Rick L. Holmes PhD, Presenter: Nothing to Disclose

Chengyu Shi, Abstract Co-Author: Nothing to Disclose

Carlos Esquivel, Abstract Co-Author: Nothing to Disclose

Alonso Gutierrez PhD, Abstract Co-Author: Nothing to Disclose

Sotirios L. Stathakis PhD, Abstract Co-Author: Nothing to Disclose

Niko Papanikolaou PhD, Abstract Co-Author: Nothing to Disclose

Weihong L. He, Abstract Co-Author: Nothing to Disclose

Purpose/Objective(s): To determine the impact of gantry arc angle and multiple dose rates utilizing the SmartArc? optimization technique integrated with the Philips Pinnacle treatment planning system. Materials/Methods: This study used the Philips? Pinnacle version 8.9 Alpha treatment planning system (TPS) with the SmartArc optimization technique. The Pinnacle system uses an 810 Opteron? hardware platform with two AMD? dual processors and software multithreading techniques. A single prostate study was utilized for all plan trials with only the gantry angle and dose rate changed. Treatment plans consisted of arc distances of 360§, 300§, 240§, and 180§. Dose rates applied to each arc consisted of 200, 300, 400, 500, and 600 monitor units per minute (MU/min) resulting in 20 trials. The collimator angle was set at 225§ and the table remained at 180§. All plans were optimized with the collapsed cone convolution algorithm. Completed plans were copied and recalculated to a calibrated water-equivalent cube phantom. A 180§ single beam planar dose was calculated. The phantom treatment plan was programmed into a Varian? 2100 C/D treatment unit using the Dynamic Arc modality. The treatment was delivered to a cube phantom with a film at isocenter to capture the dose image. Dose was measure via an ionization chamber 1 cm below the isocenter of the cube phantom. Cube phantom calculated and measure doses were compared as well as the film and planar image comparison results. Results: Dose comparison results revealed agreement ñ 3% on all trials. Film and Planar images vertical and horizontal profiles agreed within ñ 3%. All trials distance to agreement was within ñ 3 mm. Multiple MLC errors occurred at arc distances of 300§ and 360§ degrees at dose rates of 500 and 600 MU/min. MLC tolerance limits were set at 0.5 cm instead of the default 0.2 cm. Noteworthy of mention is the vast improvement of optimization calculations (÷ 80 to ÷ 11.5 minutes), when compared to the previous clinical version of software on the Sun Blade 2300 platform. Conclusions: Large arc distance with their small field shapes and higher dose rates result in MLC errors. It is concluded that the errors occur because of the gantry speed at higher dose rates causing MLC tolerance faults. Utilizing dose rates that do not exceed 400 MU/min with the 300 and 360 degree arc distance resulted in favorable results. The new hardware/software platform improved optimization calculation time by almost 700%.

Holmes, R, Shi, C, Esquivel, C, Gutierrez, A, Stathakis, S, Papanikolaou, N, He, W, Impact of Arc Angle and Dose Rate on IMRT Delivery.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8501822.html