Planning, treatment and in vivo thermo-luminescent dosimetry in patients with breast cancer treated with a VMAT technique: Geneva experience

Abstract

Objective To assess the dose delivered to the skin during a volumetric modulated arc therapy (VMAT) irradiation in patients with breast cancer. Material and method Ten patients with breast cancer were treated using RAPIDARC ® (6 with supraclavicular and/or axillary regions). Decision to perform VMAT treatments was made when the dose received to the contra-lateral breast, lungs or heart exceeds protocol limits with usual 3D conformal techniques. Ten plans were created on Eclipse ® with a daily dose prescription of 2.25 Gy to the mean target volume. Dose planning technique used 1 cm expansion of the target in air (Nicolini et al., Med. Phys. 2011) to take into account patient's breathing movements. Boluses were added in 7 plans for proper target coverage. Patients were treated supine on a 2100C/D Varian linac equipped with on-board imaging. Daily patient positioning was performed by orthogonal images alone or with cone-beam computed tomography scans (CBCT). Thermoluminescent dosimeters (TLD) were located at specific distances on the skin respect to the isocenter, and expected doses were estimated. Measurements reproducibility using bolus was tested on 28 points by repeating several times TLD in vivo dosimetry. Results A total of 116 measurements (100 with bolus, 16 without) were analyzed. Mean standard deviations representing TLD reproducibility was 4.8 cGy. For 52 points measured with bolus the mean measured/expected dose ratio was 1.018 (SD 0.037), while for 16 points without bolus 1.044 (SD 0.065). Conclusion Results indicate that the overall treatment process is adequate and that 1 cm target expansion is sufficient to take into account breast motion during treatment. This technique is well-indicated for patients with convex thoracic wall to avoid important lungs volume irradiated at high dose and also for patients with left breast tumor to reduce heart doses.