Purpose: Adaptive Radiation Therapy (ART) is a feedback treatment process that optimizes a patient's treatment according to the patient specific information measured during the course of treatment. Utilizing an electronic portal imaging device (EPID) and a computer-controlled multileaf collimator (MLC), the ART process is currently being implemented in our clinic to improve the treatment accuracy by compensating for the treatment setup error. A prospective study was conducted to evaluate the feasibility and efficacy of the ART process for clinical use.
Methods and materials: The prospective study included 20 patients who underwent conventional radiotherapy on a linear accelerator equipped with an EPID and a MLC. No specific changes were made in the routine clinical procedures except daily portal images were obtained for each treatment field. Two-dimensional setup error for each treatment field was then measured offline using a software tool. The measured setup errors from initial treatment days were used to predict the systematic and random setup errors for each treatment field. An adjustment decision was made if the predicted systematic error was larger than or equal to 2 mm. Furthermore, the treatment field was extended if the predicted random setup error could not be effectively compensated by the predefined treatment setup margin. Instead of the conventional approach of patient repositioning, setup adjustment was implemented by reshaping the MLC field. The entire process from measuring setup error to reshaping the MLC field was performed offline through a computer network. After completion of a patient's treatment, the systematic and random setup errors after adjustment were compared with those predicted prior to the adjustment. The accuracy of the adjustment, and the reliability and stability of the process were analyzed.
Results: Treatment fields of 13 patients were modified to correct for systematic errors. The mean systematic error was 4 mm with a range of 2 to 7 mm before adjustment. It was reduced to 0.5 mm with a range of 0.2 to 1.4 mm after adjustment. There was no significant difference in random setup errors before and after adjustment. The ART process was found to be stable, as more than 95% of patient specific setup margins were predictable within 1 mm using the first four to nine fractions of treatment, confirming the feasibility of treatment plan reoptimization with the ART process.
Conclusions: The prospective study demonstrates that the ART process can be effectively implemented in routine clinical practice to improve treatment accuracy. This process is also ready to be further extended to reoptimize the treatment plan by incorporating the predicted patient specific setup variation.