[Evaluation of absorbed dose in respiratory-gated radiotherapy using a phantom system that simulates patient respiration].

Respiratory-gated (RG) radiotherapy is useful for minimizing the irradiated volume of normal tissues resulting from the shifting of internal structures caused by respiratory movement. In this technique, although improvement in the dose distribution of the target can be expected, the actual absorbed dose distribution is not clearly determined. Therefore, it is important to clarify the absorbed dose at the tumor and at the evaluation points according to the patient's respiration. We have developed a phantom system that simulates patient respiration (TNK Co., Ltd.), to evaluate the absorbed dose and ensure precise RG radiotherapy. Actual patient respiratory signals were obtained using a respiratory synchronization and gating system (AZ-733V, Anzai Medical). The acquired data were then transferred to a phantom system driven by a ball screw to simulate the shifting of internal structures caused by respiratory movement. We measured the absorbed dose using a micro-ionization chamber dosimeter and the dose distribution using the film method for RG irradiation at expiratory phase by using Linac (PRIMUS, Toshiba Medical Systems Corp.) X-rays. When the distance of phantom movement was set to the average patient respiratory movement distance of 1.5 cm, we first compared absorbed dose with RG irradiation with a gating signal of 50% or less, and without RG irradiation. The absorbed dose at the iso-center was improved by 6.0% and 4.4% at a field size of 4×4 cm2, and by 1.3% and 0.7% at a field size of 5×5 cm2 with an X-ray energy of 6 MV and 10 MV, respectively. There was, however, no dose change at a field size of 10×10 cm2 and 15×15 cm2. When the gating signal was reduced to 25% and 10%, absorbed dose was also improved. With regard to the flatness of the dose profile, no changes in dose distribution were observed in the lateral direction, e.g., beam flatness was within 1.4% and 1.6% at field sizes of 5×5 cm2 and 10×10 cm2, respectively, with an X-ray energy of 6 MV. In the cranial-caudal direction, the dose profile was relatively large even if a gating signal of 50% was applied, i.e., 8.1% and 10.4% at field sizes of 5×5 cm2 and 10×10 cm2, respectively. Beam flatness without RG was much worse, i.e., 37.8% and 38.2%, at field sizes of 5×5 cm2 and 10×10 cm2, respectively. In both cases, the dose was insufficient in the expiratory direction. Although RG radiotherapy is quite useful, the margins in the inspiratory and expiratory phases should be considered based on the level of gating signal and field size in order to formulate appropriate radiotherapy planning in terms of the shifting of internal structures. To ensure accurate radiotherapy, the characteristics of the RG irradiation technique and the radiotherapy equipment must be clearly understood when this technique is to be employed in clinical practice.