Evaluating efficiency of Vertical MLC VMAT plan for nasopharyngeal carcinoma
Seung hoon Chae,
Korea Republic of
PO-2331
Abstract
Evaluating efficiency of Vertical MLC VMAT plan for nasopharyngeal carcinoma
Authors: Seung hoon Chae1, Sang jun Son1
1Seoul national university hospital, Radiation oncology, Seoul, Korea Republic of
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Purpose or Objective
The purpose of the study is to evaluate the efficiency of Vertical MLC VMAT plan(VMV plan) Using 273° and 350° collimator angle compare to Complemental MLC VMAT plan(CMV plan) using 20° and 340° collimator angle for nasopharyngeal carcinoma.
Material and Methods
Thirty patients treated for nasopharyngeal carcinoma with the VMV plans were retrospectively selected. Those cases were planned by Varian Eclipse with PO and AcurosXB Algorithm. The geometry of VMAT plans were two 6MV 360° arcs using Varian Vitalbeam. All plans were created with same parameters, but only the collimator angles of 273° and 350° for the VMV plans and 20° and 340° for the CMV plans were different. For dosimetric evaluation, the dose-volumetric(DV) parameters of the PTVs and OARs were calculated for all VMAT plans. Modulation complexity score of VMAT(MCSv), MU and treatment time were also compared. In addition, Pearson's correlation analysis was performed to confirm whether there was a correlation between the difference in the MCSv and the difference in each evaluation index of the two treatment plans.
Results
In the case of PTV evaluation index, the CI was improved by 3.76% in the VMV Plan, then for OAR, the dose reduction effect of the spinal cord (-14.05%) and brain stem (-9.34%) was remarkable. In addition, the parotid glands (left parotid : -5.38%, right : -5.97%) and visual organs (left optic nerve: -4.88%, right optic nerve: -5.80%, optic chiasm : -6.12%, left lens: -6.12%, right lens: -5.26%), auditory organs (left: -11.74%, right: -12.31%) and thyroid gland (-2.02%) were confirmed. Also, the VMV Plan showed 5.31% higher MCSv and 6.11% lower MU. The difference in MCSv of the two treatment plans showed a significant negative (-) correlation with the difference in CI (r=-0.55) of PTV. Spinal cord (r=0.40), brain stem (r=0.34), and both salivary glands (left: r=0.36, right: r=0.37) showed a positive (+) correlation. (For all the values, p<.05)
Conclusion
Compared to the CMV plan, the VMV plan is considered to be helpful in improving the quality of the treatment plan for nasopharyngeal carcinoma by allowing the MLC to be modulated more efficiently.