Abstract
The purpose of this study was to investigate the setup and positioning uncertainty of a custom cushion/mask/bite‐block (CMB) immobilization system and determine PTV margin for image‐guided head and neck stereotactic ablative radiotherapy (HN‐SABR). We analyzed 105 treatment sessions among 21 patients treated with HN‐SABR for recurrent head and neck cancers using a custom CMB immobilization system. Initial patient setup was performed using the ExacTrac infrared (IR) tracking system and initial setup errors were based on comparison of ExacTrac IR tracking system to corrected online ExacTrac X‐rays images registered to treatment plans. Residual setup errors were determined using repeat verification X‐ray. The online ExacTrac corrections were compared to cone‐beam CT (CBCT) before treatment to assess agreement. Intrafractional positioning errors were determined using prebeam X‐rays. The systematic and random errors were analyzed. The initial translational setup errors were −0.8±1.3 mm, −0.8±1.6 mm, and 0.3±1.9 mm in AP, CC, and LR directions, respectively, with a three‐dimensional (3D) vector of 2.7±1.4 mm. The initial rotational errors were up to 2.4° if 6D couch is not available. CBCT agreed with ExacTrac X‐ray images to within 2 mm and 2.5°. The intrafractional uncertainties were 0.1±0.6 mm, 0.1±0.6 mm, and 0.2±0.5 mm in AP, CC, and LR directions, respectively, and 0.0∘±0.5°, 0.0∘±0.6°, and −0.1∘±0.4∘ in yaw, roll, and pitch direction, respectively. The translational vector was 0.9±0.6 mm. The calculated PTV margins mPTV(90,95) were within 1.6 mm when using image guidance for online setup correction. The use of image guidance for online setup correction, in combination with our customized CMB device, highly restricted target motion during treatments and provided robust immobilization to ensure minimum dose of 95% to target volume with 2.0 mm PTV margin for HN‐SABR.PACS number(s): 87.55.ne
Highlights
Stereotactic ablative radiotherapy for the head and neck (HN-SABR) is increasingly used for treatment of locally recurrent cancers.[1]
cone-beam CT (CBCT) is available on most modern linacs and used for setup verification because of its capability to acquire volumetric images.[3,4,5,6] In-room orthogonal X-ray systems, such as the Brainlab ExacTrac X-ray 6D system (Brainlab AG, Feldkirchen, Germany), are common add-ons that allow for 6D setup verification and corrections prior to and during treatment.[7,8]
We report on the reproducibility and robustness of our unique customized frameless immobilization system utilizing a custom cushion, mask, and bite-block (CMB)
Summary
Stereotactic ablative radiotherapy for the head and neck (HN-SABR) is increasingly used for treatment of locally recurrent cancers.[1]. Recent advances in image guidance have allowed delivery of high-dose radiation with increased confidence and safety in HN-SABR. While image guidance is normally used to correct initial setup errors, the robustness of the immobilization device is essential in maintaining the patient’s position during the actual treatment (intrafractional) to constrain voluntary or involuntary motion. Several studies comparing post-treatment to pretreatment imaging[6,8] have observed increasing setup errors in the posttreatment images, indicating a potential for significant patient motion during treatment. These uncertainties need to be investigated to generate a confident planning target volume (PTV). The inter- and intrafractional setup errors for head and neck patients immobilized with thermoplastic masks have been extensively investigated.[5,9,10] Other immobilization devices, such as customizable head cushions and mouth pieces, can be coupled to the thermoplastic mask to further restrict patient motion.[5]
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