Background: This study aimed to evaluate the inter-fraction set-up error and intra-fraction motion during reverse semi-decubitus (RSD) breast radiotherapy, and to determine a planning target volume (PTV) margin.
Material and methods: Pre- and post-treatment cone-beam computed tomography (CBCT) scans were prospectively acquired at fractions 1, 4, 7, 8, 11, and 14 for 30 patients who underwent RSD breast radiotherapy. Online correction for initial set-up error greater than 5 mm or 2° was performed and post-correction CBCT was acquired. An off-line analysis was performed to quantify initial and residual inter-fraction set-up errors and intra-fraction motion in three-dimensions. Patient inter-fraction errors were analysed for time trends during the course of radiotherapy. PTV margins were calculated from the systematic and random errors.
Results: The initial inter-fraction population systematic errors were 1.8-3.3 mm (translation) and 0.5° (rotation); random errors were 1.8-2.1 mm (translation) and 0.3-0.5° (rotation). After online correction, the residual inter-fraction population systematic errors were 1.2-1.8 mm (translation) and 0.3-0.4° (rotation); random errors were 1.4-1.6 mm (translation) and 0.3-0.4° (rotation). Intra-fraction population systematic and random errors were ≤ 1.3 mm (translation) and ≤ 0.2° (rotation). The magnitude of inter-fraction set-up errors in the anterior-posterior direction, roll, and yaw were significantly correlated with higher body weight and body mass index (BMI). The inter-fraction set-up error did not change significantly as a function of time during the course of radiotherapy. The magnitude of intra-fraction motion was not correlated with patient characteristics and treatment time. The total PTV margins accounting for pre-correction and intra-fraction errors were 6.5-10.2 mm; those accounting for post-correction and intra-fraction errors were 4.7-6.3 mm.
Conclusions: CBCT is an effective modality to evaluate and improve the inter-fraction set-up reproducibility in RSD breast radiotherapy, particularly for patients with higher BMI. Intra-fraction motion was minimal during RSD breast radiotherapy.
Keywords: Breast radiotherapy; Inter-fraction reproducibility; Intra-fraction motion; Reverse semi-decubitus technique.