,4.35mm, and 4.16mm in X, Y and Z directions, respectively.CONCLUSIONS: 'Occlusal splint method' is a convenient and efficient method formeasuring setup error in radiotherapy of nasopharyngeal carcinoma. Using thismethod each institution can precisely calculate setup error and stipulate the size ofPTV (planning target volume) margin adequate to one's own situation.