Dosimetric analysis of patient setup and mechanical uncertainties in RapidArc prostate treatments

Abstract

Over the course of a full radiotherapy treatment, patient setup errors result in differences between the calculated, optimised dose distribution and the delivered distribution. In addition, there are mechanical tolerances associated with the linac components, treatment table, and positioning lasers together with allowed dosimetric variations in the radiation beams over the timeframe of weeks in which a treatment course is completed. The aim of this study is to investigate the geometric and dosimetric variations that result from such uncertainties and which will have an impact on the dose distribution to the target volume and organs at risk. To do this, the Rando anthropomorphic phantom was treated as a standard prostate patient undergoing a course of radiotherapy. A full RapidArc treatment was designed for a typical structure set delineated on a CT scan of the phantom. EBT3 gafchromic film was placed between two axial slices of Rando for each fraction, which was then placed in the treatment position and irradiated. The dose actually delivered to the film could then be evaluated for that fraction and compared to the optimised distribution that was originally prescribed. From this, the institutional baseline uncertainty in the distribution due to patient setup, mechanical, and dosimetric uncertainties for each individual fraction over the course of a patient's treatment could be determined as well as the cumulative impact that these variations can have on the dose distribution actually delivered to the patient. This will aid in defining appropriate treatment margins.