[Placement repeatability of individual oral stent used in radiotherapy of nasopharyngeal carcinoma].

Abstract

Background and objective: Individual oral stent used in nasopharyngeal carcinoma (NPC) patients during radiotherapy can spare the oral mucosa and tongue. This study was to analyze the placement repeatability and feasibility of individual oral stent used in radiotherapy of NPC.

Methods: Individual oral stents were made for 17 naive NPC patients, and three lead markers of 2.0 mm diameter were embedded in each oral stent. The patients wearing individual oral stent were immobilized by thermoplast mask and underwent CT scan. Before and every week during radiotherapy, orthogonal (antero-posterior and lateral) X-ray films were taken in conventional simulator, respectively. Displacements of leader markers in medio-lateral (M-L),antero-posterior (A-P) and cranio-caudal (C-C) directions were respectively calculated and compared on each patient's X-ray films. At the dose of 40 Gy, a repeated CT scan was done with the same thermoplast mask and positioning markers. The displacements of co-ordinating gravity center of three lead markers and three selected bony structures between two sets of CT images for each patient were calculated and compared by ANOVA.

Results: A total of 240 X-ray films were taken. The displacements of lead markers were (0.69+/-0.54) mm in M-L direction, (0.49+/-0.62) mm in A-P direction, and (0.56+/-0.57) mm in C-C direction, and the overall vector displacement was (1.2+/-0.77) mm (range, 0-4.98 mm). In three-dimensional method, the displacements of co-ordinating gravity center of three lead markers were (0.75+/-0.68) mm in M-L direction, (1.25+/-1.12) mm in A-P direction, and (1.06+/-0.77) mm in C-C direction, and the overall vector displacement was (2.15+/-0.90) mm (range, 1-4.24 mm). The displacements between gravity center of three lead markers and three bony structures had no significant difference in M-L, A-P, and C-C directions (P>0.05). The overall vector displacements also had no significant difference (P=0.083).

Conclusion: The placement repeatability of individual oral dental stent in the mouth is very good during radiotherapy of NPC, and dental stent has a fixed position relationship with bony structures.