Study on Optimal Plan Approach for Dual Arcs Volumetric Modulated Arc Therapy for Oropharyngeal Cancer

Abstract

Volumetric modulated arc therapy (VMAT) is a high conformal technique widely used for the treatment of oropharyngeal cancer. A proper setting of the treatment parameters can improve target dose coverage and organs at risks (OARs) sparing. The aim of this study is to find an optimal planning approach for dual arcs VMAT oropharyngeal cancer treatments. Ten patients admitted and treated in our hospital for oropharyngeal carcinoma were included in this retrospective study. Computed tomography scans were acquired with 3mm slice thickness and used by an experienced radiation oncologist to define targets, OARs and a normal tissue region of the neck (nTis) between the lymph nodes drainage areas. The primary oropharyngeal tumor and the lymph nodes (PGTV), low risk volume (PCTV) and lymph nodes drainage areas (PCTVln) received 70Gy, 60Gy and 54Gy respectively in 33 fractions. A 6 MV photons beam by a linear accelerator with a treatment planning system were used for the treatment. For each patient two different 360° dual arcs VMAT plans were performed: i) a standard plan (SP) setting large field fixed jaws and ii) a new plan (NP) using only half jaw with the margin in the middle of the lymph nodes drainage area in order to accurately protect nTis region with the leaves of the collimator. Dose volume histogram cut-off point Dmean was used for OARs and nTis dose comparison while D98 and homogeneity index (HI) were used for targets. A paired T test, was used to compare the dosimetric results obtained for the two treatment planning groups (SP vs NP). A total of 20 treatment plans over the SP and NP groups were analyzed. SP and NP respected the clinical requirements, the targets results presented little difference. For what is concerning the OARs, NP performed with half jaws can significantly decrease the oral cavity, larynx and nTis mean dose. In the table below the results obtained for NP and SP for targets, larynx, oral cavity, nTis and the results of the statistical analysis (t and p values) were shown.Abstract MO_32_2769; Table 1PGTVPCTVPCTVlnLarynxOral cavitynTisHID98(Gy)HID98(Gy)HID98(Gy)Dmean (Gy)Dmean (Gy)Dmean (Gy)SP0.06±0.0269.0±0.30.18±0.0459.0 ±0.40.06±0.0252.0±0.929.6±1.730.0±2.834.8±6.6NP0.05±0.0169.4±0.60.16±0.0259.7±0.10.05±0.0153.5±0.225.9±1.228.0±2.831.7±6.0t2.302.753.030.834.573.878.972.583.33P0.0280.0490.0120.0210.0030.0100.0000.0230.000 Open table in a new tab Dual arcs VMAT plans performed with the use of half jaw can be a valid option to decrease the dose to the OARs without compromise the targets coverage. Moreover, the dose to normal tissue in the middle region between the lymph nodes drainage areas can be significantly decreased.