A Research On DMLC-based Real-time Tumor Tracking And Gating Radiotherapy Technology

For traditional tumor radiation treatment technology, the motion of thoracic and abdominal tumor increases the risk of target omission, and reduces the tumor control probability(TCP). In order to improve the control probability of mobile tumor, generally a large margin is always added to the clinical target volume(CTV) to avoid the tumor dose insufficiency. But the irradiation dose and irradiated volume of the surrounding normal tissues and organs at risk(OAR), which results in increasing of normal tissue complication probability, would be significantly increased. Worldwide, the three kinds of cancer with the highest mortalities and the three kinds of cancer with the most incidences are all located in thorax and abdomen. So it is of paramount necessity to further improve the received dose accuracy of mobile tumor, and make the mobile tumor receiving the same dose as static tumor, meanwhile give more effective protection to the surrounding normal tissue to minimize their irradiation dose.In this paper, dynamic multi-leaf collimator(DMLC) based real-time tumor tracking was intensively studied, and a gating technique which uses the overlapped projection ratio(OPR) of OAR as beam toggling signal was proposed. The integration of the two techniques formulates a novel tracking and gating radiotherapy for mobile tumor, where the tumor motion can be effectively compensated and more protection can be given to OAR. In this kind of radiation modality, the spatial motion of tumor and its adjacent organ are detected in real time; DMLC aperture tracks the projection of tumor in beam’s eye view(BEV), which ensures beam and tumor keep relatively static, to reduce or even eliminate the margin which be used to compensate tumor motion. During irradiation delivery, OPR in BEV is computed online, irradiation beam switches off when the ratio is greater than a pre-defined threshold.To verify the validity and reliability of the proposed method, intensive studies have been investigated on the DMLC-based real-time tracking and gating radiotherapy. Firstly, the effects of spatial relationship of tumor and OAR on dose distribution of OAR and treatment efficiency were analyzed in detail by employing the experimental motion measurements and simulated structure models of tumor and OAR. Then, the proposed radiotherapy method was applied to real cases to calculate and evaluate dose distribution. In dose calculation process, the tumor motion, OAR motion, and their relative motion information are taken into consideration at the same time, which leads to better dose accuracy and reflect the actual absorbed dose of tissues and organs more accurately. Finally, the beam orientation optimization of the treatment planning based on proposed OPR curve was investigated. Experimental results show that the proposed method is reliable and effective; the margin can be effectively reduced under the premise of tumor receiving the same prescription dose in this kind of treatment modality; Delivery gating can effectively improve the protection ability to OAR; Compared with traditional gating techniques, treatment plans with the proposed beam direction optimization has the possibility of much higher executive efficiency.