Development of adaptive dose constraints templates for dose optimization in intensity-modulated radiation therapy (IMRT) treatment planning advanced-stage nasopharyngeal cancer.

摘要

Advanced-stage nasopharyngeal carcinoma (NPC) presents very difficult scenarios for radiation therapy (RT) planning. The infiltration of tumor to the skull base and beyond means that the tumor is very close to critical normal organs (organs at risk, OARs). Despite the advent of intensity-modulated radiotherapy (IMRT) treatment technique---the state-of-art RT technique, conflicting requirements between organ protection and target dose conformity is still problematic. The objectives of the present research are (1) to investigate the dosimetry properties of IMRT treatment in advanced-stage NPC in respect of its dosimetric limitations and planning problems, (2) to develop new methods and tools to resolve such problems, in particular to improve the quality of treatment plans and efficiency of the dose planning and optimization process. A series of four inter-linked studies were conducted to address these issues.; The first study was to improve the efficacy in target coverage and organs sparing using an "organ-splitting" approach. The OARs which overlapped with targets were split into target-overlapping and non-overlapping segments and each segment was assigned with different constraints parameters to increase the degree of flexibility during optimization. As a result, a steep gradient in the dose distribution at the regions of interface between the targets and normal critical organs could be achieved and treatment quality was improved. In the second study, a thorough dosimetric comparison between conventional 2-dimensional (2D) RT and IMRT plans was conducted to determine, with reference to outcome of 2D treatments, the extended tolerance dose limits for the critical organs, especially that of the brainstem and spinal cord, and their planning organ at risk volume. Such data could then serve as reference in IMRT planning when the dose of critical organs need be exceeded in order to allow adequate dose to a very close by target. In the third study, the feasibility of using interpolated contours for segmentation of targets and OARs in IMRT planning was investigated. The result indicated that the use of interpolated contours in IMRT planning could significantly reduce the contouring time by about 50% without degrading the target coverage and OARS sparing. In the final study, an array of dose constraint templates that could accommodate different degrees of overlap between the targets and OARs, together with a template selection program, were developed to improve the efficiency of IMRT planning. By applying the methods and tools developed, IMRT treatment planning of advanced NPC could become more efficient and less dependent on planner's experience.; In conclusion, the solutions to several major problems in IMRT planning for advanced-stage NPC were investigated and established. It has been demonstrated in this research that, by applying these methods and tools, significant improvement in the dosimetry and efficiency of IMRT treatment planning can be accomplished as compared with conventional IMRT planning techniques. It is expected that such would translate into an improvement in treatment throughput, better tumor control and reduction in normal tissues complications. The methods developed have potential to be applied to all stages of NPC and to other tumor sites.