[Measurement of setup error in conformal radiotherapy for nasopharyngeal carcinoma]

Setup error is the key factor in determining the margin of planning target volume (M(PTV)) for conformal radiotherapy of nasopharyngeal carcinoma (NPC). This study was to establish a method using radiopaque fiducial markers which were embedded in occlusal splint ("occlusal splint method") to acquire setup errors to calculate M(PTV) in CRT for NPC.

The occlusal splint method was performed in 22 NPC patients who received CRT. Setup errors were calculated by comparing the coordinates of 3 fiducial markers read from portal images with those read from computed tomography (CT) data. According to the formulation, the size of MPTV with or without consideration of organ motion was also calculated.

The standard deviations of systematic errors (Sigma) were 1.13 mm, 1.47 mm, and 1.31 mm in X (medio-lateral), Y (antero-posterior) and Z (cranio-caudal) axes, respectively; the standard deviations of random errors were 0.81 mm, 0.45 mm, and 0.80 mm in X, Y, and Z axes, respectively. Without consideration of organ motion, the sizes of M(PTV) were 3.40 mm, 3.98 mm, and 3.83 mm in X, Y, and Z axes, respectively; with consideration of organ motion, the sizes of M(PTV) were 3.75 mm, 4.35 mm, and 4.16 mm in X, Y, and Z axes, respectively.

Using the "occlusal splint method", each institution can precisely calculate setup error data, and stipulate the size of M(PTV) adequate to one's own situation.