ESTRO 2023 - Abstract Book

S2099

Digital Posters

ESTRO 2023

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. Materials 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. 1 Hospital Clínic de Barcelona, Oncologia Radioteràpica (ICMHO), Barcelona, Spain; 2 Hospita Clínic de Barcelona, Oncologia Radioteràpica (ICMHO), Barcelona, Spain Purpose or Objective In lung SBRT, the inverse planning process can lead to undesired fluence peaks between the ITV and PTV to compensate the lack of lateral equilibrium and a low density tissue. On one hand, the use of more advanced calculation engines in the optimization process, further increases this effect. On the other hand, when using modulated delivery techniques, the desired steep dose gradient is no longer directly correlated with the presence of large maximum doses within the PTV. The purpose of this study is to analyze the impact of the basic optimization parameters prevalent in this treatment, and asses its impact independently. Materials and Methods We optimized and calculated 51 VMAT treatment plans (Eclipse v 16.1, PO, Acuros TPS, dose to medium) using a SBRT lung phantom (CIRS) with the Hounsfield Units (HU) of the ITV overridden with water (Figure 1 a ). The variables in the optimization process were: • Maximum PTV dose: 105%, 110%, 115%, 120% of the prescribed dose. • Maximum dose priorities: 0, 50, 100, 150, 200. • NTO priorities: 0, 100, 200. For each plan the optimization process was started with a specific combination of parameters and remained unmodified and calculating the intermediate dose. For all cases the dose was normalized to ensure that the 100% isodose covered 95% of the PTV. The analyzed dosimetric parameters were: • Mean dose to the ITV D _meanITV • Mean dose to a ring enclosing 0.5 cm ( D _mean0.5) beyond the PTV and 1.5 cm ( D _mean1.5) beyond the PTV, excluding the first 0.5 cm. • Mean dose to the ITV to PTV ring, calculated in a phantom with a 2 cm sphere surrounding the PTV with water HU (Figure 1 b ) ( D _meanP-I). The last parameter was used, compared to D _meanITV to evaluate the fluence peaks. We also defined Dmax strength as the D max objective priority divided by the difference between the D max value and the prescribed dose. PO-2332 Should we limit the maximum dose in inverse-planning-based lung SBRT plans? C. Castro 1 , E. Escudero 2 , C. Cases 1 , F. Casas 1 , G. Antelo 1 , M. Mollà 1