Set-up verification and 2-dimensional electronic portal imaging device dosimetry during breath hold compared with free breathing in breast cancer radiation therapy.

Purpose: To compare set-up and 2-dimensional (2D) electronic portal imaging device (EPID) dosimetry data of breast cancer patients treated during voluntary moderately deep inspiration breath hold (vmDIBH) and free breathing (FB).
Methods and Materials: Set-up data were analyzed for 29 and 51 consecutively treated patients, irradiated during FB and vmDIBH, respectively. Of the 51 vmDIBH patients, the first 25 had undergone an extra trained computed tomography (CT) scan and used an additional "breathing stick" (vmDIBH_trained). The last 26 patients did not use the breathing stick and did not undergo a trained CT (vmDIBH_untrained). The delivered 2D transit dose was measured with EPID in 15 FB and 28 vmDIBH patients and compared with a 2D predicted dose by calculating global gamma values γ using 5% and 5 mm as dose difference and distance-to-agreement criteria, respectively. Measurements with a percentage of pixels with an absolute gamma value > 1 (|γ| > 1) greater than 10% were classified as deviating.
Results: Only small, sub-millimeter differences were seen in the set-up data between the different patient groups. The mean of means, systematic error, and random error ranged from - 0.6 mm to 3.3 mm. The percentage of pixels with |γ| > 1 for all patients was 9.8% (2-25.8). No statistically significant differences were observed between the patient groups. In total, 38% of the gamma images were classified as deviating: 43.6% in vmDIBH_untrained patients compared with 38.0% in vmDIBH_trained patients and 33.3% in FB patients (P > .05).
Conclusion: Both set-up and 2D EPID dosimetry data indicate that reproducibility of radiation therapy for patients treated during FB and vmDIBH is similar. Small but not significant differences in 2D EPID dosimetry were observed. Further investigation with 3-dimensional EPID dosimetry is recommended to investigate the clinical relevance of deviant gamma images.