Breast board combined with a thermoplastic head mask immobilization can improve the reproducibility of the treatment setup for breast cancer patients receiving whole breast and supraclavicular nodal region irradiation

Abstract

Objective To quantify the setup errors measured with kV cone-beam CT (CBCT) using breast board with or without a thermoplastic head mask in breast cancer patients who received whole breast and supraclavicular nodal region irradiation. Clinical target volume (CTV) to planning target volume (PTV) in 3 directions were also calculated. Methods The study included thirty patient receiving both whole breast and supraclavicular nodal region irradiation on Elekta Versa HD linear accelerators (Elekta Oncology Systems, Crawley, UK) between June 2016 and January 2018.The setup error data were retrospectively analyzed. All the patients were immobilized in the supine position on a breast board system (Carbon fibre breast board, Civco, Iowa, USA) with both arms raised. Twenty of the patients added an extra thermoplastic head mask to immobilize the neck. A CBCT scan was taken before treatment at the first 3 to 5 fractions and then once every week. Registration with the planning CT was performed and adjusted to match the target volume of the supraclavicular nodal region and the cervical vertebra by experienced medical staff, the position of the arms and the breast were also considered. For all patients the couch shifts in left-right (x), superior-inferior (y), anterior-posterior (z) were recorded. Statistical analysis included two-tails significance tests (t-Student and Manne-Whitney test for means, χ2 test for variances). Population of the two groups was compared in terms of distribution of the mean shift (systematic error) and their standard deviations (random error). Van Herk′s setup margin defined as MPTV=2.5Σ+ 0.7δ was calculated in patients with or without mask immobilization. Results Altogether, 56 images and 109 images were acquired in breast board only group and head mask group, respectively. Shifts of the breast board only group and the head mask group in x, y, z were 0.212±0.174 cm vs.0.272±0.242 cm (P=0.070); 0.364±0.246 cm vs.0.242±0.171 cm (P=0.001); 0.423±0.302 cm vs.0.364±0.269 cm (P=0.204). Proportion of the shift less than 5 mm in the breast board only group and the head mask group were 91.07% vs. 85.32%(P=0.294); 67.86% vs.89.91%(P=0.001); 67.86% vs.74.31%(P=0.381). The CTV to PTV margin in x, y, z were 0.645 cm, 0.981 cm, 1.317 cm in breast board only group and 0.873 cm, 0.709 cm, 0.961 cm in head mask group, respectively. Setup error in the x direction was significantly correlated with BMI (P=0.001). Conclusions For the alignment and immobilization of patients who received whole breast and supraclavicular nodal region irradiation, using a breast board in combination of a thermoplastic head mask may significantly help to reduce the shift variance in superior-inferior direction compared to using breast board only. The anterior-posterior error is relatively large, other immobilization device or patient alignment methods are needed to be further explored to improve the accuracy. Key words: Breast neoplasms/radiotherapy; Setup error; Cone-beam CT