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Aim We sought to improve error detection ability during volume modulated arc therapy (VMAT) by dividing and evaluating the treatment plan. Background VMAT involves moving a beam source delivering radiation to tumor tissue through an arc, which significantly decreases treatment time. Treatment planning for VMAT involves many parameters. Quality assurance before treatment is a major focus of research. Materials and methods We used an established VMAT prostate treatment plan and divided it into 12° × 30° sections. In all the sections, only image data that generated errors in one segment and those that were integrally acquired were evaluated by a gamma analysis. This was done with five different patient plans. Results The integrated image data resulting from errors in each section was 100% (tolerance 0.5 mm/0.5%) in the gamma analysis result in all image data. Division of the treatment plans produced a shift in the mean value of each gamma analysis in the cranial, left, and ventral directions of 94.59%, 98.83%, 96.58%, and the discrimination ability improved. Conclusion The error discrimination ability was improved by dividing and verifying the portal imaging.

Aim This study aimed to assess the utility and stability of intraoral stent during intensity-modulated radiation therapy (IMRT). Background The benefits of intraoral stents in radiotherapy are unclear. Materials and methods We analyzed 386 setup errors in 12 patients who received IMRT for head and neck cancers without intraoral stents (intraoral stent [−]) and 183 setup errors in 6 patients who received IMRT with intraoral stents (intraoral stent [+]). All patients were matched according to the immobilization method (masks and boards). Setup errors were measured as the distance from the initial setup based on the marking on the skin and mask to the corrected position based on bone matching on cone beam computed tomography. Results The mean interfractional setup errors in the right–left, craniocaudal, anterior–posterior (AP), and three-dimensional (3D) directions were −0.33, 0.08, −0.25, and 2.75 mm in the intraoral stent (−) group and −0.37, 0.24, −0.63, and 2.42 mm in the intraoral stent (+) group, respectively ( P = 0.50, 0.65, 0.01, and 0.02, respectively). The systematic errors for the same directions were 0.89, 1.46, 1.15, and 0.88 mm in the intraoral stent (−) group and 0.62, 1.69, 0.68, and 0.56 mm in the intraoral stents (+) group, respectively. The random errors were 1.43, 1.43, 1.44, and 1.22 mm in the intraoral stent (−) group and 1.06, 1.11, 1.05, and 0.92 mm in the intraoral stents (+) group, respectively. Conclusion Setup errors can be significantly reduced in the AP and 3D-directions by using intraoral stents.