Feasibility Study of Robust Treatment Planning in VMAT for Head and Neck Cancer

Abstract

In general, patient positional uncertainty is considered by adding a geometrically expanded margin to clinical target volume (CTV) in photon radiation therapy. However, this method may not be suitable because image-guided radiotherapy is available. In intensity modulated proton beam therapy, robust treatment planning is currently common to take patient positional uncertainty into account in optimization rather than in margins. The purpose of this study is to assess the feasibility of clinical implementation of the method in volumetric modulated arc therapy (VMAT) for head and neck cancer. We quantitatively evaluated whether the plans with the robust optimization (Robust plans) can adequately cover CTV against patients' positional uncertainties and body shape change throughout a treatment course. Ten head and neck cancer patients were chosen, who were treated with PTV-based VMAT plans in our hospital between 2021.5-2022.4. RayStation V10A (RaySearch Laboratories, Stockholm, Sweden) was used for the robust optimization, which was applied to the CTVs with patient positional uncertainty of 5 mm in the 6-axis direction. Dose prescribed to the high- and low-risk CTVs were to 70 and 56 Gy in 35 fractions, respectively. To create the patients' CT images with residual set-up errors and body shape change at the treatment, pseudo simulation-CT images were created by deformable image registration with CBCT and simulation-CT. Dose distribution at the treatment was re-calculated by applying the plan to the pseudo simulation-CT images. The variation of D98 for the high-risk CTV from the time of treatment planning was evaluated on a weekly basis. For comparison, planning target volume (PTV) -based plans (5 mm margin circumference) were created and a similar evaluation was performed. D98 for the high-risk CTV varied between -3∼2% in the robust plan and between -5∼1% in the PTV-based plan during the treatment course. There was no significant difference in the amount of D98 variation between the two plans by t-test, except for one case with hypopharyngeal cancer. In this case, D98 for the high-risk CTV varied within ±1% with the PTV-based plan, whereas the value decreased up to 3% with the robust plan (p < 0.05). This case often had a residual setup error of approximately 5 mm at the sites related to the pitch rotation of head, suggesting that the dose distribution for the robust plan was affected by non-rigid positional errors. Patient weight loss during the treatment period was -3.5±2.4 kg, showing a weak correlation (r = -0.33) with the variation in D98 for the high-risk CTV. The robust treatment planning exhibits comparable CTV coverage to the conventional PTV-based planning against positional uncertainty and body shape change throughout a treatment period. In order to overcome set-up baseline shift by the non-rigid positional errors, re-planning should be recommended. Further planning studies will be conducted to promote clinical implementation of the method.