Comparative analysis of residual setup errors in head and neck patients from upright versus supine radiotherapy postures.

Background: Carbon-ion rotating gantries use is limited by its large size, weight, and high cost. Gantry-free modality enables the reduction of the overall size, weight, and cost. Among them, upright treatment, which utilizes fixed ion beamlines, in combination with a treatment chair capable of 360° rotation and adjustable pitch angle (enabling non-coplanar beam delivery), provides a wider range of beam entry angles compared to conventional couch-based setups and has already been applied in particle radiotherapy for head and neck cancer patients.

Purpose: In this study, we analyzed clinical data from the Shanghai Proton and Heavy Ion Center (SPHIC) to quantify residual setup errors across various regions of interest (ROIs) for both upright and supine treatments.

Methods: A total of 402 treatment fractions from 28 patients (median 5 fractions, range: 5-16 fractions per posture per patient) were enrolled in this study. All these patients were immobilized and scanned in supine posture and received both supine and upright radiotherapy. Three rectangular-shaped ROIs were delineated based on bone structures, encompassing the mandible, orbit, and neck vertebrae C1-C3. Box-based registration, focusing solely on the anatomical structures within the specific ROIs was performed to subtract the correction vector used in treatment, thereby obtaining the residual setup error for each ROI. Margins for each ROIs were calculated.

Results: For both postures, the median values of residual setup error for all translational directions were less than 1 mm. The median values did not exceed 0.2 degrees for rotational errors. More than 78% of the fractions for upright treatment fell within the 1 mm/° threshold, while 94% were within the 2 mm/° threshold. In contrast, for supine treatment, over 61% fell within the 1 mm/° threshold, while 86% were within the 2 mm/° threshold. The maximum margin was 3.3 mm in the AP direction of the C1-C3 region for the supine posture.

Conclusions: Upright treatments demonstrated comparable residual setup errors to supine treatments, with most errors falling within clinically acceptable thresholds. This study provides valuable clinical evidence for the continued development and implementation of upright radiotherapy.