A new irradiation system for lung cancer: patient’s self-breath-hold and self-switching radiation-beam on and off without any respiratory monitoring devices

Abstract

Purpose: It is helpful to irradiate a lung tumor under a breath-hold for a reduction of planning target volume(PTV). To develop a simple and efficient breath-hold radiation technique for lung cancer, we made three purposes. 1) To compare the reproducibility of tumor-position under a breath-hold by a patient’s self-estimation with that under a breath-hold by a radiation technologist’s instruction. 2) To develop a switch which enable a patient to turn the radiation-beam on and off. 3) To evaluate the reproducibility of a tumor-position in the radiation field with a patient’s self-breath-hold and self-switching radiation-beam. Materials and Methods: Twenty patients with lung cancer were enrolled in this study. All patients were taught sufficiently how to hold the breath at a same inspiration phase with showing a fluoroscopy of respiratory motion before evaluation. CT images were obtained with 2mm-thickness in the vicinity of the tumor under a series of the breath-hold by a radiation technologist’s instruction and another series of the breath-hold by a patient’s self-estimation. CT was repeated three times with each breath-hold method. Differences of tumor position on CT images were measured in three dimensions with a CT analyzing menu. We have newly developed a switch which was connected directly to the console box of linac ( EXL-15DP; Mitsubishi Electric, Tokyo, Japan). It enables a patient to turn radiation beam on and off freely and repeatedly during a term designated by a radiation technologist. Ten real-time portal images were taken for each patient during irradiation under a patient’s self-breath-hold and self-switching radiation-beam, and the accuracy of reproducibility of the tumor position in the radiation field was visually evaluated. Results: The average of maximum differences of the tumor position in three series of 20 patients under the breath-hold by a radiation technologist’s instruction was 3.0 mm in cranio-caudal direction, 2.1mm in antero-posterior direction and 2.3mm in right-left direction. And that in three series under the breath-hold by a patient’s self-estimation was 2.3mm in cranio-caudal direction, 1.4mm in antero-posterior direction and 1.4mm in right-left direction, respectively. A standard deviation of these differences was larger in the method of breath-hold by a radiation technologist’s instruction than in the method of breath-hold by a patient’s self-estimation. And these differences were larger in tumors of lower lung field than upper lung field. There was statistically significant difference between two methods of breath-hold. The actual switching of the radiation beam was delayed less than 0.1 second behind the patient’s switching. All portal images of 20 patients had a visually sufficient accuracy of tumor position in the radiation field with a difference less than 3mm between a planned position and actual tumor positions. Conclusion: The reproducibility of tumor position under patient’s self breath-hold without any respiratory monitoring devices had a satisfactory accuracy with a difference of tumor position less than 3mm in all directions. The method of breath-hold by a patient’s self-estimation was more accurate than that by a radiation technologist’s instruction. Our newly developed switch which enabled patients to turn the radiation-beam on and off was useful with a good reproducibility under a breath-hold at patients’ pace. And it has an excellent time-efficiency because the irradiation starts at the initiation of patients’ breath-hold and continues irradiation as long as patients keep the breath-hold. This new irradiation system is simple (no need for a respiratory monitoring device) and useful for irradiation of lung cancer with reduced PTV and a sufficient reproducibility.