Study On The Conformal Irradiation Method Of Moving Targets In Heavy Ion Radiotherapy

Heavy-ion beam due to its inversed depth dose distribution profile(i.e. Bragg peak) and relative high biological effectiveness(RBE), it kills the tumor cells effectively while sparing the surrounding healthy tissues, and is considered to be the optimal radiotherapy modality. For the tumor with a relatively fixed position and shape, good treatment accuracy could be ensured as long as the patient was positioned and fixed properly before the irradiation. However, tumors located in the thoracic and abdominal regions are dynamic, which brought many troubles in treatment planning and implementation. The interplay between target motion in the tumor and dynamic beam delivery can cause unwanted geometric misses of the target and/or irradiation of normal structures, limiting the normal tissue sparing potential of heavy-ion radiotherapy.To take full advantage of the particle beam irradiation, and further improve the efficacy of the particle beam therapy and reduce the healthy tissue damage, a series of research work were carried out for moving tumor target conformal radiotherapy based on the passive and active heavy-ion beam delivery system of the Cooling Storage Ring at the Heavy Ion Research Facility in Lanzhou(HIRFL-CSR) and Heavy Ion Medical Machine(HIMM). The main research approaches and content of this dissertation are as follows,(1) Constructed the 4D-CT(time resolved computed tomography) scanning method for the moving target, developed a moving tumor target positioning device and established the CT-WEPL(water-equivalent path length) calibration curve. Based on the Siemens Sensation Open CT and AZ-733 V respiratory gating system, the 4D-CTscanning method for the moving tumor target was achieved. The invention of the positioning device was aimed to locate the moving tumor target in the thoracic and abdominal region. The device allows the patient CT scans, treatment planning and patient positioning under the same coordinate system. Thus, the precision of patient positioning and treatment delivery could be guaranteed. In order to obtain the internal target volume(ITV) of the moving tumor target, the CT-WEPL calibration curve was established based on the experiments at the HIRFL-CSR.(2) Proposed an alternative way to compensate tumor target motion by means of adjustable beam slow extraction of the synchrotron in carbon ion therapy with pencil beam scanning, and the method was verified by computer simulations. The results showed that the dose homogeneity within the target volume gradually tended to be better with the increase of beam extraction time. Volumetric rescanning on the basis of the tunable beam slow extraction might be very useful in dose homogeneity improvement for a moving target if the beam slow extraction time is carefully selected. However, the adjustable beam extraction duration in conjunction with slice-by-slice rescanning would not mitigate the effect of target motion significantly, even with 10 times rescans. For the proposed method combined with lateral beam overlap increment, an increase of the beam spot width seems to be more efficient than reduction of the lateral grid spacing in improving the dose homogeneity.(3) Developed a human respiratory simulation system, and based on this system a series of target motion compensation experiments were carried out. The simulation system consists of four parts: 2-Dimensional motion platform, double-wedge system, multi-wedge system and system frame. The motion patterns for each sub-system include: special function mode, series mode, sensor mode, accelerator external control mode and compensation mode. Based on this system we have conducted several dose verification tests such as moving target rescanning test, lateral beam overlap increment test, manual multi-leaf collimator(MLC) active tracking test and longitudinal motion compensation test. Methods of rescanning and lateral beam overlap increment could improve the dose homogeneity within the moving target volume significantly, but the dose penumbra increased correspondingly. If thesemethods were applied to clinical practice, the radiation damage to the surrounding normal tissues would be large.(4) Established respiratory gating system in the deep-seated tumor treatment terminal at the HIRFL-CSR. The target motion information was detected by opto NCDT1700-750 laser displacement sensor. Since a time delay between target position detection and beam irradiation was exist, we have developed an interacting multi-model algorithm for target motion prediction. The results showed that the dose homogeneity within the target volume was increased significantly with the integration of the prediction algorithm. The test results of the respiratory gating system showed that the dose penumbra was decreased significantly under the passive beam delivery system, and almost recovered to the static case like dose distribution. However, the treatment time was increased substantially with the respiratory gating system, and the effective dose rate was decreased correspondingly.(5) Developed an audio-visual biofeedback(BFB) system, and practical simulation was performed to evaluate the feasibility and effectiveness of this method. To efficiently deliver respiratory-gated radiation during synchrotron-based pulsed heavy-ion radiotherapy, a novel respiratory guidance method combing a personalized BFB system, breath hold, and synchrotron-based gating was designed to help patients synchronize their respiratory patterns with synchrotron pulses and to overcome typical limitations such as low efficiency, residual motion and discomfort. Using the respiratory guidance methods under the passive beam delivery system, the magnitude of residual motion was almost ten times less than during nongated irradiation, and increases in the average effective dose rate by factors of 1.73~4.65 were observed in contrast with conventional free breathing-base gated irradiation. Also the repeatability of the method between treatment fractions was conformed. We have simulated the dose distribution within the moving target volume of the respiratory guidance method under the active beam delivery system. The results showed that the dose distribution was recovered to static case like homogenous distribution. The proposed respiratory guidance method with personalized BFB was conformed to be feasible. Increased effective dose rate and improved overall treatment precision were observed comparedto conventional free breathing-based, respiratory-gated irradiation.